2.0.5
ApplicationContext
Resource
interfaceResource
implementationsResourceLoader
ResourceLoaderAware
interfaceResources
as dependenciesResource
pathsBeanWrapper
, and PropertyEditors
Advice
types<tx:advice/>
settings@Transactional
@Transactional
with AspectJSessionFactory
setup in a Spring containerHibernateTemplate
JpaDialect
DispatcherServlet
MBeanInfoAssembler
InterfaceAutodetectCapableMBeanInfoAssembler
interfaceMethodNameBasedMBeanInfoAssembler
ObjectName
s for your beansutil
schemajee
schema<jee:jndi-lookup/>
(simple)<jee:jndi-lookup/>
(with single JNDI environment setting)<jee:jndi-lookup/>
(with multiple JNDI environment settings)<jee:jndi-lookup/>
(complex)<jee:local-slsb/>
(simple)<jee:local-slsb/>
(complex)<jee:remote-slsb/>
lang
schematx
(transaction) schemaaop
schematool
schemabeans
schemaspring-beans-2.0.dtd
Developing software applications is hard enough even with good tools and technologies. Implementing applications using platforms which promise everything but turn out to be heavy-weight, hard to control and not very efficient during the development cycle makes it even harder. Spring provides a light-weight solution for building enterprise-ready applications, while still supporting the possibility of using declarative transaction management, remote access to your logic using RMI or web services, and various options for persisting your data to a database. Spring provides a full-featured MVC framework, and transparent ways of integrating AOP into your software.
Spring could potentially be a one-stop-shop for all your enterprise applications; however, Spring is modular, allowing you to use just those parts of it that you need, without having to bring in the rest. You can use the IoC container, with Struts on top, but you could also choose to use just the Hibernate integration code or the JDBC abstraction layer. Spring has been (and continues to be) designed to be non-intrusive, meaning dependencies on the framework itself are generally none (or absolutely minimal, depending on the area of use).
This document provides a reference guide to Spring's features. Since this document is still to be considered very much work-in-progress, if you have any requests or comments, please post them on the user mailing list or on the support forums at http://forum.springframework.org/.
Before we go on, a few words of gratitude are due to Christian Bauer (of the Hibernate team), who prepared and adapted the DocBook-XSL software in order to be able to create Hibernate's reference guide, thus also allowing us to create this one. Also thanks to Russell Healy for doing an extensive and valuable review of some of the material.
Java applications (a loose term which runs the gamut from constrained applets to full-fledged n-tier server-side enterprise applications) typically are composed of a number of objects that collaborate with one another to form the application proper. The objects in an application can thus be said to have dependencies between themselves.
The Java language and platform provides a wealth of functionality for architecting and building applications, ranging all the way from the very basic building blocks of primitive types and classes (and the means to define new classes), to rich full-featured application servers and web frameworks. One area that is decidedly conspicuous by its absence is any means of taking the basic building blocks and composing them into a coherent whole; this area has typically been left to the purvey of the architects and developers tasked with building an application (or applications). Now to be fair, there are a number of design patterns devoted to the business of composing the various classes and object instances that makeup an all-singing, all-dancing application. Design patterns such as Factory, Abstract Factory, Builder, Decorator, and Service Locator (to name but a few) have widespread recognition and acceptance within the software development industry (presumably that is why these patterns have been formalized as patterns in the first place). This is all very well, but these patterns are just that: best practices given a name, typically together with a description of what the pattern does, where the pattern is typically best applied, the problems that the application of the pattern addresses, and so forth. Notice that the last paragraph used the phrase “... a description of what the pattern does...”; pattern books and wikis are typically listings of such formalized best practice that you can certainly take away, mull over, and then implement yourself in your application.
The IoC component of the Spring Framework addresses the enterprise concern of taking the classes, objects, and services that are to compose an application, by providing a formalized means of composing these various disparate components into a fully working application ready for use. The Spring Framework takes best practices that have been proven over the years in numerous applications and formalized as design patterns, and actually codifies these patterns as first class objects that you as an architect and developer can take away and integrate into your own application(s). This is a Very Good Thing Indeed as attested to by the numerous organizations and institutions that have used the Spring Framework to engineer robust, maintainable applications.
The Spring Framework contains a lot of features, which are well-organized in seven modules shown in the diagram below. This chapter discusses each of the modules in turn.
Overview of the Spring Framework
The Core package
is the most fundamental part of the framework and provides the IoC and Dependency
Injection features. The basic concept here is the BeanFactory
,
which provides a sophisticated implementation of the factory pattern which removes
the need for programmatic singletons and allows you to decouple the configuration and
specification of dependencies from your actual program logic.
The Context package build on the solid base provided by the Core package: it provides a way to access objects in a framework-style manner in a fashion somewhat reminiscent of a JNDI-registry. The context package inherits its features from the beans package and adds support for internationalization (I18N) (using for example resource bundles), event-propagation, resource-loading, and the transparent creation of contexts by, for example, a servlet container.
The DAO package provides a JDBC-abstraction layer that removes the need to do tedious JDBC coding and parsing of database-vendor specific error codes. Also, the JDBC package provides a way to do programmatic as well as declarative transaction management, not only for classes implementing special interfaces, but for all your POJOs (plain old Java objects).
The ORM package provides integration layers for popular object-relational mapping APIs, including JPA, JDO, Hibernate, and iBatis. Using the ORM package you can use all those O/R-mappers in combination with all the other features Spring offers, such as the simple declarative transaction management feature mentioned previously.
Spring's AOP package provides an AOP Alliance-compliant aspect-oriented programming implementation allowing you to define, for example, method-interceptors and pointcuts to cleanly decouple code implementing functionality that should logically speaking be separated. Using source-level metadata functionality you can also incorporate all kinds of behavioral information into your code, in a manner similar to that of .NET attributes.
Spring's Web package provides basic web-oriented integration features, such as multipart file-upload functionality, the initialization of the IoC container using servlet listeners and a web-oriented application context. When using Spring together with WebWork or Struts, this is the package to integrate with.
Spring's MVC package provides a Model-View-Controller (MVC) implementation for web-applications. Spring's MVC framework is not just any old implementation; it provides a clean separation between domain model code and web forms, and allows you to use all the other features of the Spring Framework.
With the building blocks described above you can use Spring in all sorts of scenarios, from applets up to fully-fledged enterprise applications using Spring's transaction management functionality and web framework integration.
Typical full-fledged Spring web application
By using Spring's
declarative transaction management features
the web application is fully transactional, just as it would be when using container
managed transactions as provided by Enterprise JavaBeans. All your custom business logic
can be implemented using simple POJOs, managed by Spring's IoC container. Additional
services include support for sending email, and validation that is independent of the
web layer enabling you to choose where to execute validation rules. Spring's ORM
support is integrated with JPA, Hibernate, JDO and iBatis; for example, when using
Hibernate, you can continue to use your existing mapping files and standard Hibernate
SessionFactory
configuration. Form controllers seamlessly
integrate the web-layer with the domain model, removing the need for
ActionForms
or other classes that transform HTTP parameters to
values for your domain model.
Spring middle-tier using a third-party web framework
Sometimes the current circumstances do not allow you to completely switch
to a different framework. The Spring Framework does not force
you to use everything within it; it is not an all-or-nothing
solution. Existing front-ends built using WebWork, Struts, Tapestry, or other UI
frameworks can be integrated perfectly well with a Spring-based middle-tier,
allowing you to use the transaction features that Spring offers. The only thing
you need to do is wire up your business logic using an
ApplicationContext
and integrate your web layer using a
WebApplicationContext
.
Remoting usage scenario
When you need to access existing code via web services, you can use Spring's
Hessian-
, Burlap-
, Rmi-
or JaxRpcProxyFactory
classes. Enabling remote access to
existing applications suddenly is not that hard anymore.
EJBs - Wrapping existing POJOs
The Spring Framework also provides an access- and abstraction- layer for Enterprise JavaBeans, enabling you to reuse your existing POJOs and wrap them in Stateless Session Beans, for use in scalable, failsafe web applications that might need declarative security.
If you have been using the Spring Framework for some time, you will be aware that Spring has just undergone a major revision.
This revision includes a host of new features, and a lot of the existing functionality has been reviewed and improved. In fact, so much of Spring is shiny and improved that the Spring development team decided that the next release of Spring merited an increment of the version number; and so Spring 2.0 was announced in December 2005 at the Spring Experience conference in Florida.
This chapter is a guide to the new and improved features of Spring 2.0. It is intended to provide a high-level summary so that seasoned Spring architects and developers can become immediately familiar with the new Spring 2.0 functionality. For more in-depth information on the features, please refer to the corresponding sections hyperlinked from within this chapter.
Some of the new and improved functionality described below has been (or will be) backported into the Spring 1.2.x release line. Please do consult the changelogs for the 1.2.x releases to see if a feature has been backported.
One of the areas that contains a considerable number of 2.0 improvements is Spring's IoC container.
Spring XML configuration is now even easier, thanks to the advent of the new XML configuration syntax based on XML Schema. If you want to take advantage of the new tags that Spring provides (and the Spring team certainly suggest that you do because they make configuration less verbose and easier to read), then do read the section entitled Appendix A, XML Schema-based configuration.
On a related note, there is a new, updated DTD for Spring 2.0 that
you may wish to reference if you cannot take advantage of the XML Schema-based
configuration. The DOCTYPE declaration is included below for your convenience,
but the interested reader should definitely read the
'spring-beans-2.0.dtd'
DTD included in the
'dist/resources'
directory of the Spring
2.0 distribution.
<!DOCTYPE beans PUBLIC "-//SPRING//DTD BEAN 2.0//EN" "http://www.springframework.org/dtd/spring-beans-2.0.dtd">
Previous versions of Spring had IoC container level support for exactly two distinct bean scopes (singleton and prototype). Spring 2.0 improves on this by not only providing a number of additional scopes depending on the environment in which Spring is being deployed (for example, request and session scoped beans in a web environment), but also by providing integration points so that Spring users can create their own scopes.
It should be noted that although the underlying (and internal) implementation for singleton- and prototype-scoped beans has been changed, this change is totally transparent to the end user... no existing configuration needs to change, and no existing configuration will break.
Both the new and the original scopes are detailed in the section entitled Section 3.4, “Bean scopes”.
Not only is XML configuration easier to write, it is now also extensible.
What 'extensible' means in this context is that you, as an application developer, or (more likely) as a third party framework or product vendor, can write custom tags that other developers can then plug into their own Spring configuration files. This allows you to have your own domain specific language (the term is used loosely here) of sorts be reflected in the specific configuration of your own components.
Implementing custom Spring tags may not be of interest to every single application developer or enterprise architect using Spring in their own projects. We expect third-party vendors to be highly interested in developing custom configuration tags for use in Spring configuration files.
The extensible configuration mechanism is documented in Appendix B, Extensible XML authoring.
Spring 2.0 has a much improved AOP offering. The Spring AOP framework itself is markedly easier to configure in XML, and significantly less verbose as a result; and Spring 2.0 integrates with the AspectJ pointcut language and @AspectJ aspect declaration style. The chapter entitled Chapter 6, Aspect Oriented Programming with Spring is dedicated to describing this new support.
Spring 2.0 introduces new schema support for defining aspects backed by
regular Java objects. This support takes advantage of the AspectJ pointcut
language and offers fully typed advice (i.e. no more casting and
Object[]
argument manipulation). Details of this support
can be found in the section entitled Section 6.3, “Schema-based AOP support”.
Spring 2.0 also supports aspects defined using the @AspectJ annotations. These aspects can be shared between AspectJ and Spring AOP, and require (honestly!) only some simple configuration. Said support for @AspectJ aspects is discussed in Section 6.2, “@AspectJ support”.
The way that transactions are configured in Spring 2.0 has been changed significantly. The previous 1.2.x style of configuration continues to be valid (and supported), but the new style is markedly less verbose and is the recommended style. Spring 2.0 also ships with an AspectJ aspects library that you can use to make pretty much any object transactional - even objects not created by the Spring IoC container.
The chapter entitled Chapter 9, Transaction management contains all of the details.
Spring 2.0 ships with a JPA abstraction layer that is similar in intent to Spring's JDBC abstraction layer in terms of scope and general usage patterns.
If you are interested in using a JPA-implementation as the backbone of your persistence layer, the section entitled Section 12.6, “JPA” is dedicated to detailing Spring's support and value-add in this area.
Prior to Spring 2.0, Spring's JMS offering was limited to
sending messages and the synchronous receiving of
messages. This functionality (encapsulated in the
JmsTemplate
class) is great, but it doesn't
address the requirement for the asynchronous
receiving of messages.
Spring 2.0 now ships with full support for the reception of messages in an asynchronous fashion, as detailed in the section entitled Section 19.4.2, “Asynchronous Reception - Message-Driven POJOs”.
There are some small (but nevertheless notable) new classes in the
Spring Framework's JDBC support library. The first,
NamedParameterJdbcTemplate
,
provides support for programming JDBC statements using named parameters (as
opposed to programming JDBC statements using only classic placeholder
('?'
) arguments.
Another of the new classes, the
SimpleJdbcTemplate
,
is aimed at making using the JdbcTemplate
even easier to use when
you are developing against Java 5+ (Tiger).
The web tier support has been substantially improved and expanded in Spring 2.0.
A rich JSP tag library for Spring MVC was the JIRA issue that garnered the most votes from Spring users (by a wide margin).
Spring 2.0 ships with a full featured JSP tag library that makes the job of authoring JSP pages much easier when using Spring MVC; the Spring team is confident it will satisfy all of those developers who voted for the issue on JIRA. The new tag library is itself covered in the section entitled Section 13.9, “Using Spring's form tag library”, and a quick reference to all of the new tags can be found in the appendix entitled Appendix E, spring-form.tld.
For a lot of projects, sticking to established conventions and
having reasonable defaults is just what the projects need...
this theme of convention-over-configuration now has explicit support in
Spring MVC. What this means is that if you establish a set of naming
conventions for your Controllers
and views, you can
substantially cut down on the amount of XML configuration
that is required to setup handler mappings, view resolvers,
ModelAndView
instances, etc. This is a great boon
with regards to rapid prototyping, and can also lend a degree of (always
good-to-have) consistency across a codebase.
Spring MVC's convention-over-configuration support is detailed in the section entitled Section 13.11, “Convention over configuration”
Spring 2.0 ships with a Portlet framework that is conceptually similar to the Spring MVC framework. Detailed coverage of the Spring Portlet framework can be found in the section entitled Chapter 16, Portlet MVC Framework.
This final section outlines all of the other new and improved Spring 2.0 features and functionality.
Spring 2.0 now has support for beans written in languages other than Java, with the currently supported dynamic languages being JRuby, Groovy and BeanShell. This dynamic language support is comprehensively detailed in the section entitled Chapter 24, Dynamic language support.
The Spring Framework now has support for Notifications
;
it is also possible to exercise declarative control over the registration
behavior of MBeans with an MBeanServer
.
Spring 2.0 offers an abstraction around the scheduling of tasks.
For the interested developer, the section entitled Section 23.4, “The Spring TaskExecutor
abstraction”
contains all of the details.
Find below pointers to documentation describing some of the new Java 5 support in Spring 2.0.
This final section details issues that may arise during any migration from Spring 1.2.x to Spring 2.0. Feel free to take this next statement with a pinch of salt, but upgrading to Spring 2.0 from a Spring 1.2 application should simply be a matter of dropping the Spring 2.0 jar into the appropriate location in your application's directory structure.
The keyword from the last sentence was of course the “should”. Whether the upgrade is seamless or not depends on how much of the Spring APIs you are using in your code. Spring 2.0 removed pretty much all of the classes and methods previously marked as deprecated in the Spring 1.2.x codebase, so if you have been using such classes and methods, you will of course have to use alternative classes and methods (some of which are summarized below).
With regards to configuration, Spring 1.2.x style XML configuration is 100%, satisfaction-guaranteed compatible with the Spring 2.0 library. Of course if you are still using the Spring 1.2.x DTD, then you won't be able to take advantage of some of the new Spring 2.0 functionality (such as scopes and easier AOP and transaction configuration), but nothing will blow up.
The suggested migration strategy is to drop in the Spring 2.0 jar(s) to benefit from the improved code present in the release (bug fixes, optimizations, etc.). You can then, on an incremental basis, choose to start using the new Spring 2.0 features and configuration. For example, you could choose to start configuring just your aspects in the new Spring 2.0 style; it is perfectly valid to have 90% of your configuration using the old-school Spring 1.2.x configuration (which references the 1.2.x DTD), and have the other 10% using the new Spring 2.0 configuration (which references the 2.0 DTD or XSD). Bear in mind that you are not forced to upgrade your XML configuration should you choose to drop in the Spring 2.0 libraries.
For a comprehensive list of changes, consult the 'changelog.txt'
file that is located in the top level directory of the Spring Framework 2.0 distribution.
The packaging of the Spring Framework jars has changed quite substantially
between the 1.2.x and 2.0 releases. In particular, there are now dedicated jars for the
JDO, Hibernate 2/3, TopLink ORM integration classes: they are no longer bundled in the
core 'spring.jar'
file anymore.
Spring 2.0 ships with XSDs that describe Spring's XML metadata format in a much richer fashion than the DTD that shipped with previous versions. The old DTD is still fully supported, but if possible you are encouraged to reference the XSD files at the top of your bean definition files.
One thing that has changed in a (somewhat) breaking fashion is the way that
bean scopes are defined. If you are using the Spring 1.2 DTD you can continue to use
the 'singleton'
attribute. You can however choose to
reference the new Spring 2.0 DTD
which does not permit the use of the 'singleton'
attribute, but
rather uses the 'scope'
attribute to define the bean lifecycle scope.
A number of classes and methods that previously were marked as
@deprecated
have been removed from the Spring 2.0 codebase.
The Spring team decided that the 2.0 release marked a fresh start of sorts, and that any
deprecated 'cruft' was better excised now instead of continuing to haunt the codebase for
the foreseeable future.
As mentioned previously, for a comprehensive list of changes, consult the
'changelog.txt'
file that is located in the top level directory of
the Spring Framework 2.0 distribution.
The following classes/interfaces have been removed from the Spring 2.0 codebase:
ResultReader
: Use the
RowMapper
interface instead.
BeanFactoryBootstrap
: Consider using a
BeanFactoryLocator
or a custom bootstrap class instead.
Please note that support for Apache OJB was totally removed from the main Spring source tree; the Apache OJB integration library is still available, but can be found in it's new home in the Spring Modules project.
Please note that support for iBATIS SQL Maps 1.3 has been removed. If you haven't done so already, upgrade to iBATIS SQL Maps 2.0/2.1.
The view name that is determined by the UrlFilenameViewController
now takes into account the nested path of the request. This is a breaking change from
the original contract of the UrlFilenameViewController
, and means
that if you are upgrading to Spring 2.0 from Spring 1.x and you are using this
class you might have to change your Spring Web MVC
configuration slightly. Refer to the class level Javadocs of the UrlFilenameViewController
to see examples of the new contract for view name determination.
A number of the sample applications have also been updated to showcase the new and
improved features of Spring 2.0, so do take the time to investigate them. The aforementioned
sample applications can be found in the 'samples'
directory of the full Spring distribution
('spring-with-dependencies.[zip|tar.gz]'
), and are documented
(in part) in the chapter entitled Chapter 26, Showcase applications.
The Spring reference documentation has also substantially been updated to reflect all of the above features new in Spring 2.0. While every effort has been made to ensure that there are no errors in this documentation, some errors may nevertheless have crept in. If you do spot any typos or even more serious errors, and you can spare a few cycles during lunch, please do bring the error to the attention of the Spring team by raising an issue.
Special thanks to Arthur Loder for his tireless proofreading of the Spring Framework reference documentation and Javadocs.
This initial part of the reference documentation covers all of those technologies that are absolutely integral to the Spring Framework.
Foremost amongst these is the Spring Framework's Inversion of Control (IoC) container. A thorough treatment of the Spring Framework's IoC container is closely followed by comprehensive coverage of Spring's Aspect-Oriented Programming (AOP) technologies. The Spring Framework has its own AOP framework, which is conceptually easy to understand, and which successfully addresses the 80% sweet spot of AOP requirements in Java enterprise programming.
Coverage of Spring's integration with AspectJ (currently the richest - in terms of features - and certainly most mature AOP implementation in the Java enterprise space) is also provided.
Finally, the adoption of the test-driven-development (TDD) approach to software development is certainly advocated by the Spring team, and so coverage of Spring's support for integration testing is covered (alongside best practices for unit testing). The Spring team have found that the correct use of IoC certainly does make both unit and integration testing easier (in that the presence of setter methods and appropriate constructors on classes makes them easier to wire together on a test without having to set up service locator registries and suchlike)... the chapter dedicated solely to testing will hopefully convince you of this as well.
This chapter covers the Spring Framework's implementation of the Inversion of Control (IoC) [1] principle.
The org.springframework.beans
and
org.springframework.context
packages provide the basis
for the Spring Framework's IoC container. The
BeanFactory
interface provides an advanced configuration mechanism capable of managing
objects of any nature. The
ApplicationContext
interface builds on top of the BeanFactory
(it is a sub-interface) and adds other functionality such as easier integration
with Spring's AOP features, message resource handling (for use in
internationalization), event propagation, and application-layer specific contexts
such as the WebApplicationContext
for use in web
applications.
In short, the BeanFactory
provides the
configuration framework and basic functionality, while the
ApplicationContext
adds more enterprise-centric
functionality to it. The ApplicationContext
is a
complete superset of the BeanFactory
, and any
description of BeanFactory
capabilities and
behavior is to be considered to apply to the
ApplicationContext
as well.
This chapter is divided into two parts, with the
first part covering the basic principles
that apply to both the BeanFactory
and
ApplicationContext
, and with the
second part covering those features
that apply only to the ApplicationContext
interface.
In Spring, those objects that form the backbone of your application and that are managed by the Spring IoC container are referred to as beans. A bean is simply an object that is instantiated, assembled and otherwise managed by a Spring IoC container; other than that, there is nothing special about a bean (it is in all other respects one of probably many objects in your application). These beans, and the dependencies between them, are reflected in the configuration metadata used by a container.
The org.springframework.beans.factory.BeanFactory
is the actual representation of the Spring IoC container that is
responsible for containing and otherwise managing the aforementioned beans.
The BeanFactory
interface is the central IoC container
interface in Spring. Its responsibilities include instantiating or sourcing application
objects, configuring such objects, and assembling the dependencies between these objects.
There are a number of implementations of the BeanFactory
interface that come supplied straight out-of-the-box with Spring. The most commonly used
BeanFactory
implementation is the
XmlBeanFactory
class. This implementation allows you to express the
objects that compose your application, and the doubtless rich interdependencies between such
objects, in terms of XML. The XmlBeanFactory
takes this
XML configuration metadata and uses it to create a
fully configured system or application.
The Spring IoC container
As can be seen in the above image, the Spring IoC container consumes some form of configuration metadata; this configuration metadata is nothing more than how you (as an application developer) inform the Spring container as to how to “instantiate, configure, and assemble [the objects in your application]”. This configuration metadata is typically supplied in a simple and intuitive XML format. When using XML-based configuration metadata, you write bean definitions for those beans that you want the Spring IoC container to manage, and then let the container do it's stuff.
![]() | Note |
---|---|
XML-based metadata is by far the most commonly used form of configuration metadata. It is not however the only form of configuration metadata that is allowed. The Spring IoC container itself is totally decoupled from the format in which this configuration metadata is actually written. At the time of writing, you can supply this configuration metadata using either XML, the Java properties format, or programmatically (using Spring's public API). The XML-based configuration metadata format really is simple though, and so the remainder of this chapter will use the XML format to convey key concepts and features of the Spring IoC container. |
Please be advised that in the vast majority of application scenarios,
explicit user code is not required to instantiate one or more instances
of a Spring IoC container. For example, in a web application scenario, a simple
eight (or so) lines of absolutely boilerplate J2EE web descriptor XML in the
web.xml
file of the application will typically suffice
(see Section 3.8.4, “Convenient ApplicationContext
instantiation for web applications”).
Spring configuration consists of at least one bean definition that the
container must manage, but typically there will be more than one bean definition.
When using XML-based configuration metadata, these beans are configured as
<bean/>
elements inside a top-level <beans/>
element.
These bean definitions correspond to the actual objects that make up your
application. Typically you will have bean definitions for your service layer
objects, your data access objects (DAOs), presentation objects such as Struts
Action
instances, infrastructure objects such as
Hibernate SessionFactory
instances, JMS
Queue
references, etc. (the possibilities are of
course endless, and are limited only by the scope and complexity of your application).
(Typically one does not configure fine-grained domain objects in the container.)
Find below an example of the basic structure of XML-based configuration metadata.
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.0.xsd"> <bean id="..." class="..."> <!-- collaborators and configuration for this bean go here --> </bean> <bean id="..." class="..."> <!-- collaborators and configuration for this bean go here --> </bean> <!-- more bean definitions go here... --> </beans>
Instantiating a Spring IoC container is easy; find below some examples of how to do just that:
Resource resource = new FileSystemResource("beans.xml"); BeanFactory factory = new XmlBeanFactory(resource);
... or...
ClassPathResource resource = new ClassPathResource("beans.xml"); BeanFactory factory = new XmlBeanFactory(resource);
... or...
ApplicationContext context = new ClassPathXmlApplicationContext(
new String[] {"applicationContext.xml", "applicationContext-part2.xml"});
// of course, an ApplicationContext
is just a BeanFactory
BeanFactory factory = (BeanFactory) context;
It can often be useful to split up container definitions into multiple
XML files. One way to then load an application context which is configured
from all these XML fragments is to use the application context constructor
which takes multiple Resource
locations. With
a bean factory, a bean definition reader can be used multiple times to read
definitions from each file in turn.
Generally, the Spring team prefers the above approach, since it
keeps container configuration files unaware of the fact that they are
being combined with others. An alternate approach is to use one or more
occurrences of the <import/>
element to load bean definitions
from another file (or files). Any <import/>
elements must be
placed before <bean/>
elements in the file doing the importing.
Let's look at a sample:
<beans> <import resource="services.xml"/> <import resource="resources/messageSource.xml"/> <import resource="/resources/themeSource.xml"/> <bean id="bean1" class="..."/> <bean id="bean2" class="..."/> </beans>
In this example, external bean definitions are being loaded from 3
files, services.xml
,
messageSource.xml
, and
themeSource.xml
. All location paths are considered
relative to the definition file doing the importing, so
services.xml
in this case must be in the same directory
or classpath location as the file doing the importing, while
messageSource.xml
and
themeSource.xml
must be in a
resources
location below the location of the importing
file. As you can see, a leading slash is actually ignored, but given that
these are considered relative paths, it is probably better form not to use
the slash at all.
The contents of the files being imported must be fully valid XML
bean definition files according to the Schema or DTD, including the
top level <beans/>
element.
As mentioned previously, a Spring IoC container manages one or more
beans. These beans are created using the instructions
defined in the configuration metadata that has been supplied to the container
(typically in the form of XML <bean/>
definitions).
Within the container itself, these bean definitions are represented as
BeanDefinition
objects, which contain (among other
information) the following metadata:
a package-qualified class name: this is
normally the actual implementation class of the bean being defined.
However, if the bean is to be instantiated by invoking a
static
factory method
instead of using a normal constructor, this will actually be the
class name of the factory class.
bean behavioral configuration elements, which state how the bean should behave in the container (prototype or singleton, autowiring mode, initialization and destruction callbacks, and so forth).
constructor arguments and property values to set in the newly created bean. An example would be the number of connections to use in a bean that manages a connection pool (either specified as a property or as a constructor argument), or the pool size limit.
other beans which are needed for the bean to do its work, that is collaborators (also called dependencies).
The concepts listed above directly translate to a set of properties that each bean definition consists of. Some of these properties are listed below, along with a link to further documentation about each of them.
Table 3.1. The bean definition
Feature | Explained in... |
---|---|
class | |
name | |
scope | |
constructor arguments | |
properties | |
autowiring mode | |
dependency checking mode | |
lazy-initialization mode | |
initialization method | |
destruction method |
Besides bean definitions which contain information on how to
create a specific bean, certain BeanFactory
implementations also permit the registration of existing objects that have
been created outside the factory (by user code). The
DefaultListableBeanFactory
class supports this
through the registerSingleton(..)
method. Typical applications
solely work with beans defined through metadata bean definitions, though.
Every bean has one or more ids (also called identifiers, or names; these terms refer to the same thing). These ids must be unique within the container the bean is hosted in. A bean will almost always have only one id, but if a bean has more than one id, the extra ones can essentially be considered aliases.
When using XML-based configuration metadata, you use the 'id'
or 'name'
attributes to specify the bean identifier(s). The
'id'
attribute allows you to specify exactly one id, and as
it is a real XML element ID attribute, the XML parser is able to do
some extra validation when other elements reference the id; as such, it is the
preferred way to specify a bean id. However, the XML specification does limit
the characters which are legal in XML IDs. This is usually not a constraint, but
if you have a need to use one of these special XML characters, or want to introduce
other aliases to the bean, you may also or instead specify one or more bean ids,
separated by a comma (,
), semicolon (;
), or
whitespace in the 'name'
attribute.
Please note that you are not required to supply a name for a bean. If no name is supplied explicitly, the container will generate a (unique) name for that bean. The motivations for not supplying a name for a bean will be discussed later (one use case is inner beans).
In a bean definition itself, you may supply more than one name for
the bean, by using a combination of up to one name specified via the
id
attribute, and any number of other names via the
name
attribute. All these names can be considered
equivalent aliases to the same bean, and are useful for some situations,
such as allowing each component used in an application to refer to a
common dependency using a bean name that is specific to that component
itself.
Having to specify all aliases when the bean is actually defined is not
always adequate however. It is sometimes desirable to introduce an alias
for a bean which is defined elsewhere. In XML-based configuration metadata
this may be accomplished via the use of the standalone
<alias/>
element. For example:
<alias name="fromName" alias="toName"/>
In this case, a bean in the same container which is named
'fromName'
, may also after the use of this alias
definition, be referred to as 'toName'
.
As a concrete example, consider the case where component A defines a DataSource bean called componentA-dataSource, in its XML fragment. Component B would however like to refer to the DataSource as componentB-dataSource in its XML fragment. And the main application, MyApp, defines its own XML fragment and assembles the final application context from all three fragments, and would like to refer to the DataSource as myApp-dataSource. This scenario can be easily handled by adding to the MyApp XML fragment the following standalone aliases:
<alias name="componentA-dataSource" alias="componentB-dataSource"/> <alias name="componentA-dataSource" alias="myApp-dataSource" />
Now each component and the main app can refer to the dataSource via a name that is unique and guaranteed not to clash with any other definition (effectively there is a namespace), yet they refer to the same bean.
A bean definition can be seen as a recipe for creating one or more actual objects. The container looks at the recipe for a named bean when asked, and uses the configuration metadata encapsulated by that bean definition to create (or acquire) an actual object.
If you are using XML-based configuration metadata, you can specify
the type (or class) of object that is to be instantiated using the
'class'
attribute of the <bean/>
element. This 'class'
attribute (which internally
eventually boils down to being a Class
property on a
BeanDefinition
instance) is normally
mandatory (see Section 3.2.3.2.3, “Instantiation using an instance factory method” and
Section 3.6, “Bean definition inheritance” for the two exceptions)
and is used for one of two purposes. The class property specifies the
class of the bean to be constructed in the much more common case where the
container itself directly creates the bean by calling its constructor
reflectively (somewhat equivalent to Java code using the
'new' operator). In the less common case where the
container invokes a static
, factory
method on a class to create the bean, the class property specifies the actual
class containing the static
factory method that is to
be invoked to create the object (the type of the object returned from the
invocation of the static
factory method may be the same
class or another class entirely, it doesn't matter).
When creating a bean using the constructor approach, all normal classes are usable by and compatible with Spring. That is, the class being created does not need to implement any specific interfaces or be coded in a specific fashion. Just specifying the bean class should be enough. However, depending on what type of IoC you are going to use for that specific bean, you may need a default (empty) constructor.
Additionally, the Spring IoC container isn't limited to just managing true JavaBeans, it is also able to manage virtually any class you want it to manage. Most people using Spring prefer to have actual JavaBeans (having just a default (no-argument) constructor and appropriate setters and getters modeled after the properties) in the container, but it is also possible to have more exotic non-bean-style classes in your container. If, for example, you need to use a legacy connection pool that absolutely does not adhere to the JavaBean specification, Spring can manage it as well.
When using XML-based configuration metadata you can specify your bean class like so:
<bean id="exampleBean" class="examples.ExampleBean"/> <bean name="anotherExample" class="examples.ExampleBeanTwo"/>
The mechanism for supplying arguments to the constructor (if required), or setting properties of the object instance after it has been constructed, will be described shortly.
When defining a bean which is to be created using a static
factory method, along with the class
attribute
which specifies the class containing the static
factory method,
another attribute named factory-method
is needed to
specify the name of the factory method itself. Spring expects to be
able to call this method (with an optional list of arguments as
described later) and get back a live object, which from that point on
is treated as if it had been created normally via a constructor. One
use for such a bean definition is to call static
factories in legacy code.
The following example shows a bean definition which specifies
that the bean is to be created by calling a factory-method. Note that
the definition does not specify the type (class) of the returned
object, only the class containing the factory method. In this example,
the createInstance()
method must be a
static method.
<bean id="exampleBean" class="examples.ExampleBean2" factory-method="createInstance"/>
The mechanism for supplying (optional) arguments to the factory method, or setting properties of the object instance after it has been returned from the factory, will be described shortly.
In a fashion similar to instantiation via a static factory method, instantiation using an instance factory method is where the factory method of an existing bean from the container is invoked to create the new bean.
To use this mechanism, the 'class'
attribute
must be left empty, and the 'factory-bean'
attribute
must specify the name of a bean in the current (or parent/ancestor) container
that contains the factory method. The factory method itself must still be set
via the 'factory-method'
attribute (as seen in the example
below).
<!-- the factory bean, which contains a method called createInstance()
-->
<bean id="myFactoryBean" class="...">
...
</bean>
<!-- the bean to be created via the factory bean -->
<bean id="exampleBean"
factory-bean="myFactoryBean"
factory-method="createInstance"/>
Although the mechanisms for setting bean properties are still to be discussed, one implication of this approach is that the factory bean itself can be managed and configured via DI.
A BeanFactory
is essentially nothing more
than the interface for an advanced factory capable of maintaining a registry
of different beans and their dependencies. The BeanFactory
enables you to read bean definitions and access them using the bean factory.
When using just the BeanFactory
you would create
one and read in some bean definitions in the XML format as follows:
InputStream is = new FileInputStream("beans.xml"); BeanFactory factory = new XmlBeanFactory(is);
Basically that's all there is to it. Using getBean(String)
you can retrieve instances of your beans; the client-side view of the
BeanFactory
is surprisingly simple. The
BeanFactory
interface has only six methods for
client code to call:
boolean containsBean(String)
: returns true
if the BeanFactory
contains a bean
definition or bean instance that matches the given name
Object getBean(String)
: returns an instance
of the bean registered under the given name. Depending on how the
bean was configured by the BeanFactory
configuration, either a
singleton and thus shared instance or a newly created bean will be
returned. A BeansException
will be thrown when
either the bean could not be found (in which case it'll be a
NoSuchBeanDefinitionException
), or an exception
occurred while instantiating and preparing the bean
Object getBean(String, Class)
: returns a
bean, registered under the given name. The bean returned will be
cast to the given Class. If the bean could not be cast,
corresponding exceptions will be thrown
(BeanNotOfRequiredTypeException
). Furthermore,
all rules of the getBean(String)
method apply (see above)
Class getType(String name)
: returns the
Class
of the bean with the given name. If no bean
corresponding to the given name could be found, a
NoSuchBeanDefinitionException
will be thrown
boolean isSingleton(String)
: determines
whether or not the bean definition or bean instance registered under
the given name is a singleton (bean scopes such as singleton are explained
later). If no bean corresponding
to the given name could be found, a
NoSuchBeanDefinitionException
will be thrown
String[] getAliases(String)
: Return the
aliases for the given bean name, if any were defined in the bean
definition
Your typical enterprise application is not made up of a single object (or bean in the Spring parlance). Even the simplest of applications will no doubt have at least a handful of objects that work together to present what the end-user sees as a coherent application. This next section explains how you go from defining a number of bean definitions that stand-alone, each to themselves, to a fully realized application where objects work (or collaborate) together to achieve some goal (usually an application that does what the end-user wants).
The basic principle behind Dependency Injection (DI) is that objects define their dependencies (that is to say the other objects they work with) only through constructor arguments, arguments to a factory method, or properties which are set on the object instance after it has been constructed or returned from a factory method. Then, it is the job of the container to actually inject those dependencies when it creates the bean. This is fundamentally the inverse, hence the name Inversion of Control (IoC), of the bean itself being in control of instantiating or locating its dependencies on its own using direct construction of classes, or something like the Service Locator pattern.
It becomes evident upon usage that code gets much cleaner when the DI principle is applied, and reaching a higher grade of decoupling is much easier when beans do not look up their dependencies, but are provided with them (and additionally do not even know where the dependencies are located and of what actual class they are).
As touched on in the previous paragraph, DI exists in two major variants, namely Setter Injection, and Constructor Injection.
Setter-based DI is realized by calling setter methods
on your beans after invoking a no-argument constructor or no-argument
static
factory method to instantiate your bean.
Find below an example of a class that can only be dependency injected using pure setter injection. Note that there is nothing special about this class... it is plain old Java.
public class SimpleMovieLister { // theSimpleMovieLister
has a dependency on theMovieFinder
private MovieFinder movieFinder; // a setter method so that the Spring container can 'inject' aMovieFinder
public void setMovieFinder(MovieFinder movieFinder) { this.movieFinder = movieFinder; } // business logic that actually 'uses' the injectedMovieFinder
is omitted... }
Constructor-based DI
is realized by invoking a constructor with a number of arguments,
each representing a collaborator. Additionally,
calling a static
factory method with specific
arguments to construct the bean, can be considered almost equivalent,
and the rest of this text will consider arguments to a constructor and
arguments to a static
factory method similarly.
Find below an example of a class that could only be dependency injected using constructor injection. Again, note that there is nothing special about this class.
public class SimpleMovieLister { // theSimpleMovieLister
has a dependency on theMovieFinder
private MovieFinder movieFinder; // a constructor so that the Spring container can 'inject' aMovieFinder
public SimpleMovieLister(MovieFinder movieFinder) { this.movieFinder = movieFinder; } // business logic that actually 'uses' the injectedMovieFinder
is omitted... }
The BeanFactory
supports both of these
variants for injecting dependencies into beans it manages. (It in fact
also supports injecting setter-based dependencies after some
dependencies have already been supplied via the constructor approach.)
The configuration for the dependencies comes in the form of a
BeanDefinition
, which is used together with
PropertyEditor
instances to know how to
convert properties from one format to another. However, most users of
Spring will not be dealing with these classes directly
(that is programmatically), but rather with an XML definition file which
will be converted internally into instances of these classes, and used
to load an entire Spring IoC container instance.
Bean dependency resolution generally happens as follows:
The BeanFactory
is created
and initialized with a configuration which describes all
the beans. (Most Spring users use a
BeanFactory
or
ApplicationContext
implementation
that supports XML format configuration files.)
Each bean has dependencies expressed in the form of properties, constructor arguments, or arguments to the static-factory method when that is used instead of a normal constructor. These dependencies will be provided to the bean, when the bean is actually created.
Each property or constructor argument is either an actual definition of the value to set, or a reference to another bean in the container.
Each property or constructor argument which is a value must
be able to be converted from whatever format it was specified
in, to the actual type of that property or constructor argument.
By default Spring can convert a value supplied in string format
to all built-in types, such as int
,
long
, String
,
boolean
, etc.
The Spring container validates the configuration of each bean as the container
is created, including the validation that properties which are bean references
are actually referring to valid beans. However, the bean properties themselves
are not set until the bean is actually created. For those
beans that are singleton-scoped and set to be pre-instantiated (such as
singleton beans in an ApplicationContext
),
creation happens at the time that the container is created, but otherwise this is
only when the bean is requested. When a bean actually has to be created, this will
potentially cause a graph of other beans to be created, as its dependencies and its
dependencies' dependencies (and so on) are created and assigned.
You can generally trust Spring to do the right thing. It will detect
mis-configuration issues, such as references to non-existent beans and
circular dependencies, at container load-time. It will actually set
properties and resolve dependencies as late as possible, which is when
the bean is actually created. This means that
a Spring container which has loaded correctly can later generate an
exception when you request a bean if there is a problem creating that
bean or one of its dependencies. This could happen if the bean throws
an exception as a result of a missing or invalid property, for example.
This potentially delayed visibility of some configuration issues is why
ApplicationContext
implementations by default
pre-instantiate singleton beans. At the cost of some upfront time
and memory to create these beans before they are actually needed,
you find out about configuration issues when the
ApplicationContext
is created, not later.
If you wish, you can still override this default behavior and set any of these
singleton beans to lazy-initialize (that is not be pre-instantiated).
Finally, if it is not immediately apparent, it is worth mentioning that when one or more collaborating beans are being injected into a dependent bean, each collaborating bean is totally configured prior to being passed (via one of the DI flavors) to the dependent bean. This means that if bean A has a dependency on bean B, the Spring IoC container will totally configure bean B prior to invoking the setter method on bean A; you can read 'totally configure' to mean that the bean will be instantiated (if not a pre-instantiated singleton), all of its dependencies will be set, and the relevant lifecycle methods (such as a configured init method or the IntializingBean callback method) will all be invoked.
First, an example of using XML-based configuration metadata for setter-based DI. Find below a small part of a Spring XML configuration file specifying some bean definitions.
<bean id="exampleBean" class="examples.ExampleBean">
<!-- setter injection using the nested <ref/>
element -->
<property name="beanOne"><ref bean="anotherExampleBean"/></property>
<!-- setter injection using the neater 'ref' attribute -->
<property name="beanTwo" ref="yetAnotherBean"/>
<property name="integerProperty" value="1"/>
</bean>
<bean id="anotherExampleBean" class="examples.AnotherBean"/>
<bean id="yetAnotherBean" class="examples.YetAnotherBean"/>
public class ExampleBean { private AnotherBean beanOne; private YetAnotherBean beanTwo; private int i; public void setBeanOne(AnotherBean beanOne) { this.beanOne = beanOne; } public void setBeanTwo(YetAnotherBean beanTwo) { this.beanTwo = beanTwo; } public void setIntegerProperty(int i) { this.i = i; } }
As you can see, setters have been declared to match against the properties specified in the XML file.
Now, an example of using constructor-based DI. Find below a snippet from an XML configuration that specifies constructor arguments, and the corresponding Java class.
<bean id="exampleBean" class="examples.ExampleBean">
<!-- constructor injection using the nested <ref/>
element -->
<constructor-arg><ref bean="anotherExampleBean"/></constructor-arg>
<!-- constructor injection using the neater 'ref' attribute -->
<constructor-arg ref="yetAnotherBean"/>
<constructor-arg type="int" value="1"/>
</bean>
<bean id="anotherExampleBean" class="examples.AnotherBean"/>
<bean id="yetAnotherBean" class="examples.YetAnotherBean"/>
public class ExampleBean { private AnotherBean beanOne; private YetAnotherBean beanTwo; private int i; public ExampleBean( AnotherBean anotherBean, YetAnotherBean yetAnotherBean, int i) { this.beanOne = anotherBean; this.beanTwo = yetAnotherBean; this.i = i; } }
As you can see, the constructor arguments specified in the
bean definition will be used to pass in as arguments to the constructor
of the ExampleBean
.
Now consider a variant of this where instead of using a
constructor, Spring is told to call a static
factory
method to return an instance of the object:
<bean id="exampleBean" class="examples.ExampleBean" factory-method="createInstance"> <constructor-arg ref="anotherExampleBean"/> <constructor-arg ref="yetAnotherBean"/> <constructor-arg value="1"/> </bean> <bean id="anotherExampleBean" class="examples.AnotherBean"/> <bean id="yetAnotherBean" class="examples.YetAnotherBean"/>
public class ExampleBean { // a private constructor private ExampleBean(...) { ... } // a static factory method; the arguments to this method can be // considered the dependencies of the bean that is returned, // regardless of how those arguments are actually used. public static ExampleBean createInstance ( AnotherBean anotherBean, YetAnotherBean yetAnotherBean, int i) { ExampleBean eb = new ExampleBean (...); // some other operations... return eb; } }
Note that arguments to the static
factory method
are supplied via constructor-arg
elements, exactly
the same as if a constructor had actually been used. Also, it is
important to realize that the type of the class being returned by
the factory method does not have to be of the same type as the class
which contains the static
factory method, although
in this example it is. An instance (non-static) factory method would
be used in an essentially identical fashion (aside from the use of the
factory-bean
attribute instead of the
class
attribute), so details will not be discussed here.
Constructor argument resolution matching occurs using the argument's type. If there is no potential for ambiguity in the constructor arguments of a bean definition, then the order in which the constructor arguments are defined in a bean definition is the order in which those arguments will be supplied to the appropriate constructor when it is being instantiated. Consider the following class:
package x.y;
public class Foo {
public Foo(Bar bar, Baz baz) {
// ...
}
}
There is no potential for ambiguity here (assuming of course that Bar
and Baz
classes are not related in an inheritance hierarchy).
Thus the following configuration will work just fine, and you do not need to
specify the constructor argument indexes and / or types explicitly.
<beans> <bean name="foo" class="x.y.Foo"> <constructor-arg> <bean class="x.y.Bar"/> </constructor-arg> <constructor-arg> <bean class="x.y.Baz"/> </constructor-arg> </bean> </beans>
When another bean is referenced, the type is known, and
matching can occur (as was the case with the preceding example).
When a simple type is used, such as
<value>true<value>
, Spring cannot
determine the type of the value, and so cannot match by type without
help. Consider the following class:
package examples; public class ExampleBean { // No. of years to the calculate the Ultimate Answer private int years; // The Answer to Life, the Universe, and Everything private String ultimateAnswer; public ExampleBean(int years, String ultimateAnswer) { this.years = years; this.ultimateAnswer = ultimateAnswer; } }
The above scenario can use type matching
with simple types by explicitly specifying the type of the constructor
argument using the 'type'
attribute. For example:
<bean id="exampleBean" class="examples.ExampleBean"> <constructor-arg type="int" value="7500000"/> <constructor-arg type="java.lang.String" value="42"/> </bean>
Constructor arguments can have their index specified explicitly by use of
the index
attribute. For example:
<bean id="exampleBean" class="examples.ExampleBean"> <constructor-arg index="0" value="7500000"/> <constructor-arg index="1" value="42"/> </bean>
As well as solving the ambiguity problem of multiple simple values, specifying an index also solves the problem of ambiguity where a constructor may have two arguments of the same type. Note that the index is 0 based.
As mentioned in the previous section, bean properties and
constructor arguments can be defined as either references to other
managed beans (collaborators), or values defined inline. Spring's XML-based
configuration metadata supports a number of sub-element types
within its <property/>
and
<constructor-arg/>
elements for just this purpose.
The <value/>
element specifies a property or
constructor argument as a human-readable string representation.
As mentioned previously,
JavaBeans PropertyEditors
are used to convert these
string values from a String
to the actual type of the
property or argument.
<bean id="myDataSource" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close">
<!-- results in a setDriverClassName(String)
call -->
<property name="driverClassName">
<value>com.mysql.jdbc.Driver</value>
</property>
<property name="url">
<value>jdbc:mysql://localhost:3306/mydb</value>
</property>
<property name="username">
<value>root</value>
</property>
<property name="password">
<value>masterkaoli</value>
</property>
</bean>
The <property/>
and <constructor-arg/>
elements also support the use of the 'value'
attribute, which can lead
to much more succinct configuration. When using the 'value'
attribute,
the above bean definition reads like so:
<bean id="myDataSource" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close">
<!-- results in a setDriverClassName(String)
call -->
<property name="driverClassName" value="com.mysql.jdbc.Driver"/>
<property name="url" value="jdbc:mysql://localhost:3306/mydb"/>
<property name="username" value="root"/>
<property name="password" value="masterkaoli"/>
</bean>
The Spring team generally prefer the attribute style over the use of nested
<value/>
elements. If you are reading this reference manual
straight through from top to bottom (wow!) then we are getting slightly ahead of ourselves here,
but you can also configure a java.util.Properties
instance like so:
<bean id="mappings" class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
<!-- typed as a java.util.Properties
-->
<property name="properties">
<value>
jdbc.driver.className=com.mysql.jdbc.Driver
jdbc.url=jdbc:mysql://localhost:3306/mydb
</value>
</property>
</bean>
Can you see what is happening? The Spring container is converting the text inside
the <value/>
element into a java.util.Properties
instance using the JavaBeans PropertEditor
mechanism.
This is a nice shortcut, and is one of a few places where the Spring team do favor the
use of the nested <value/>
element over the 'value'
attribute style.
The idref
element is simply an error-proof way to
pass the id of another bean in the container (to
a <constructor-arg/>
or <property/>
element).
<bean id="theTargetBean" class="..."/> <bean id="theClientBean" class="..."> <property name="targetName"> <idref bean="theTargetBean" /> </property> </bean>
The above bean definition snippet is exactly equivalent (at runtime) to the following snippet:
<bean id="theTargetBean" class="..."/> <bean id="client" class="..."> <property name="targetName"> <value>theTargetBean</value> </property> </bean>
The main reason the first form is preferable to
the second is that using the idref
tag allows the
container to validate at deployment time that the
referenced, named bean actually exists. In the second variation,
no validation is performed on the value that is passed to the
'targetName'
property of the 'client'
bean. Any typo will only be discovered (with most likely fatal results)
when the 'client'
bean is actually instantiated.
If the 'client'
bean is a
prototype bean, this typo
(and the resulting exception) may only be discovered long after the
container is actually deployed.
Additionally, if the bean being referred to is in the same XML unit, and
the bean name is the bean id, the 'local'
attribute may be used, which allows the XML parser itself to validate the bean
id even earlier, at XML document parse time.
<property name="targetName">
<!-- a bean with an id of 'theTargetBean
' must exist,
otherwise an XML exception will be thrown -->
<idref local="theTargetBean"/>
</property>
By way of an example, one common place (at least in pre-Spring 2.0
configuration) where the <idref/> element brings value is in the
configuration of AOP interceptors in a
ProxyFactoryBean
bean definition. If you use
<idref/> elements when specifying the interceptor names, there is
no chance of inadvertently misspelling an interceptor id.
The ref
element is the final element allowed
inside a <constructor-arg/>
or
<property/>
definition element. It is used to
set the value of the specified property to be a reference to another
bean managed by the container (a collaborator). As
mentioned in a previous section, the referred-to bean is considered to
be a dependency of the bean who's property is being set, and will be
initialized on demand as needed (if it is a singleton bean it may have
already been initialized by the container) before the property is set.
All references are ultimately just a reference to another object, but
there are 3 variations on how the id/name of the other object may be
specified, which determines how scoping and validation is handled.
Specifying the target bean by using the bean
attribute of the <ref/>
tag is the most general form,
and will allow creating a reference to any bean in the same
container (whether or not in the same XML file), or parent container.
The value of the 'bean'
attribute may be the same as either the
'id'
attribute of the target bean, or one of the
values in the 'name'
attribute of the target bean.
<ref bean="someBean"/>
Specifying the target bean by using the local
attribute leverages the ability of the XML parser to validate XML id
references within the same file. The value of the
local
attribute must be the same as the
id
attribute of the target bean. The XML parser
will issue an error if no matching element is found in the same file.
As such, using the local variant is the best choice (in order to know
about errors are early as possible) if the target bean is in the same
XML file.
<ref local="someBean"/>
Specifying the target bean by using the 'parent'
attribute allows a reference to be created to a bean which is in a
parent container of the current container. The value of the
'parent'
attribute may be the same as either the
'id'
attribute of the target bean, or one of the
values in the 'name'
attribute of the target bean,
and the target bean must be in a parent container to the current one.
The main use of this bean reference variant is when you have a hierarchy
of containers and you want to wrap an existing bean in a parent container
with some sort of proxy which will have the same name as the parent bean.
<!-- in the parent context --> <bean id="accountService" class="com.foo.SimpleAccountService"> <!-- insert dependencies as required as here --> </bean>
<!-- in the child (descendant) context -->
<bean id="accountService" <-- notice that the name of this bean is the same as the name of the 'parent'
bean
class="org.springframework.aop.framework.ProxyFactoryBean">
<property name="target">
<ref parent="accountService"/> <-- notice how we refer to the parent bean
</property>
<!-- insert other configuration and dependencies as required as here -->
</bean>
A <bean/>
element inside the
<property/>
or <constructor-arg/>
elements is used to define a so-called inner bean. An
inner bean definition does not need to have any id or name defined, and it is
best not to even specify any id or name value because the id or name value
simply will be ignored by the container.
<bean id="outer" class="..."> <!-- instead of using a reference to a target bean, simply define the target bean inline --> <property name="target"> <bean class="com.mycompany.Person"> <!-- this is the inner bean --> <property name="name" value="Fiona Apple"/> <property name="age" value="25"/> </bean> </property> </bean>
Note that in the specific case of inner beans, the 'scope'
flag and any 'id'
or 'name'
attribute are
effectively ignored. Inner beans are always anonymous and they
are always scoped as
prototypes. Please also note
that it is not possible to inject inner beans into
collaborating beans other than the enclosing bean.
The <list/>
, <set/>
,
<map/>
, and <props/>
elements
allow properties and arguments of the Java Collection
type List
, Set
,
Map
, and Properties
,
respectively, to be defined and set.
<bean id="moreComplexObject" class="example.ComplexObject"> <!-- results in a setAdminEmails(java.util.Properties
) call --> <property name="adminEmails"> <props> <prop key="administrator">administrator@somecompany.org</prop> <prop key="support">support@somecompany.org</prop> <prop key="development">development@somecompany.org</prop> </props> </property> <!-- results in a setSomeList(java.util.List
) call --> <property name="someList"> <list> <value>a list element followed by a reference</value> <ref bean="myDataSource" /> </list> </property> <!-- results in a setSomeMap(java.util.Map
) call --> <property name="someMap"> <map> <entry> <key> <value>yup an entry</value> </key> <value>just some string</value> </entry> <entry> <key> <value>yup a ref</value> </key> <ref bean="myDataSource" /> </entry> </map> </property> <!-- results in a setSomeSet(java.util.Set) call --> <property name="someSet"> <set> <value>just some string</value> <ref bean="myDataSource" /> </set> </property> </bean>
Note that the value of a map key or value, or a set value, can also again be any of the following elements:
bean | ref | idref | list | set | map | props | value | null
As of Spring 2.0, the container also supports the merging
of collections. This allows an application developer to define a parent-style
<list/>
, <map/>
,
<set/>
or <props/>
element, and
have child-style <list/>
, <map/>
,
<set/>
or <props/>
elements
inherit and override values from the parent collection; that is to say the child collection's
values will be the result obtained from the merging of the elements of the parent
and child collections, with the child's collection elements overriding values
specified in the parent collection.
Please note that this section on merging makes use of the parent-child bean mechanism. This concept has not yet been introduced, so readers unfamiliar with the concept of parent and child bean definitions may wish to read the relevant section before continuing.
Find below an example of the collection merging feature:
<beans>
<bean id="parent" abstract="true" class="example.ComplexObject">
<property name="adminEmails">
<props>
<prop key="administrator">administrator@somecompany.com</prop>
<prop key="support">support@somecompany.com</prop>
</props>
</property>
</bean>
<bean id="child" parent="parent">
<property name="adminEmails">
<!-- the merge is specified on the *child* collection definition -->
<props merge="true">
<prop key="sales">sales@somecompany.com</prop>
<prop key="support">support@somecompany.co.uk</prop>
</props>
</property>
</bean>
<beans>
Notice the use of the merge=true
attribute on the
<props/>
element of the
adminEmails
property of the child
bean definition. When the child
bean is actually resolved
and instantiated by the container, the resulting instance will have an
adminEmails
Properties
collection
that contains the result of the merging of the child's
adminEmails
collection with the parent's
adminEmails
collection.
administrator=administrator@somecompany.com sales=sales@somecompany.com support=support@somecompany.co.uk
Notice how the child Properties
collection's
value set will have inherited all the property elements from the parent
<props/>
. Notice also how the child's value for
the support
value overrides the value in the parent collection.
This merging behavior applies similarly to the <list/>
,
<map/>
, and <set/>
collection
types. In the specific case of the <list/>
element, the
semantics associated with the List
collection type, that is the
notion of an ordered
collection of values, is maintained;
the parent's values will precede all of the child list's values. In the
case of the Map
, Set
,
and Properties
collection types, there is no notion
of ordering and hence no ordering semantics are in effect for the collection types that
underlie the associated Map
,
Set
and Properties
implementation types used internally by the container.
Finally, some minor notes about the merging support are in order; you
cannot merge different collection types (e.g. a Map
and
a List
), and if you do attempt to do so an appropriate
Exception
will be thrown; and in case it is not
immediately obvious, the 'merge'
attribute must be specified
on the lower level, inherited, child definition; specifying the
'merge'
attribute on a parent collection definition is
redundant and will not result in the desired merging; and (lastly), please
note that this merging feature is only available in Spring 2.0 (and later
versions).
If you are one of the lucky few to be using Java5 (Tiger), you will be aware
that it is possible to have strongly typed collections. That is, it is possible to
declare a Collection
type such that it can only
contain String
elements (for example).
If you are using Spring to dependency inject a strongly-typed
Collection
into a bean, you can take advantage
of Spring's type-conversion support such that the elements of your strongly-typed
Collection
instances will be converted to the
appropriate type prior to being added to the Collection
.
public class Foo { private Map<String, Float> accounts; public void setAccounts(Map<String, Float> accounts) { this.accounts = accounts; } }
<beans> <bean id="foo" class="x.y.Foo"> <property name="accounts"> <map> <entry key="one" value="9.99"/> <entry key="two" value="2.75"/> <entry key="six" value="3.99"/> </map> </property> </bean> </beans>
When the 'accounts'
property of the 'foo'
bean is being prepared for injection, the generics information about the element
type of the strongly-typed Map<String, Float>
is
actually available via reflection, and so Spring's type conversion infrastructure
will actually recognize the various value elements as being of type
Float
and so the string values '9.99', '2.75'
,
and '3.99'
will be converted into an actual Float
type.
The <null/>
element is used to handle
null
values. Spring treats empty arguments for
properties and the like as empty Strings
. The following
XML-based configuration metadata snippet results in the email property
being set to the empty String
value ("")
<bean class="ExampleBean"> <property name="email"><value/></property> </bean>
This is equivalent to the following Java code:
exampleBean.setEmail("")
. The special
<null>
element may be used to indicate a
null
value. For example:
<bean class="ExampleBean"> <property name="email"><null/></property> </bean>
The above configuration is equivalent to the following Java code:
exampleBean.setEmail(null)
.
The configuration metadata shown so far is a tad verbose. That is why there
are several options available for you to limit the amount of XML you have to write to configure your
components. The first is a shortcut to define values and references to other beans
as part of a <property/>
definition. The second is slightly different format
of specifying properties alltogether.
The <property/>
, <constructor-arg/>
, and
<entry/>
elements all support a 'value'
attribute which may be used instead of embedding a full <value/>
element. Therefore, the following:
<property name="myProperty"> <value>hello</value> </property>
<constructor-arg> <value>hello</value> </constructor-arg>
<entry key="myKey"> <value>hello</value> </entry>
are equivalent to:
<property name="myProperty" value="hello"/>
<constructor-arg value="hello"/>
<entry key="myKey" value="hello"/>
The <property/>
and
<constructor-arg/>
elements support a similar
shortcut 'ref'
attribute which may be used instead
of a full nested <ref/>
element. Therefore,
the following:
<property name="myProperty"> <ref bean="myBean"> </property>
<constructor-arg> <ref bean="myBean"> </constructor-arg>
... are equivalent to:
<property name="myProperty" ref="myBean"/>
<constructor-arg ref="myBean"/>
Note however that the shortcut form is equivalent to a
<ref bean="xxx">
element; there is no
shortcut for <ref local="xxx"
>. To enforce a
strict local reference, you must use the long form.
Finally, the entry element allows a shortcut form to specify the
key and/or value of the map, in the form of the 'key'
/ 'key-ref'
and 'value'
/
'value-ref'
attributes. Therefore, the following:
<entry> <key> <ref bean="myKeyBean" /> </key> <ref bean="myValueBean" /> </entry>
is equivalent to:
<entry key-ref="myKeyBean" value-ref="myValueBean"/>
Again, the shortcut form is equivalent to a
<ref bean="xxx">
element; there is no shortcut for
<ref local="xxx"
>.
The second option you have to limit the amount of XML you have to write to configure your components
is to use the special "p-namespace". Spring 2.0 and later features support for extensible configuration
formats using namespaces. Those namespaces are
all based on an XML Schema definition. In fact, the beans
configuration format that you've been
reading about is defined in an XML Schema document.
One special namespace is not defined in an XSD file, and only exists in the core of Spring itself. The so-called
p-namespace doesn't need a schema definition and is an alternative way of configuring your properties differently
than the way you have seen so far. Instead of using nested property
elements, using the p-namespace you can use attributes as part of the bean
element that
describe your property values. The values of the attributes will be taken as the values for your properties.
The following two XML snippets boil down to the same thing in the end: the first is using the format
you're familiar with (the property
elements) whereas the second example is using the p-namespace
<beans xmlns="http://www.springframework.org/schema/beans" xmlns:p="http://www.springframework.org/schema/p" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.springframework.org/schema/beans/spring-beans-2.0.xsd"> <bean name="classic" class="com.mycompany.ExampleBean"> <property name="email" value="foo@bar.com/> </bean> <bean name="p-namespace" class="com.mycompany.ExampleBean" p:email="foo@bar.com"/> </beans>
As you can see, we are including an attribute from the p-namespace called email in the bean definition. This is telling Spring that it should include a property declaration. As previously mentioned, the p-namespace doesn't have a schema definition, so the name of the attribute can be set to whatever name your property has.
This next example includes two more bean definitions that both have a reference to another bean:
<beans xmlns="http://www.springframework.org/schema/beans" xmlns:p="http://www.springframework.org/schema/p" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.springframework.org/schema/beans/spring-beans-2.0.xsd"> <bean name="john-classic" class="com.mycompany.Person"> <property name="name" value="John Doe"/> <property name="spouse" ref="jane"/> </bean> <bean name="john-modern" class="com.mycompany.Person" p:name="John Doe" p:spouse-ref="jane"/> <bean name="jane" class="com.mycompany.Person"> <property name="name" value="Jane Doe"/> </bean> </beans>
As you can see, this example doesn't only include a property value using the p-namespace, but also uses a special
format to declare property references. Whereas the first bean definition uses <property name="spouse" ref="jane"/>
to create a reference from bean john to bean jane, the second bean definition uses
p:spouse-ref="jane"
as an attribute to do the exact same thing. In this case spouse
is the property name whereas the -ref
part tells Spring this is not a value but a bean reference.
![]() | Note |
---|---|
Note that we recommend you to choose carefully which approach you are going to use in your project. You should also communicate this to your team members so you won't end up with XML documents using all three approaches at the same time. This will prevent people from not understanding the application because of different ways of configuring it, and will add to the consistency of your codebase. Also note that this functionality is only available as of Spring 2.0. |
Compound or nested property names are perfectly legal when setting bean properties, as long as all components of the path except the final property name are not null. For example, in this bean definition:
<bean id="foo" class="foo.Bar"> <property name="fred.bob.sammy" value="123" /> </bean>
The foo
bean has a fred
property
which has a bob
property, which has a
sammy
property, and that final sammy
property is being set to the value 123
. In order
for this to work, the fred
property of foo
,
and the bob
property of fred
must both
be non-null after the bean is constructed, or a
NullPointerException
will be thrown.
For most situations, the fact that a bean is a dependency of another is
expressed by the fact that one bean is set as a property of another.
This is typically accomplished with the <ref/>
element
in XML-based configuration metadata. For the relatively infrequent situations
where dependencies between beans are less direct (for example, when a static
initializer in a class needs to be triggered, such as database driver registration),
the 'depends-on'
attribute may be used to explicitly
force one or more beans to be initialized before the bean using this
element is initialized. Find below an example of using the
'depends-on'
attribute to express a dependency on a single bean.
<bean id="beanOne" class="ExampleBean" depends-on="manager"/> <bean id="manager" class="ManagerBean" />
If you need to express a dependency on multiple beans, you can supply a
list of bean names as the value of the 'depends-on'
attribute, with commas, whitespace and semi-colons all valid delimiters, like so:
<bean id="beanOne" class="ExampleBean" depends-on="manager,accountDao"> <property name="manager" ref="manager" /> </bean> <bean id="manager" class="ManagerBean" /> <bean id="accountDao" class="x.y.jdbc.JdbcAccountDao" />
![]() | Note |
---|---|
The ' |
The default behavior for ApplicationContext
implementations is to eagerly pre-instantiate all singleton
beans
at startup. Pre-instantiation means that an ApplicationContext
will eagerly create and configure all of its singleton
beans as part of its initialization process. Generally this is a good thing,
because it means that any errors in the configuration or in the surrounding environment
will be discovered immediately (as opposed to possibly hours or even days down the line).
However, there are times when this behavior is not what is wanted.
If you do not want a singleton bean to be pre-instantiated when using
an ApplicationContext
, you can selectively
control this by marking a bean definition as lazy-initialized. A lazily-initialized
bean indicates to the IoC container whether or not a bean instance should be
created at startup or when it is first requested.
When configuring beans via XML, this lazy loading is controlled by the
'lazy-init'
attribute on the <bean/>
element; for example:
<bean id="lazy" class="com.foo.ExpensiveToCreateBean" lazy-init="true"/>
<bean name="not.lazy" class="com.foo.AnotherBean"/>
When the above configuration is consumed by an
ApplicationContext
, the bean named
'lazy'
will not be eagerly pre-instantiated
when the ApplicationContext
is starting up, whereas
the 'not.lazy'
bean will be eagerly pre-instantiated.
One thing to understand about lazy-initialization is that even though a bean
definition may be marked up as being lazy-initialized, if the lazy-initialized bean
is the dependency of a singleton bean that is not lazy-initialized, when the
ApplicationContext
is eagerly pre-instantiating the
singleton, it will have to satisfy all of the singletons dependencies,
one of which will be the lazy-initialized bean! So don't be confused if the IoC
container creates one of the beans that you have explicitly configured as
lazy-initialized at startup; all that means is that the lazy-initialized bean
is being injected into a non-lazy-initialized singleton bean elsewhere.
It is also possible to control lazy-initialization at the container level by using
the 'default-lazy-init'
attribute on the <beans/>
element; for example:
<beans default-lazy-init="true">
<!-- no beans will be pre-instantiated... -->
</beans>
The Spring container is able to autowire
relationships between collaborating beans. This means that it is possible to
automatically let Spring resolve collaborators (other beans) for your
bean by inspecting the contents of the BeanFactory
.
The autowiring functionality has five modes. Autowiring is specified
per bean and can thus be enabled for some beans,
while other beans will not be autowired. Using autowiring, it is possible
to reduce or eliminate the need to specify properties or constructor
arguments, thus saving a significant amount of typing.
[2]
When using XML-based configuration metadata, the autowire mode for a bean definition
is specified by using the autowire
attribute of the
<bean/>
element. The following values are allowed:
Table 3.2. Autowiring modes
Mode | Explanation |
---|---|
no |
No autowiring at all. Bean references must be defined
via a |
byName |
Autowiring by property name. This option will inspect
the container and look for a bean named exactly the same as
the property which needs to be autowired. For example, if you
have a bean definition which is set to autowire by name, and
it contains a master property (that is,
it has a setMaster(..) method), Spring
will look for a bean definition named |
byType |
Allows a property to be autowired if there is exactly
one bean of the property type in the container. If there is
more than one, a fatal exception is thrown, and this indicates
that you may not use byType autowiring
for that bean. If there are no matching beans, nothing
happens; the property is not set. If this is not desirable,
setting the |
constructor |
This is analogous to byType, but applies to constructor arguments. If there isn't exactly one bean of the constructor argument type in the container, a fatal error is raised. |
autodetect |
Chooses constructor or byType through introspection of the bean class. If a default constructor is found, the byType mode will be applied. |
Note that explicit dependencies in property
and
constructor-arg
settings always
override autowiring. Please also note that it is not currently possible to autowire
so-called simple properties such as primitives,
Strings
, and Classes
(and arrays of such
simple properties).(This is by-design and should be considered a feature.)
Autowire behavior can be combined with dependency checking, which will
be performed after all autowiring has been completed.
It is important to understand the various advantages and disadvantages of autowiring. Some advantages of autowiring include:
Autowiring can significantly reduce the volume of configuration required. However, mechanisms such as the use of a bean template (discussed elsewhere in this chapter) are also valuable in this regard.
Autowiring can cause configuration to keep itself up to date as your objects evolve. For example, if you need to add an additional dependency to a class, that dependency can be satisfied automatically without the need to modify configuration. Thus there may be a strong case for autowiring during development, without ruling out the option of switching to explicit wiring when the code base becomes more stable.
Some disadvantages of autowiring:
Autowiring is more magical than explicit wiring. Although, as noted in the above table, Spring is careful to avoid guessing in case of ambiguity which might have unexpected results, the relationships between your Spring-managed objects are no longer documented explicitly.
Wiring information may not be available to tools that may generate documentation from a Spring container.
Autowiring by type will only work when there is a single bean definition of the type specified by the setter method or constructor argument. You need to use explicit wiring if there is any potential ambiguity.
There is no wrong or right answer in all cases. A degree of consistency across a project is best though; for example, if autowiring is not used in general, it might be confusing to developers to use it just to wire one or two bean definitions.
You can also (on a per-bean basis) totally exclude a bean from being an
autowire candidate. When configuring beans using Spring's XML format, the
'autowire-candidate'
attribute of the
<bean/>
element can be set to
'false'
; this has the effect of making the container
totally exclude that specific bean definition from being available
to the autowiring infrastructure.
This can be useful when you have a bean that you absolutely never ever want to have injected into other beans via autowiring. It does not mean that the excluded bean cannot itself be configured using autowiring... it can, it is rather that it itself will not be considered as a candidate for autowiring other beans.
The Spring IoC container also has the ability to check for the existence of unresolved dependencies of a bean deployed into the container. These are JavaBeans properties of the bean, which do not have actual values set for them in the bean definition, or alternately provided automatically by the autowiring feature.
This feature is sometimes useful when you want to ensure that all
properties (or all properties of a certain type) are set on a bean. Of
course, in many cases a bean class will have default values for many
properties, or some properties do not apply to all usage scenarios, so
this feature is of limited use. Dependency checking can also be enabled
and disabled per bean, just as with the autowiring functionality. The
default is to not check dependencies. Dependency
checking can be handled in several different modes. When using
XML-based configuration metadata, this is specified via the
'dependency-check'
attribute in a bean definition,
which may have the following values.
Table 3.3. Dependency checking modes
Mode | Explanation |
---|---|
none |
No dependency checking. Properties of the bean which have no value specified for them are simply not set. |
simple |
Dependency checking is performed for primitive types and collections (everything except collaborators). |
object |
Dependency checking is performed for collaborators only. |
all |
Dependency checking is done for collaborators, primitive types and collections. |
If you are using Java 5 (Tiger) and thus have access to source level annotations,
you may find the section entitled Section 25.3.1, “@Required
”
to be of interest.
For most application scenarios, the majority of the beans in the container will be singletons. When a singleton bean needs to collaborate with another singleton bean, or a non-singleton bean needs to collaborate with another non-singleton bean, the typical and common approach of handling this dependency by defining one bean to be a property of the other is quite adequate. There is a problem when the bean lifecycles are different. Consider a singleton bean A which needs to use a non-singleton (prototype) bean B, perhaps on each method invocation on A. The container will only create the singleton bean A once, and thus only get the opportunity to set the properties once. There is no opportunity for the container to provide bean A with a new instance of bean B every time one is needed.
One solution to this issue is to forgo some inversion of
control. Bean A can be
made aware of the container
by implementing the BeanFactoryAware
interface, and
use programmatic means to
ask the container via a getBean("B")
call for
(a typically new) bean B instance every time it needs it. Find below an
admittedly somewhat contrived example of this approach:
// a class that uses a stateful Command-style class to perform some processing package fiona.apple; // lots of Spring-API imports import org.springframework.beans.BeansException; import org.springframework.beans.factory.BeanFactory; import org.springframework.beans.factory.BeanFactoryAware; public class CommandManager implements BeanFactoryAware { private BeanFactory beanFactory; public Object process(Map commandState) { // grab a new instance of the appropriateCommand
Command command = createCommand(); // set the state on the (hopefully brand new)Command
instance command.setState(commandState); return command.execute(); } // theCommand
returned here could be an implementation that executes asynchronously, or whatever protected Command createCommand() { return (Command) this.beanFactory.getBean("command"); // notice the Spring API dependency } public void setBeanFactory(BeanFactory beanFactory) throws BeansException { this.beanFactory = beanFactory; } }
The above example is generally is not a desirable solution since the business code is then aware of and coupled to the Spring Framework. Method Injection, a somewhat advanced feature of the Spring IoC container, allows this use case to be handled in a clean fashion.
Lookup method injection refers to the ability of the container to override methods on container managed beans, to return the result of looking up another named bean in the container. The lookup will typically be of a prototype bean as in the scenario described above. The Spring Framework implements this method injection by dynamically generating a subclass overriding the method, using bytecode generation via the CGLIB library.
So if you look at the code from previous code snippet (the
CommandManager
class), the Spring container
is going to dynamically override the implementation of the
createCommand()
method. Your
CommandManager
class is not going to have any
Spring dependencies, as can be seen in this reworked example below:
package fiona.apple; // no more Spring imports! public abstract class CommandManager { public Object process(Object commandState) { // grab a new instance of the appropriateCommand
interface Command command = createCommand(); // set the state on the (hopefully brand new)Command
instance command.setState(commandState); return command.execute(); } // okay... but where is the implementation of this method? protected abstract Command createCommand(); }
In the client class containing the method to be injected (the
CommandManager
in this case), the method that
is to be 'injected' must have a signature of the following form:
<public|protected> [abstract] <return-type> theMethodName(no-arguments);
If the method is abstract
, the dynamically-generated
subclass will implement the method. Otherwise, the dynamically-generated subclass
will override the concrete method defined in the original class. Let's look at an
example:
<!-- a stateful bean deployed as a prototype (non-singleton) --> <bean id="command" class="fiona.apple.AsyncCommand" scope="prototype"> <!-- inject dependencies here as required --> </bean> <!--commandProcessor
usesstatefulCommandHelper
--> <bean id="commandManager" class="fiona.apple.CommandManager"> <lookup-method name="createCommand" bean="command"/> </bean>
The bean identified as commandManager will call its
own method createCommand()
whenever it needs
a new instance of the command bean. It
is important to note that the person deploying the beans must be
careful to deploy the command
bean as a
prototype (if that is actually what is needed). If it is deployed
as a singleton, the same instance of
the command
bean will be returned each time!
Please be aware that in order for this dynamic subclassing to work,
you will need to have the CGLIB jar(s) on your classpath. Additionally, the
class that the Spring container is going to subclass cannot be
final
, and the method that is being overridden cannot be
final
either. Also, testing a class that has an
abstract
method can be somewhat odd in that you will have
to subclass the class yourself and supply a stub implementation of the
abstract
method. Finally, objects that have been the target
of method injection cannot be serialized.
![]() | Tip |
---|---|
The interested reader may also find the |
A less commonly useful form of method injection than Lookup Method Injection is the ability to replace arbitrary methods in a managed bean with another method implementation. Users may safely skip the rest of this section (which describes this somewhat advanced feature), until this functionality is actually needed.
When using XML-based configuration metadata, the replaced-method
element may be used to replace an existing method implementation with
another, for a deployed bean. Consider the following class, with a
method computeValue, which we want to override:
public class MyValueCalculator { public String computeValue(String input) { // some real code... } // some other methods... }
A class implementing the
org.springframework.beans.factory.support.MethodReplacer
interface provides the new method definition.
/** meant to be used to override the existingcomputeValue(String)
implementation inMyValueCalculator
*/ public class ReplacementComputeValue implements MethodReplacer { public Object reimplement(Object o, Method m, Object[] args) throws Throwable { // get the input value, work with it, and return a computed result String input = (String) args[0]; ... return ...; } }
The bean definition to deploy the original class and specify the method override would look like this:
<bean id="myValueCalculator class="x.y.z.MyValueCalculator">
<!-- arbitrary method replacement -->
<replaced-method name="computeValue" replacer="replacementComputeValue">
<arg-type>String</arg-type>
</replaced-method>
</bean>
<bean id="replacementComputeValue" class="a.b.c.ReplacementComputeValue"/>
One or more contained <arg-type/>
elements
within the <replaced-method/>
element may be used to
indicate the method signature of the method being overridden. Note
that the signature for the arguments is actually only needed in the
case that the method is actually overloaded and there are multiple
variants within the class. For convenience, the type string for an
argument may be a substring of the fully qualified type name. For
example, all the following would match java.lang.String
.
java.lang.String String Str
Since the number of arguments is often enough to distinguish between each possible choice, this shortcut can save a lot of typing, by allowing you to type just the shortest string that will match an argument type.
When you create a bean definition what you are actually creating is a recipe for creating actual instances of the class defined by that bean definition. The idea that a bean definition is a recipe is important, because it means that, just like a class, you can potentially have many object instances created from a single recipe.
You can control not only the various dependencies and
configuration values that are to be plugged into an object that is created
from a particular bean definition, but also the scope
of the objects created from a particular bean definition. This approach is very
powerful and gives you the flexibility to choose the scope
of the objects you create through configuration instead of having to 'bake in'
the scope of an object at the Java class level. Beans can be defined to be
deployed in one of a number of scopes: out of the box, the Spring Framework
supports exactly five scopes (of which three are available only if you are using
a web-aware ApplicationContext
).
The scopes supported out of the box are listed below:
Table 3.4. Bean scopes
Scope | Description |
---|---|
Scopes a single bean definition to a single object instance per Spring IoC container. | |
Scopes a single bean definition to any number of object instances. | |
Scopes a single bean definition to the lifecycle of
a single HTTP request; that is each and every HTTP request will
have its own instance of a bean created off the back of a
single bean definition. Only valid in the context of a
web-aware Spring | |
Scopes a single bean definition to the lifecycle of
a HTTP | |
Scopes a single bean definition to the lifecycle of a
global HTTP |
When a bean is a singleton, only one shared instance of the bean will be managed, and all requests for beans with an id or ids matching that bean definition will result in that one specific bean instance being returned by the Spring container.
To put it another way, when you define a bean definition and it is scoped as a singleton, then the Spring IoC container will create exactly one instance of the object defined by that bean definition. This single instance will be stored in a cache of such singleton beans, and all subsequent requests and references for that named bean will result in the cached object being returned.
Please be aware that Spring's concept of a singleton bean is quite
different from the Singleton pattern as defined in the seminal Gang of
Four (GoF) patterns book. The GoF Singleton hardcodes the scope of
an object such that one and only one instance of a
particular class will ever be created per
ClassLoader
. The scope of the Spring singleton
is best described as per container and per bean.
This means that if you define one bean for a particular class in a single
Spring container, then the Spring container will create one
and only one instance of the class defined by that
bean definition. The singleton scope is the default scope in Spring.
To define a bean as a singleton in XML, you would write configuration like so:
<bean id="accountService" class="com.foo.DefaultAccountService"/> <!-- the following is equivalent, though redundant (singleton scope is the default); usingspring-beans-2.0.dtd
--> <bean id="accountService" class="com.foo.DefaultAccountService" scope="singleton"/> <!-- the following is equivalent and preserved for backward compatibility inspring-beans.dtd
--> <bean id="accountService" class="com.foo.DefaultAccountService" singleton="true"/>
The non-singleton, prototype scope of bean deployment results in
the creation of a new bean instance every time a
request for that specific bean is made (that is, it is injected into another
bean or it is requested via a programmatic getBean()
method call on the container). As a rule of thumb, you should
use the prototype scope for all beans that are stateful, while the
singleton scope should be used for stateless beans.
The following diagram illustrates the Spring prototype scope. Please note that a DAO would not typically be configured as a prototype, since a typical DAO would not hold any conversational state; it was just easier for this author to reuse the core of the singleton diagram.
To define a bean as a prototype in XML, you would write configuration like so:
<!-- usingspring-beans-2.0.dtd
--> <bean id="accountService" class="com.foo.DefaultAccountService" scope="prototype"/> <!-- the following is equivalent and preserved for backward compatibility inspring-beans.dtd
--> <bean id="accountService" class="com.foo.DefaultAccountService" singleton="false"/>
There is one quite important thing to be aware of when deploying a bean in the prototype scope, in that the lifecycle of the bean changes slightly. Spring cannot (and hence does not) manage the complete lifecycle of a prototype bean: the container instantiates, configures, decorates and otherwise assembles a prototype object, hands it to the client and then has no further knowledge of that prototype instance. This means that while initialization lifecycle callback methods will be called on all objects regardless of scope, in the case of prototypes, any configured destruction lifecycle callbacks will not be called. It is the responsibility of the client code to clean up prototype scoped objects and release any expensive resources that the prototype bean(s) are holding onto. (One possible way to get the Spring container to release resources used by singleton-scoped beans is through the use of a custom bean post processor which would hold a reference to the beans that need to be cleaned up.)
In some respects, you can think of the Spring containers role when talking
about a prototype-scoped bean as somewhat of a replacement for the Java
'new'
operator. Any lifecycle aspects past that point have to
be handled by the client. The lifecycle of a bean in a Spring IoC container is
further described in the section entitled
Section 3.5.1, “Lifecycle interfaces”.
![]() | Backwards compatibility note: specifying the lifecycle scope in XML |
---|---|
If you are referencing the To be totally clear about this, this means that if you use the
" |
The other scopes, namely request
, session
,
and global session
are for use only in web-based applications
(and can be used irrespective of which particular web application framework you are
using, if indeed any). In the interest of keeping related concepts together in
one place in the reference documentation, these scopes are described here.
![]() | Note |
---|---|
The scopes that are described in the following paragraphs are
only available if you are using a web-aware
Spring |
In order to effect the scoping of beans at the request
,
session
, and global session
levels
(web-scoped beans), some minor initial configuration is required
before you can set about defining your bean definitions. Please note that
this extra setup is not required if you just want to use the
'standard' scopes (namely singleton and prototype).
Now as things stand, there are a couple of ways to effect this
initial setup depending on your particular servlet environment. If you are
using a Servlet 2.4+ web container, then you need only add the following
ContextListener
to the XML declarations in
your web application's 'web.xml'
file.
<web-app> ... <listener> <listener-class>org.springframework.web.context.request.RequestContextListener</listener-class> </listener> ... </web-app>
If you are using an older web container (before Servlet 2.4), you will need to use
a (provided) javax.servlet.Filter
implementation.
Find below a snippet of XML configuration that has to be included
in the 'web.xml'
file of your web application if you
want to have access to web-scoped beans (the filter settings depend on
the surrounding web application configuration and so you will have to change
them as appropriate).
<web-app> .. <filter> <filter-name>requestContextFilter</filter-name> <filter-class>org.springframework.web.filter.RequestContextFilter</filter-class> </filter> <filter-mapping> <filter-name>requestContextFilter</filter-name> <url-pattern>/*</url-pattern> </filter-mapping> ... </web-app>
That's it. The RequestContextListener
and
RequestContextFilter
classes both do exactly the
same thing, namely bind the HTTP request object to the
Thread
that is servicing that request. This makes
beans that are request- and session-scoped available further down the
call chain.
Consider the following bean definition:
<bean id="loginAction" class="com.foo.LoginAction" scope="request"/>
With the above bean definition in place, the Spring container will create
a brand new instance of the LoginAction
bean
using the 'loginAction'
bean definition for each and
every HTTP request. That is, the 'loginAction'
bean will be
effectively scoped at the HTTP request level. You can change or dirty
the internal state of the instance that is created as much as you want,
safe in the knowledge that other requests that are also using instances created
off the back of the same 'loginAction'
bean definition
will not be seeing these changes in state since they are particular to an individual
request. When the request is finished processing, the bean that is scoped
to the request will be discarded.
Consider the following bean definition:
<bean id="userPreferences" class="com.foo.UserPreferences" scope="session"/>
With the above bean definition in place, the Spring container will create
a brand new instance of the UserPreferences
bean
using the 'userPreferences'
bean definition for the
lifetime of a single HTTP Session
.
In other words, the 'userPreferences'
bean will be
effectively scoped at the HTTP Session
level.
Just like request-scoped
beans, you can change the internal
state of the instance that is created as much as you want, safe in the
knowledge that other HTTP Session
instances
that are also using instances created off the back of the same
'userPreferences'
bean definition
will not be seeing these changes in state since they are particular to an individual
HTTP Session
. When the HTTP
Session
is eventually discarded, the bean
that is scoped to that particular HTTP Session
will also be discarded.
Consider the following bean definition:
<bean id="userPreferences" class="com.foo.UserPreferences" scope="globalSession"/>
The global session
scope is similar to the
standard HTTP Session
scope
(described immediately above),
and really only makes sense in the context of portlet-based web applications.
The portlet specification defines the notion of a global
Session
that is shared amongst all
of the various portlets that make up a single portlet web application.
Beans defined at the global session
scope are scoped
(or bound) to the lifetime of the global portlet
Session
.
Please note that if you are writing a standard Servlet-based
web application and you define one or more beans as having
global session
scope, the standard HTTP
Session
scope will be used, and no
error will be raised.
Being able to define a bean scoped to a HTTP request or
Session
(or indeed
a custom scope of
your own devising) is all very well, but one of the
main value-adds of the Spring IoC container is that it manages
not only the instantiation of your objects (beans), but also
the wiring up of collaborators (or dependencies). If you want to inject
a (for example) HTTP request scoped bean into another bean, you will need
to inject an AOP proxy in place of the scoped bean. That is, you need to
inject a proxy object that exposes the same public interface as the scoped
object, but that is smart enough to be able to retrieve the real, target
object from the relevant scope (for example a HTTP request) and delegate
method calls onto the real object.
![]() | Note |
---|---|
You do not need to use the
|
Let's look at the configuration that is required to effect this; the configuration is not hugely complex (it takes just one line), but it is important to understand the “why” as well as the “how” behind it.
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:aop="http://www.springframework.org/schema/aop" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.0.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-2.0.xsd"> <!-- a HTTPSession
-scoped bean exposed as a proxy --> <bean id="userPreferences" class="com.foo.UserPreferences" scope="session"> <!-- this next element effects the proxying of the surrounding bean --> <aop:scoped-proxy/> </bean> <!-- a singleton-scoped bean injected with a proxy to the above bean --> <bean id="userService" class="com.foo.SimpleUserService"> <!-- a reference to the proxied'userPreferences'
bean --> <property name="userPreferences" ref="userPreferences"/> </bean> </beans>
To create such a proxy, you need only to insert a child
<aop:scoped-proxy/>
element into a scoped bean definition
(you may also need the CGLIB library on your classpath so that the container can
effect class-based proxying; you will also need to be using Appendix A, XML Schema-based configuration).
So, just why do you need this <aop:scoped-proxy/>
element in the
definition of beans scoped at the request
,
session
, globalSession
and
'insert your custom scope here' level? The reason is best explained
by picking apart the following bean definition (please note that the following
'userPreferences'
bean definition as it stands is
incomplete):
<bean id="userPreferences" class="com.foo.UserPreferences" scope="session"/> <bean id="userManager" class="com.foo.UserManager"> <property name="userPreferences" ref="userPreferences"/> </bean>
From the above configuration it is evident that the singleton bean
'userManager'
is being injected with a reference to the HTTP
Session
-scoped bean 'userPreferences'
.
The salient point here is that the 'userManager'
bean is a
singleton... it will be instantiated exactly once per container,
and its dependencies (in this case only one, the 'userPreferences'
bean) will also only be injected (once!). This means that the
'userManager'
will (conceptually) only ever operate on the exact
same 'userPreferences'
object, that is the one that it was originally
injected with. This is not what you want when you inject a HTTP
Session
-scoped bean as a dependency into a
collaborating object (typically). Rather, what we do want is a
single 'userManager'
object, and then, for the lifetime of a HTTP
Session
, we want to see and use a
'userPreferences'
object that is specific to said HTTP
Session
.
Rather what you need then is to inject some sort of object that exposes the
exact same public interface as the UserPreferences
class (ideally
an object that is a UserPreferences
instance)
and that is smart enough to be able to go off and fetch the
real
UserPreferences
object from
whatever underlying scoping mechanism we have chosen (HTTP request,
Session
, etc.). We can then safely inject this proxy
object into the 'userManager'
bean, which will be blissfully unaware
that the UserPreferences
reference that it is holding onto is a
proxy. In the case of this example, when a UserManager
instance invokes a method on the dependency-injected UserPreferences
object, it is really invoking a method on the proxy... the proxy will then go off and
fetch the real UserPreferences
object from (in this case) the HTTP
Session
, and delegate the method invocation onto the
retrieved real UserPreferences
object.
That is why you need the following, correct and complete, configuration
when injecting request-
, session-
, and
globalSession-scoped
beans into collaborating objects:
<bean id="userPreferences" class="com.foo.UserPreferences" scope="session">
<aop:scoped-proxy/>
</bean>
<bean id="userManager" class="com.foo.UserManager">
<property name="userPreferences" ref="userPreferences"/>
</bean>
By default, when the Spring container is creating a proxy for a bean
that is marked up with the <aop:scoped-proxy/>
element, a CGLib-based class proxy will be created.
This means that you will need to have the CGLib library on the classapth
for your application.
You can choose to have the Spring container create 'standard'
JDK interface-based proxies for such scoped beans by specifying
'false
' for the value of the 'proxy-target-class
'
attribute of the <aop:scoped-proxy/>
element.
Using JDK interface-based proxies does mean that you don't need any additional
libraries on your application's classpath to effect such proxying, but it
does mean that the class of the scoped bean must implement at least one
interface, and all of the collaborators into which
the scoped bean is injected must be referencing the bean via one of its
interfaces.
<!--DefaultUserPreferences
implements theUserPreferences
interface --> <bean id="userPreferences" class="com.foo.DefaultUserPreferences" scope="session"> <aop:scoped-proxy proxy-target-class="false"/> </bean> <bean id="userManager" class="com.foo.UserManager"> <property name="userPreferences" ref="userPreferences"/> </bean>
The section entitled Section 6.6, “Proxying mechanisms” may also be of some interest with regard to understanding the nuances of choosing whether class-based or interface-based proxying is right for you.
As of Spring 2.0, the bean scoping mechanism in Spring is extensible. This means
that you are not limited to just the bean scopes that Spring provides out of the box;
you can define your own scopes, or even redefine the existing scopes (although that last
one would probably be considered bad practice - please note that you
cannot override the built-in singleton
and
prototype
scopes).
Scopes are defined by the
org.springframework.beans.factory.config.Scope
interface. This is the interface that you will need to implement in order
to integrate your own custom scope(s) into the Spring container, and is described
in detail below. You may wish to look at the Scope
implementations that are supplied with the Spring Framework itself for an idea of
how to go about implementing your own. The Scope JavaDoc
explains the main class to implement when you need your own scope in more detail too.
The Scope
interface has four methods dealing with getting objects
from the scope, removing them from the scope and allowing them to be 'destroyed' if needed.
The first method should return the object from the underlying scope. The session scope implementation for example will return the session-scoped bean (and if it does not exist, return a new instance of the bean, after having bound it to the session for future reference).
Object get(String name, ObjectFactory objectFactory)
The second method should remove the object from the underlying scope. The session scope implementation for example, removes the session-scoped bean from the underlying session. The object should be returned (you are allowed to return null if the object with the specified name wasn't found)
Object remove(String name)
The third method is used to register callbacks the scope should execute when it is destroyed or when the specified object in the scope is destroyed. Please refer to the JavaDoc or a Spring scope implementation for more information on destruction callbacks.
void registerDestructionCallback(String name, Runnable destructionCallback)
The last method deals with obtaining the conversation identifier for the underlying scope. This identifier is different for each scope. For a session for example, this can be the session identifier.
String getConversationId()
After you have written and tested one or more custom Scope
implementations, you then need to make the Spring container aware of your new scope(s).
The central method to register a new Scope
with the
Spring container is declared on the ConfigurableBeanFactory
interface (implemented by most of the concrete BeanFactory
implementations that ship with Spring); this central method is displayed below:
void registerScope(String scopeName, Scope scope);
The first argument to the registerScope(..)
method is the
unique name associated with a scope; examples of such names in the Spring container
itself are 'singleton'
and 'prototype'
.
The second argument to the registerScope(..)
method is an
actual instance of the custom Scope
implementation
that you wish to register and use.
Let's assume that you have written your own custom
Scope
implementation, and you have registered it
like so:
// note: the ThreadScope
class does not ship with the Spring Framework
Scope customScope = new ThreadScope();
beanFactory.registerScope("thread", scope);
You can then create bean definitions that adhere to the scoping rules of your
custom Scope
like so:
<bean id="..." class="..." scope="thread"/>
If you have your own custom Scope
implementation(s),
you are not just limited to only programmatic registration of the custom scope(s).
You can also do the Scope
registration declaratively,
using the CustomScopeConfigurer
class.
The declarative registration of custom Scope
implementations using the CustomScopeConfigurer
class is
shown below:
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:aop="http://www.springframework.org/schema/aop" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.0.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-2.0.xsd"> <bean class="org.springframework.beans.factory.config.CustomScopeConfigurer"> <property name="scopes"> <map> <entry key="thread"> <bean class="com.foo.ThreadScope"/> </entry> </map> </property> </bean> <bean id="bar" class="x.y.Bar" scope="thread"> <property name="name" value="Rick"/> <aop:scoped-proxy/> </bean> <bean id="foo" class="x.y.Foo"> <property name="bar" ref="bar"/> </bean> </beans>
The Spring Framework provides several marker interfaces to change the behavior
of your bean in the container; they include
InitializingBean
and
DisposableBean
. Implementing these interfaces will
result in the container calling
afterPropertiesSet()
for the former and
destroy()
for the latter to allow the bean to perform
certain actions upon initialization and destruction.
Internally, the Spring Framework uses
BeanPostProcessor
implementations to
process any marker interfaces it can find and call the appropriate
methods. If you need custom features or other lifecycle behavior Spring
doesn't offer out-of-the-box, you can implement a
BeanPostProcessor
yourself. More information about
this can be found in the section entitled Section 3.7, “Container extension points”.
All the different lifecycle marker interfaces are described below. In one of the appendices, you can find diagram that show how Spring manages beans and how those lifecycle features change the nature of your beans and how they are managed.
Implementing the
org.springframework.beans.factory.InitializingBean
interface allows a bean to perform initialization work after all necessary
properties on the bean are set by the container. The
InitializingBean
interface specifies exactly one
method:
void afterPropertiesSet() throws Exception;
Generally, the use of the InitializingBean
interface can be avoided (and is discouraged since it unnecessarily couples
the code to Spring). A bean definition provides support for a generic
initialization method to be specified. In the case of XML-based configuration
metadata, this is done using the 'init-method'
attribute.
For example, the following definition:
<bean id="exampleInitBean" class="examples.ExampleBean" init-method="init"/>
public class ExampleBean {
public void init() {
// do some initialization work
}
}
Is exactly the same as...
<bean id="exampleInitBean" class="examples.AnotherExampleBean"/>
public class AnotherExampleBean implements InitializingBean {
public void afterPropertiesSet() {
// do some initialization work
}
}
... but does not couple the code to Spring.
Implementing the
org.springframework.beans.factory.DisposableBean
interface allows a bean to get a callback when the container
containing it is destroyed. The DisposableBean
interface specifies one
method:
void destroy() throws Exception;
Generally, the use of the
DisposableBean
marker interface can be avoided (and
is discouraged since it unnecessarily couples the code to Spring). A
bean definition provides support for a generic destroy method to be
specified. When using XML-based configuration metadata this is done via the
'destroy-method'
attribute on the <bean/>
.
For example, the following definition:
<bean id="exampleInitBean" class="examples.ExampleBean" destroy-method="cleanup"/>
public class ExampleBean {
public void cleanup() {
// do some destruction work (like releasing pooled connections)
}
}
Is exactly the same as...
<bean id="exampleInitBean" class="examples.AnotherExampleBean"/>
public class AnotherExampleBean implements DisposableBean {
public void destroy() {
// do some destruction work (like releasing pooled connections)
}
}
... but does not couple the code to Spring.
When you are writing initialization and destroy method callbacks that do not
use the Spring-specific InitializingBean
and DisposableBean
callback interfaces,
one (in the experience of this author) typically finds oneself writing
methods with names such as init()
,
initialize()
, dispose()
, etc. The
names of such lifecycle callback methods are (hopefully!) standardized
across a project so that developers on a team all use the same method
names and thus ensure some level of consistency.
The Spring container can now be configured to 'look'
for named initialization and destroy callback method names on
every bean. This means that you as an application developer
can simply write your application classes, use a convention of having an
initialization callback called init()
, and then
(without having to configure each and every bean with, in the case of XML-based
configuration, an 'init-method="init"'
attribute)
be safe in the knowledge that the Spring IoC container will
call that method when the bean is being created (and in accordance with the
standard lifecycle callback contract described previously).
Let's look at an example to make the use of this feature completely clear.
For the sake of the example, let us say that one of the coding conventions on a
project is that all initialization callback methods are to be named
init()
and that destroy callback methods are to be called
destroy()
. This leads to classes like so...
public class DefaultBlogService implements BlogService {
private BlogDao blogDao;
public void setBlogDao(BlogDao blogDao) {
this.blogDao = blogDao;
}
// this is (unsurprisingly) the initialization callback method
public void init() {
if (this.blogDao == null) {
throw new IllegalStateException("The [blogDao] property must be set.");
}
}
}
<beans default-init-method="init">
<bean id="blogService" class="com.foo.DefaultBlogService">
<property name="blogDao" ref="blogDao" />
</bean>
</beans>
Notice the use of the 'default-init-method'
attribute on the
top-level <beans/>
element. The presence of this
attribute means that the Spring IoC container will recognize a method called
'init'
on beans as being the initialization method callback,
and when a bean is being created and assembled, if the bean's class has such
a method, it will be invoked at the appropriate time.
Destroy method callbacks are configured similarly (in XML that is) using the
'default-destroy-method'
attribute on the top-level
<beans/>
element.
The use of this feature can save you the (small) housekeeping chore of specifying an initialization and destroy method callback on each and every bean, and it is great for enforcing a consistent naming convention for initialization and destroy method callbacks (and consistency is something that should always be aimed for).
Consider the case where you have some existing beans where the
underlying classes already have initialization callback methods that
are named at variance with the convention. You can
always override the default by specifying (in XML that is)
the method name using the 'init-method'
and
'destroy-method'
attributes on the <bean/>
element itself.
Finally, please be aware that the Spring container guarantees that a configured initialization callback is called immediately after a bean has been supplied with all of it's dependencies. This means that the initialization callback will be called on the raw bean reference, which means that any AOP interceptors or suchlike that will ultimately be applied to the bean will not yet be in place. A target bean is fully created first, then an AOP proxy (for example) with its interceptor chain is applied. Note that, if the target bean and the proxy are defined separately, your code can even interact to the raw target bean, bypassing the proxy. Hence, it would be very inconsistent to apply the interceptors to the init method, since that would couple the lifecycle of the target bean with its proxy/interceptors, and leave strange semantics when talking to the raw target bean directly.
![]() | Note |
---|---|
This next section does not apply to web applications (in case the title of this section
did not make that abundantly clear). Spring's web-based |
If you are using Spring's IoC container in a non-web application environment, for example in a rich client desktop environment, and you want the container to shutdown gracefully and call the relevant destroy callbacks on your singleton beans, you will need to register a shutdown hook with the JVM. This is quite easy to do (see below), and will ensure that your Spring IoC container shuts down gracefully and that all resources held by your singletons are released (of course it is still up to you to both configure the destroy callbacks for your singletons and implement such destroy callbacks correctly).
So to register a shutdown hook that enables the graceful shutdown of the relevant
Spring IoC container, you simply need to call the
registerShutdownHook()
method that is declared on the
AbstractApplicationContext
class. To wit...
import org.springframework.context.support.AbstractApplicationContext; import org.springframework.context.support.ClassPathXmlApplicationContext; public final class Boot { public static void main(final String[] args) throws Exception { AbstractApplicationContext ctx = new ClassPathXmlApplicationContext(new String []{"beans.xml"}); // add a shutdown hook for the above context... ctx.registerShutdownHook(); // app runs here... // main method exits, hook is called prior to the app shutting down... } }
A class which implements the
org.springframework.beans.factory.BeanFactoryAware
interface is provided with a reference to the BeanFactory
that created
it, when it is created by that BeanFactory
.
public interface BeanFactoryAware { void setBeanFactory(BeanFactory beanFactory) throws BeansException; }
This allows beans to manipulate the BeanFactory
that created them programmatically, through the BeanFactory
interface, or by casting the reference to a known subclass of this
which exposes additional functionality. Primarily this would consist
of programmatic retrieval of other beans. While there are cases when
this capability is useful, it should generally be avoided, since it
couples the code to Spring, and does not follow the Inversion of
Control style, where collaborators are provided to beans as properties.
An alternative option that is equivalent in effect to the
BeanFactoryAware
-based approach is to use the
org.springframework.beans.factory.config.ObjectFactoryCreatingFactoryBean
.
(It should be noted that this approach still does not reduce the coupling to Spring,
but it does not violate the central principle of IoC as much as the
BeanFactoryAware
-based approach.)
The ObjectFactoryCreatingFactoryBean
is a
FactoryBean
implementation that returns a reference to an object (factory) that can in turn be used to
effect a bean lookup. The ObjectFactoryCreatingFactoryBean
class
does itself implement the BeanFactoryAware
interface;
what client beans are actually injected with is an instance of the
ObjectFactory
interface. This is a Spring-specific
interface (and hence there is still no total decoupling from Spring), but clients
can then use the ObjectFactory
's
getObject()
method to effect the bean lookup (under the hood the
ObjectFactory
implementation instance that is returned
simply delegates down to a BeanFactory
to actually
lookup a bean by name). All that you need to do is supply
the ObjectFactoryCreatingFactoryBean
with the name of the
bean that is to be looked up. Let's look at an example:
package x.y; public class NewsFeed { private String news; public void setNews(String news) { this.news = news; } public String getNews() { return this.toString() + ": '" + news + "'"; } }
package x.y; import org.springframework.beans.factory.ObjectFactory; public class NewsFeedManager { private ObjectFactory factory; public void setFactory(ObjectFactory factory) { this.factory = factory; } public void printNews() { // here is where the lookup is performed; note that there is no // need to hardcode the name of the bean that is being looked up... NewsFeed news = (NewsFeed) factory.getObject(); System.out.println(news.getNews()); } }
Find below the XML configuration to wire together the above classes
using the ObjectFactoryCreatingFactoryBean
approach.
<beans> <bean id="newsFeedManager" class="x.y.NewsFeedManager"> <property name="factory"> <bean class="org.springframework.beans.factory.config.ObjectFactoryCreatingFactoryBean"> <property name="targetBeanName"> <idref local="newsFeed" /> </property> </bean> </property> </bean> <bean id="newsFeed" class="x.y.NewsFeed" scope="prototype"> <property name="news" value="... that's fit to print!" /> </bean> </beans>
And here is a small driver program to test the fact that new (prototype)
instances of the newsFeed
bean are actually being returned for
each call to the injected ObjectFactory
inside the
NewsFeedManager
's printNews()
method.
import org.springframework.context.ApplicationContext; import org.springframework.context.support.ClassPathXmlApplicationContext; import x.y.NewsFeedManager; public class Main { public static void main(String[] args) throws Exception { ApplicationContext ctx = new ClassPathXmlApplicationContext("beans.xml"); NewsFeedManager manager = (NewsFeedManager) ctx.getBean("newsFeedManager"); manager.printNews(); manager.printNews(); } }
The output from running the above program will look like so (results will of course vary on your machine).
x.y.NewsFeed@1292d26: '... that's fit to print!' x.y.NewsFeed@5329c5: '... that's fit to print!'
If a bean implements the
org.springframework.beans.factory.BeanNameAware
interface and is deployed in a BeanFactory
, the
BeanFactory
will call the bean through this
interface to inform the bean of the id it was deployed
under. The callback will be invoked after population of normal bean properties
but before an initialization callback like InitializingBean
's
afterPropertiesSet or a custom init-method.
A bean definition potentially contains a large amount of configuration information, including container specific information (for example initialization method, static factory method name, and so forth) and constructor arguments and property values. A child bean definition is a bean definition that inherits configuration data from a parent definition. It is then able to override some values, or add others, as needed. Using parent and child bean definitions can potentially save a lot of typing. Effectively, this is a form of templating.
When working with a BeanFactory
programmatically, child bean
definitions are represented by the ChildBeanDefinition
class. Most users will never work with them on this level, instead
configuring bean definitions declaratively in something like the
XmlBeanFactory
. When using XML-based configuration metadata a child bean
definition is indicated simply by using the 'parent'
attribute, specifying the parent bean as the value of this attribute.
<bean id="inheritedTestBean" abstract="true" class="org.springframework.beans.TestBean"> <property name="name" value="parent"/> <property name="age" value="1"/> </bean> <bean id="inheritsWithDifferentClass" class="org.springframework.beans.DerivedTestBean" parent="inheritedTestBean" init-method="initialize"> <property name="name" value="override"/> <!-- the age property value of 1 will be inherited from parent --> </bean>
A child bean definition will use the bean class from the parent definition if none is specified, but can also override it. In the latter case, the child bean class must be compatible with the parent, that is it must accept the parent's property values.
A child bean definition will inherit constructor argument values,
property values and method overrides from the parent, with the option to
add new values. If any init-method, destroy-method and/or static
factory method settings are specified, they will override the corresponding parent
settings.
The remaining settings will always be taken from the child definition: depends on, autowire mode, dependency check, singleton, scope, lazy init.
Note that in the example above, we have explicitly marked the parent
bean definition as abstract by using the abstract
attribute. In the case that the parent definition does not specify a
class, and so explicitly marking the parent bean definition as
abstract
is required:
<bean id="inheritedTestBeanWithoutClass" abstract="true">
<property name="name" value="parent"/>
<property name="age" value="1"/>
</bean>
<bean id="inheritsWithClass" class="org.springframework.beans.DerivedTestBean"
parent="inheritedTestBeanWithoutClass" init-method="initialize">
<property name="name" value="override"/>
<!-- age will inherit the value of 1
from the parent bean definition-->
</bean>
The parent bean cannot get instantiated on its own since it is
incomplete, and it is also explicitly marked as abstract
.
When a definition is defined to be abstract
like this,
it is usable only as a pure template bean definition that will serve as a
parent definition for child definitions. Trying to use such an
abstract
parent bean on its own (by referring to it as
a ref property of another bean, or doing an explicit
getBean()
call with the parent bean id), will
result in an error. Similarly, the container's internal
preInstantiateSingletons()
method will completely
ignore bean definitions which are defined as abstract.
![]() | Note |
---|---|
|
The IoC component of the Spring Framework has been designed for extension.
There is typically no need for an application developer to subclass any of the various
BeanFactory
or ApplicationContext
implementation classes. The Spring IoC container can be infinitely extended by
plugging in implementations of special integration interfaces. The next few sections are
devoted to detailing all of these various integration interfaces.
The first extension point that we will look at is the
BeanPostProcessor
interface. This interface defines
a number of callback methods that you as an application
developer can implement in order to provide your own (or override the containers default)
instantiation logic, dependency-resolution logic, and so forth. If you want to do
some custom logic after the Spring container has finished instantiating, configuring
and otherwise initializing a bean, you can plug in one or more
BeanPostProcessor
implementations.
You can configure multiple BeanPostProcessors
if you wish.
You can control the order in which these BeanPostProcessors
execute by setting the 'order'
property (you can only set this property
if the BeanPostProcessor
implements the Ordered
interface; if you write your own
BeanPostProcessor
you should consider implementing the
Ordered
interface too); consult the Javadocs for the
BeanPostProcessor
and Ordered
interfaces for more details.
![]() | Note |
---|---|
If you want to change the actual bean definition (that is the recipe that
defines the bean), then you rather need to use a
Also, |
The org.springframework.beans.factory.config.BeanPostProcessor
interface consists of exactly two callback methods. When such a class is
registered as a post-processor with the container (see below for how this registration
is effected), for each bean instance that is created by the container, the post-processor
will get a callback from the container both before any container
initialization methods (such as afterPropertiesSet and any declared
init method) are called, and also afterwards. The post-processor is free to do what it
wishes with the bean instance, including ignoring the callback completely. A bean
post-processor will typically check for marker interfaces, or do something such as wrap
a bean with a proxy; some of the Spring AOP infrastructure classes are implemented as bean
post-processors and they do this proxy-wrapping logic.
It is important to know that a BeanFactory
treats bean
post-processors slightly differently than an ApplicationContext
.
An ApplicationContext
will automatically detect
any beans which are defined in the configuration metadata which is supplied to it that
implement the BeanPostProcessor
interface, and register them
as post-processors, to be then called appropriately by the container on bean creation. Nothing
else needs to be done other than deploying the post-processors in a similar fashion to any
other bean. On the other hand, when using a BeanFactory
implementation, bean post-processors explicitly have to be registered, with code like this:
ConfigurableBeanFactory factory = new XmlBeanFactory(...);
// now register any needed BeanPostProcessor
instances
MyBeanPostProcessor postProcessor = new MyBeanPostProcessor();
factory.addBeanPostProcessor(postProcessor);
// now start using the factory
This explicit registration step is not convenient, and this is one of the
reasons why the various ApplicationContext
implementations are preferred above plain BeanFactory
implementations in the vast majority of Spring-backed applications, especially
when using BeanPostProcessors
.
![]() | BeanPostProcessors and AOP auto-proxying |
---|---|
Classes that implement the For any such bean, you should see an info log message: “Bean 'foo' is not eligible for getting processed by all BeanPostProcessors (for example: not eligible for auto-proxying)”. |
Find below some examples of how to write, register, and use
BeanPostProcessors
in the context of an
ApplicationContext
.
This first example is hardly compelling, but serves to illustrate basic
usage. All we are going to do is code a custom BeanPostProcessor
implementation that simply invokes the toString()
method of each bean as it is created by the container and prints the resulting
string to the system console. Yes, it is not hugely useful, but serves to get
the basic concepts across before we move into the second example which
is actually useful.
Find below the custom BeanPostProcessor
implementation class definition:
package scripting; import org.springframework.beans.factory.config.BeanPostProcessor; import org.springframework.beans.BeansException; public class InstantiationTracingBeanPostProcessor implements BeanPostProcessor { // simply return the instantiated bean as-is public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException { return bean; // we could potentially return any object reference here... } public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException { System.out.println("Bean '" + beanName + "' created : " + bean.toString()); return bean; } }
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:lang="http://www.springframework.org/schema/lang"
xsi:schemaLocation="
http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.0.xsd
http://www.springframework.org/schema/lang http://www.springframework.org/schema/lang/spring-lang-2.0.xsd">
<lang:groovy id="messenger"
script-source="classpath:org/springframework/scripting/groovy/Messenger.groovy">
<lang:property name="message" value="Fiona Apple Is Just So Dreamy."/>
</lang:groovy>
<!--
when the above bean ('messenger') is instantiated, this custom
BeanPostProcessor
implementation will output the fact to the system console
-->
<bean class="scripting.InstantiationTracingBeanPostProcessor"/>
</beans>
Notice how the InstantiationTracingBeanPostProcessor
is
simply defined; it doesn't even have a name, and because it is a bean it can be
dependency injected just like any other bean. (The above configuration also just so
happens to define a bean that is backed by a Groovy script. The Spring 2.0 dynamic
language support is detailed in the chapter entitled
Chapter 24, Dynamic language support.)
Find below a small driver script to exercise the above code and configuration;
import org.springframework.context.ApplicationContext; import org.springframework.context.support.ClassPathXmlApplicationContext; import org.springframework.scripting.Messenger; public final class Boot { public static void main(final String[] args) throws Exception { ApplicationContext ctx = new ClassPathXmlApplicationContext("scripting/beans.xml"); Messenger messenger = (Messenger) ctx.getBean("messenger"); System.out.println(messenger); } }
The output of executing the above program will be (something like) this:
Bean 'messenger' created : org.springframework.scripting.groovy.GroovyMessenger@272961 org.springframework.scripting.groovy.GroovyMessenger@272961
Using marker interfaces or annotations in conjunction with a custom
BeanPostProcessor
implementation is a common
means of extending the Spring IoC container. This next example is a bit of
a cop-out, in that you are directed to the section entitled
Section 25.3.1, “@Required
” which demonstrates the usage of a
custom BeanPostProcessor
implementation that
ships with the Spring distribution which ensures that JavaBean properties on
beans that are marked with an (arbitrary) annotation are actually (configured to
be) dependency-injected with a value.
The next extension point that we will look at is the
org.springframework.beans.factory.config.BeanFactoryPostProcessor
.
The semantics of this interface are similar to the BeanPostProcessor
,
with one major difference. BeanFactoryPostProcessors
operate on; that is to say,
the Spring IoC container will allow BeanFactoryPostProcessors
to read the
configuration metadata and potentially change it before the
container has actually instantied any other beans.
You can configure multiple BeanFactoryPostProcessors
if you wish.
You can control the order in which these BeanFactoryPostProcessors
execute by setting the 'order'
property (you can only set this property
if the BeanFactoryPostProcessor
implements the Ordered
interface; if you write your own
BeanFactoryPostProcessor
you should consider
implementing the Ordered
interface too); consult
the Javadocs for the BeanFactoryPostProcessor
and Ordered
interfaces for more details.
![]() | Note |
---|---|
If you want to change the actual bean instances
(the objects that are created from the configuration metadata), then
you rather need to use a Also, |
A bean factory post-processor is executed manually (in the case of a
BeanFactory
) or automatically (in the case of an
ApplicationContext
) to apply changes of
some sort to the configuration metadata that defines a container. Spring
includes a number of pre-existing bean factory post-processors, such as
PropertyResourceConfigurer
and
PropertyPlaceholderConfigurer
, both described below,
and BeanNameAutoProxyCreator
, which is very useful for wrapping
other beans transactionally or with any other kind of proxy, as described
later in this manual. The BeanFactoryPostProcessor
can be used to add custom property editors.
In a BeanFactory
, the process of applying a
BeanFactoryPostProcessor
is manual, and will be
similar to this:
XmlBeanFactory factory = new XmlBeanFactory(new FileSystemResource("beans.xml"));
// bring in some property values from a Properties
file
PropertyPlaceholderConfigurer cfg = new PropertyPlaceholderConfigurer();
cfg.setLocation(new FileSystemResource("jdbc.properties"));
// now actually do the replacement
cfg.postProcessBeanFactory(factory);
This explicit registration step is not convenient, and this is one of the
reasons why the various ApplicationContext
implementations are preferred above plain BeanFactory
implementations in the vast majority of Spring-backed applications, especially
when using BeanFactoryPostProcessors
.
An ApplicationContext
will detect any beans which
are deployed into it which implement the BeanFactoryPostProcessor
interface, and automatically use them as bean factory post-processors, at the appropriate
time. Nothing else needs to be done other than deploying these post-processor in a similar
fashion to any other bean.
![]() | Note |
---|---|
Just as in the case of |
The PropertyPlaceholderConfigurer
is used to externalize property values from a
BeanFactory
definition, into another
separate file in the standard Java Properties
format.
This is useful to allow the person deploying an application to customize
enviroment-specific properties (for example database URLs, usernames and passwords), without the
complexity or risk of modifying the main XML definition file or files for the
container.
Consider the following XML-based configuration metadata fragment, where a
DataSource
with placeholder values is defined. We
will configure some properties from an external Properties
file, and at runtime, we will apply a
PropertyPlaceholderConfigurer
to the metadata which will
replace some properties of the datasource:
<bean class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer"> <property name="locations"> <value>classpath:com/foo/jdbc.properties</value> </property> </bean> <bean id="dataSource" destroy-method="close" class="org.apache.commons.dbcp.BasicDataSource"> <property name="driverClassName" value="${jdbc.driverClassName}"/> <property name="url" value="${jdbc.url}"/> <property name="username" value="${jdbc.username}"/> <property name="password" value="${jdbc.password}"/> </bean>
The actual values come from another file in the standard Java
Properties
format:
jdbc.driverClassName=org.hsqldb.jdbcDriver jdbc.url=jdbc:hsqldb:hsql://production:9002 jdbc.username=sa jdbc.password=root
The PropertyPlaceholderConfigurer
doesn't only
look for properties in the Properties
file you
specify, but also checks against the Java System
properties if it cannot find a property you are trying to use. This
behavior can be customized by setting the systemPropertiesMode
property of the configurer. It has three values, one to tell the configurer
to always override, one to let it never override and
one to let it override only if the property cannot be found in the
properties file specified. Please consult the Javadoc for the
PropertiesPlaceholderConfigurer
for more information.
![]() | Class name substitution |
---|---|
The <bean class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer"> <property name="locations"> <value>classpath:com/foo/strategy.properties</value> </property> <property name="properties"> <value>custom.strategy.class=com.foo.DefaultStrategy</value> </property> </bean> <bean id="serviceStrategy" class="${custom.strategy.class}"/> If the class is unable to be resolved at runtime to a valid class,
resolution of the the bean will fail once it is about to be created
(which is during the |
The PropertyOverrideConfigurer
, another bean
factory post-processor, is similar to the
PropertyPlaceholderConfigurer
, but in contrast to the
latter, the original definitions can have default values or no values at
all for bean properties. If an overriding Properties
file
does not have an entry for a certain bean property, the default context definition
is used.
Note that the bean factory definition is not
aware of being overridden, so it is not immediately obvious when looking
at the XML definition file that the override configurer is being used.
In case that there are multiple PropertyOverrideConfigurer
instances that define different values for the same bean property, the last one
will win (due to the overriding mechanism).
Properties file configuration lines are expected to be in the format:
beanName.property=value
An example properties file might look like this:
dataSource.driverClassName=com.mysql.jdbc.Driver dataSource.url=jdbc:mysql:mydb
This example file would be usable against a container definition which contains a bean called dataSource, which has driver and url properties.
Note that compound property names are also supported, as long as every component of the path except the final property being overridden is already non-null (presumably initialized by the constructors). In this example...
foo.fred.bob.sammy=123
... the sammy
property of the bob
property
of the fred
property of the foo
bean is being set to the scalar value 123
.
The org.springframework.beans.factory.FactoryBean
interface is to be implemented by objects that are themselves
factories.
The FactoryBean
interface is a point of pluggability
into the Spring IoC containers instantiation logic. If you have some complex
initialization code that is better expressed in Java as opposed to a (potentially)
verbose amount of XML, you can create your own FactoryBean
,
write the complex initialization inside that class, and then plug your custom
FactoryBean
into the container.
The FactoryBean
interface provides three methods:
Object getObject()
: has to return an
instance of the object this factory creates. The instance can
possibly be shared (depending on whether this factory returns
singletons or prototypes).
boolean isSingleton()
: has to return
true
if this FactoryBean
returns singletons, false
otherwise
Class getObjectType()
: has to return
either the object type returned by the
getObject()
method or null
if the type isn't known in advance
The FactoryBean
concept and interface
is used in a number of places within the Spring Framework; at the time of writing
there are over 50 implementations of the FactoryBean
interface that ship with Spring itself.
Finally, there is sometimes a need to ask a container for an actual
FactoryBean
instance itself, not the bean it produces.
This may be achieved by prepending the bean id with '&'
(sans quotes) when calling the getBean
method of
the BeanFactory
(including
ApplicationContext
). So for a given
FactoryBean
with an id of myBean
,
invoking getBean("myBean")
on the container will return the
product of the FactoryBean
, but invoking
getBean("&myBean")
will return the
FactoryBean
instance itself.
While the beans
package provides basic
functionality for managing and manipulating beans, often in a programmatic
way, the context
package adds
ApplicationContext
,
which enhances BeanFactory
functionality in a more
framework-oriented style. Many users will use
ApplicationContext
in a completely declarative fashion,
not even having to create it manually, but instead relying on support classes such as
ContextLoader
to automatically start an ApplicationContext
as part of the
normal startup process of a J2EE web-app. Of course, it is still possible
to programmatically create an ApplicationContext.
The basis for the context package is the
ApplicationContext
interface, located in the
org.springframework.context
package. Deriving from the
BeanFactory
interface, it provides all the functionality of
BeanFactory
. To allow working in a more framework-oriented
fashion, using layering and hierarchical contexts, the context package also provides the
following functionality:
MessageSource
, providing access to
messages in i18n-style
Access to resources, such as URLs and files
Event propagation to beans implementing
the ApplicationListener
interface
Loading of multiple (hierarchical) contexts, allowing each to be focused on one particular layer, for example the web layer of an application
As the ApplicationContext
includes all functionality of the
BeanFactory
, it is generally recommended that it be used over the
BeanFactory
, except for a few limited situations such as perhaps in an
Applet
, where memory consumption might be critical, and a few extra
kilobytes might make a difference. The following sections describe
functionality that ApplicationContext
adds to basic BeanFactory
capabilities.
The ApplicationContext
interface extends an interface called
MessageSource
, and therefore provides messaging (i18n
or internationalization) functionality. Together with the
HierarchicalMessageSource
, capable of resolving
hierarchical messages, these are the basic interfaces Spring provides to
do message resolution. Let's quickly review the methods defined there:
String getMessage(String code, Object[] args,
String default, Locale loc)
: the basic method used to
retrieve a message from the MessageSource
. When no message is
found for the specified locale, the default message is used. Any
arguments passed in are used as replacement values, using the
MessageFormat
functionality provided by the
standard library.
String getMessage(String code, Object[] args,
Locale loc)
: essentially the same as the previous
method, but with one difference: no default message can be
specified; if the message cannot be found, a
NoSuchMessageException
is thrown.
String getMessage(MessageSourceResolvable
resolvable, Locale locale)
: all properties used in the
methods above are also wrapped in a class named
MessageSourceResolvable
, which you can use via
this method.
When an ApplicationContext
gets loaded, it automatically searches
for a MessageSource
bean defined in the context. The bean has to have
the name messageSource
. If such a bean is found, all
calls to the methods described above will be delegated to the message
source that was found. If no message source was found, the
ApplicationContext
attempts to see if it has a parent containing a bean
with the same name. If so, it uses that bean as the MessageSource
. If it
can't find any source for messages, an empty
StaticMessageSource
will be instantiated in order to
be able to accept calls to the methods defined above.
Spring currently provides two MessageSource
implementations. These are the
ResourceBundleMessageSource
and the
StaticMessageSource
. Both implement
NestingMessageSource
in order to do nested messaging.
The StaticMessageSource
is hardly ever used but provides programmatic
ways to add messages to the source. The ResourceBundleMessageSource
is
more interesting and is the one we will provide an example for:
<beans> <bean id="messageSource" class="org.springframework.context.support.ResourceBundleMessageSource"> <property name="basenames"> <list> <value>format</value> <value>exceptions</value> <value>windows</value> </list> </property> </bean> </beans>
This assumes you have three resource bundles defined on your
classpath called format
,
exceptions
and windows
. Using the
JDK standard way of resolving messages through ResourceBundles, any
request to resolve a message will be handled. For the purposes of the example,
lets assume the contents of two of the above resource bundle files are...
# in 'format.properties'
message=Alligators rock!
# in 'exceptions.properties'
argument.required=The '{0}' argument is required.
Some (admittedly trivial) driver code to exercise the
MessageSource
functionality can be found below. Remember
that all ApplicationContext
implementations are also
MessageSource
implementations and so can be cast to the
MessageSource
interface.
public static void main(String[] args) { MessageSource resources = new ClassPathXmlApplicationContext("beans.xml"); String message = resources.getMessage("message", null, "Default", null); System.out.println(message); }
The resulting output from the above program will be...
Alligators rock!
So to summarize, the MessageSource
is defined in a file
called 'beans.xml'
(this file exists at the root of your classpath).
The 'messageSource'
bean definition refers to a number
of resource bundles via it's basenames
property; the three files
that are passed in the list to the basenames
property exist as files
at the root of your classpath (and are called
format.properties
, exceptions.properties
,
and windows.properties
respectively).
Lets look at another example, and this time we will look at passing arguments to the message lookup; these arguments will be converted into strings and inserted into placeholders in the lookup message. This is perhaps best explained with an example:
<beans> <!-- thisMessageSource
is being used in a web application --> <bean id="messageSource" class="org.springframework.context.support.ResourceBundleMessageSource"> <property name="baseName" value="WEB-INF/test-messages"/> </bean> <!-- let's inject the aboveMessageSource
into this POJO --> <bean id="example" class="com.foo.Example"> <property name="messages" ref="messageSource"/> </bean> </beans>
public class Example { private MessageSource messages; public void setMessages(MessageSource messages) { this.messages = messages; } public void execute() { String message = this.messages.getMessage("argument.required", new Object [] {"userDao"}, "Required", null); System.out.println(message); } }
The resulting output from the invocation of the execute()
method will be...
The 'userDao' argument is required.
With regard to internationalization (i18n), Spring's various
MessageResource
implementations follow the same locale resolution
and fallback rules as the standard JDK ResourceBundle. In short, and continuing with
the example 'messageSource'
defined previously, if you want to resolve
messages against the British (en-GB) locale, you would create files called
format_en_GB.properties
, exceptions_en_GB.properties
,
and windows_en_GB.properties
respectively.
Locale resolution is typically going to be managed by the surrounding environment of the application. For the purpose of this example though, we'll just manually specify the locale that we want to resolve our (British) messages against.
# in 'exceptions_en_GB.properties'
argument.required=Ebagum lad, the '{0}' argument is required, I say, required.
public static void main(final String[] args) { MessageSource resources = new ClassPathXmlApplicationContext("beans.xml"); String message = resources.getMessage("argument.required", new Object [] {"userDao"}, "Required", Locale.UK); System.out.println(message); }
The resulting output from the running of the above program will be...
Ebagum lad, the 'userDao' argument is required, I say, required.
The MessageSourceAware
interface can also be used to acquire
a reference to any MessageSource
that has been defined. Any bean
that is defined in an ApplicationContext
that implements the
MessageSourceAware
interface will be injected with the
application context's MessageSource
when it (the bean) is being
created and configured.
Event handling in the ApplicationContext
is provided
through the ApplicationEvent
class and
ApplicationListener
interface. If a bean which
implements the ApplicationListener
interface is
deployed into the context, every time an ApplicationEvent
gets published to the ApplicationContext
, that
bean will be notified. Essentially, this is the standard Observer
design pattern. Spring provides three standard events:
Table 3.5. Built-in Events
Event | Explanation |
---|---|
ContextRefreshedEvent
|
Published when the
|
ContextClosedEvent
|
Published when the |
RequestHandledEvent
|
A web-specific event telling all beans that a HTTP
request has been serviced (this will be published
after the request has been finished).
Note that this event is only applicable for web applications
using Spring's |
Implementing custom events can be done as well. Simply call the
publishEvent()
method on the ApplicationContext
,
specifying a parameter which is an instance of your custom event class
implementing ApplicationEvent
. Event listeners receive events
synchronously. This means the publishEvent()
method blocks until all
listeners have finished processing the event (it is possible to supply
an alternate event publishing strategy via a
ApplicationEventMulticaster
implementation).
Furthermore, when a listener receives an event it operates inside the transaction context of
the publisher, if a transaction context is available.
Let's look at an example. First, the ApplicationContext
:
<bean id="emailer" class="example.EmailBean"> <property name="blackList"> <list> <value>black@list.org</value> <value>white@list.org</value> <value>john@doe.org</value> </list> </property> </bean> <bean id="blackListListener" class="example.BlackListNotifier"> <property name="notificationAddress" value="spam@list.org"/> </bean>
Now, let's look at the actual classes:
public class EmailBean implements ApplicationContextAware {
private List blackList;
private ApplicationContext ctx;
public void setBlackList(List blackList) {
this.blackList = blackList;
}
public void setApplicationContext(ApplicationContext ctx) {
this.ctx = ctx;
}
public void sendEmail(String address, String text) {
if (blackList.contains(address)) {
BlackListEvent evt = new BlackListEvent(address, text);
ctx.publishEvent(evt);
return;
}
// send email...
}
}
public class BlackListNotifier implements ApplicationListener {
private String notificationAddress;
public void setNotificationAddress(String notificationAddress) {
this.notificationAddress = notificationAddress;
}
public void onApplicationEvent(ApplicationEvent evt) {
if (evt instanceof BlackListEvent) {
// notify appropriate person...
}
}
}
Of course, this particular example could probably be implemented in better ways (perhaps by using AOP features), but it should be sufficient to illustrate the basic event mechanism.
For optimal usage and understanding of application contexts, users
should generally familiarize themselves with Spring's
Resource
abstraction, as described
in the chapter entitled Chapter 4, Resources.
An application context is a ResourceLoader
,
able to be used to load Resource
s. A
Resource
is essentially a
java.net.URL
on steroids (in fact, it just wraps and
uses a URL where appropriate), which can be used to obtain low-level
resources from almost any location in a transparent fashion, including
from the classpath, a filesystem location, anywhere describable with a
standard URL, and some other variations. If the resource location string
is a simple path without any special prefixes, where those resources
come from is specific and appropriate to the actual application context
type.
A bean deployed into the application context may implement the
special marker interface, ResourceLoaderAware
, to be
automatically called back at initialization time with the application
context itself passed in as the ResourceLoader
.
A bean may also expose properties of type
Resource
, to be used to access static resources, and
expect that they will be injected into it like any other properties. The
person deploying the bean may specify those Resource
properties as simple String paths, and rely on a special JavaBean
PropertyEditor
that is automatically registered
by the context, to convert those text strings to actual Resource
objects.
The location path or paths supplied to an ApplicationContext
constructor are actually resource strings, and in simple form are
treated appropriately to the specific context implementation (
ClassPathXmlApplicationContext
treats a simple location path as a
classpath location), but may also be used with special prefixes to force
loading of definitions from the classpath or a URL, regardless of the
actual context type.
As opposed to the BeanFactory
, which will
often be created programmatically, ApplicationContext
instances can be created declaratively using for example a
ContextLoader
. Of course you can also create
ApplicationContext
instances programmatically
using one of the ApplicationContext
implementations.
First, let's examine the ContextLoader
interface
and its implementations.
The ContextLoader
interface has two
implementations: the ContextLoaderListener
and the
ContextLoaderServlet
. They both have the same
functionality but differ in that the listener version cannot be used in Servlet
2.2 compatible containers. Since the Servlet 2.4 specification, servlet context
listeners are required to execute immediately after the servlet context for the
web application has been created and is available to service the first request
(and also when the servlet context is about to be shut down): as such a
servlet context listener is an ideal place to initialize the Spring
ApplicationContext
. It is up to you
as to which one you use, but all things being equal you should probably
prefer ContextLoaderListener
; for more information on
compatibility, have a look at the Javadoc for the
ContextLoaderServlet
.
You can register an ApplicationContext
using the
ContextLoaderListener
as follows:
<context-param>
<param-name>contextConfigLocation</param-name>
<param-value>/WEB-INF/daoContext.xml /WEB-INF/applicationContext.xml</param-value>
</context-param>
<listener>
<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>
<!-- or use the ContextLoaderServlet
instead of the above listener
<servlet>
<servlet-name>context</servlet-name>
<servlet-class>org.springframework.web.context.ContextLoaderServlet</servlet-class>
<load-on-startup>1</load-on-startup>
</servlet>
-->
The listener inspects the
contextConfigLocation
parameter. If it doesn't exist,
it'll use /WEB-INF/applicationContext.xml
as a default.
When it does exist, it'll separate the String using
predefined delimiters (comma, semi-colon and whitespace) and use the values as
locations where application contexts will be searched for. The
ContextLoaderServlet
can be used instead of the
ContextLoaderListener
. The servlet will use the
'contextConfigLocation'
parameter just as the listener does.
The majority of the code inside an application is best written in a
DI style, where that code is served out of a Spring IoC container, has its own
dependencies supplied by the container when it is created, and is
completely unaware of the container. However, for the small glue layers of
code that are sometimes needed to tie other code together, there is
sometimes a need for singleton (or quasi-singleton) style access to a
Spring IoC container. For example,
third party code may try to construct new objects directly (Class.forName()
style), without the ability to force it to get these objects out of a
Spring IoC container. If the object constructed by the
third party code is just a small stub or proxy, which then uses a singleton style access to a
Spring IoC container to get a real object to delegate to, then inversion of control has
still been achieved for the majority of the code (the object coming out of the
container); thus most code is still unaware of the container or how it is accessed, and
remains uncoupled from other code, with all ensuing benefits. EJBs may also use this stub/proxy
approach to delegate to a plain Java implementation object, coming out of
a Spring IoC container. While the Spring IoC container itself ideally does not have to be a
singleton, it may be unrealistic in terms of memory usage or initialization times (when
using beans in the Spring IoC container such as a Hibernate
SessionFactory
) for each bean to use its own, non-singleton
Spring IoC container.
As another example, in a complex J2EE apps with multiple layers
(various JAR files, EJBs, and WAR files packaged as an EAR),
with each layer having its own Spring IoC container definition
(effectively forming a hierarchy), the preferred approach when
there is only one web-app (WAR) in the top hierarchy is to simply
create one composite Spring IoC container from the multiple XML definition
files from each layer. All of the various Spring IoC container implementations
may be constructed from multiple definition files in this fashion.
However, if there are multiple sibling web-applications at the root
of the hierarchy, it is problematic to create a
Spring IoC container for each web-application
which consists of mostly identical bean definitions from lower layers,
as there may be issues due to increased memory usage, issues with
creating multiple copies of beans which take a long time to initialize
(e.g. a Hibernate SessionFactory
), and
possible issues due to side-effects. As an alternative, classes such as
ContextSingletonBeanFactoryLocator
or
SingletonBeanFactoryLocator
may be used to demand-load multiple hierarchical (that is one container is the parent of
another) Spring IoC container instances in a singleton fashion,
which may then be used as the parents of the web-application Spring IoC
container instances. The result is that bean definitions for lower layers are
loaded only as needed, and loaded only once.
You can see a detailed example of their usage in SingletonBeanFactoryLocator and ContextSingletonBeanFactoryLocator by viewing their respective Javadocs.
As mentioned in the chapter on EJBs, the
Spring convenience base classes for EJBs normally use a non-singleton
BeanFactoryLocator
implementation, which is
easily replaced by the use of SingletonBeanFactoryLocator
and ContextSingletonBeanFactoryLocator
if there is
a need.
[1] See the section entitled Background
[2] See the section entitled Section 3.3.1, “Injecting dependencies”
Java's standard java.net.URL
class and
standard handlers for various URL prefixes unfortunately are not quite
adequate enough for all access to low-level resources. For example,
there is no standardized URL
implementation
that may be used to access a resource that needs to be obtained from
the classpath, or relative to a
ServletContext
. While it is possible
to register new handlers for specialized URL
prefixes (similar to existing handlers for prefixes such as
http:
), this is generally quite complicated, and the
URL
interface still lacks some desirable
functionality, such as a method to check for the existence of the
resource being pointed to.
Spring's Resource
interface is meant
to be a more capable interface for abstracting access to low-level
resources.
public interface Resource extends InputStreamSource { boolean exists(); boolean isOpen(); URL getURL() throws IOException; File getFile() throws IOException; Resource createRelative(String relativePath) throws IOException; String getFilename(); String getDescription(); }
public interface InputStreamSource { InputStream getInputStream() throws IOException; }
Some of the most important methods from the
Resource
interface are:
getInputStream()
: locates and opens the
resource, returning an InputStream
for reading
from the resource. It is expected that each invocation returns a
fresh InputStream
. It is the responsibility of
the caller to close the stream.
exists()
: returns a
boolean
indicating whether this resource actually
exists in physical form.
isOpen()
: returns a
boolean
indicating whether this resource represents
a handle with an open stream. If true
, the
InputStream
cannot be read multiple times, and
must be read once only and then closed to avoid resource leaks. Will
be false
for all usual resource implementations,
with the exception of
InputStreamResource
.
getDescription()
: returns a description
for this resource, to be used for error output when working with the
resource. This is often the fully qualified file name or the actual
URL of the resource.
Other methods allow you to obtain an actual
URL
or File
object
representing the resource (if the underlying implementation is compatible,
and supports that functionality).
The Resource
abstraction is used
extensively in Spring itself, as an argument type in many method
signatures when a resource is needed. Other methods in some Spring APIs
(such as the constructors to various
ApplicationContext
implementations), take a
String
which in unadorned or simple form is used to
create a Resource
appropriate to that
context implementation, or via special prefixes on the
String
path, allow the caller to specify that a
specific Resource
implementation must be
created and used.
While the Resource
interface is used
a lot with Spring and by Spring, it's actually very useful to use as a
general utility class by itself in your own code, for access to resources,
even when your code doesn't know or care about any other parts of Spring.
While this couples your code to Spring, it really only couples it to this
small set of utility classes, which are serving as a more capable
replacement for URL
, and can be considered
equivalent to any other library you would use for this purpose.
It is important to note that the
Resource
abstraction does not replace
functionality: it wraps it where possible. For example, a
UrlResource
wraps a URL, and uses the wrapped
URL
to do it's work.
There are a number of Resource
implementations that come supplied straight out of the box in
Spring:
The UrlResource
wraps a
java.net.URL
, and may be used to access any
object that is normally accessible via a URL, such as files, an HTTP
target, an FTP target, etc. All URLs have a standardized
String
representation, such that appropriate
standardized prefixes are used to indicate one URL type from another.
This includes file:
for accessing filesystem paths,
http:
for accessing resources via the HTTP protocol,
ftp:
for accessing resources via FTP, etc.
A UrlResource
is created by Java code
explicitly using the UrlResource
constructor, but
will often be created implicitly when you call an API method which takes
a String
argument which is meant to represent a
path. For the latter case, a JavaBeans
PropertyEditor
will ultimately decide
which type of Resource
to create. If the
path string contains a few well-known (to it, that is) prefixes such as
classpath:
, it will create an appropriate specialized
Resource
for that prefix. However, if it
doesn't recognize the prefix, it will assume the this is just a standard
URL string, and will create a UrlResource
.
This class represents a resource which should be obtained from the classpath. This uses either the thread context class loader, a given class loader, or a given class for loading resources.
This Resource
implementation
supports resolution as java.io.File
if the class
path resource resides in the file system, but not for classpath
resources which reside in a jar and have not been expanded (by the
servlet engine, or whatever the environment is) to the filesystem. To
address this the various Resource
implementations always support resolution as a
java.net.URL
.
A ClassPathResource
is created by Java code
explicitly using the ClassPathResource
constructor, but will often be created implicitly when you call an API
method which takes a String
argument which is
meant to represent a path. For the latter case, a JavaBeans
PropertyEditor
will recognize the special
prefix classpath:
on the string path, and create a
ClassPathResource
in that case.
This is a Resource
implementation
for java.io.File
handles. It obviously supports
resolution as a File
, and as a
URL
.
This is a Resource
implementation
for ServletContext
resources,
interpreting relative paths within the relevant web application's root
directory.
This always supports stream access and URL access, but only allows
java.io.File
access when the web application
archive is expanded and the resource is physically on the filesystem.
Whether or not it's expanded and on the filesystem like this, or
accessed directly from the JAR or somewhere else like a DB (it's
conceivable) is actually dependent on the Servlet container.
A Resource
implementation for a
given InputStream
. This should only be
used if no specific Resource
implementation is applicable. In particular, prefer
ByteArrayResource
or any of the file-based
Resource
implementations where
possible.
In contrast to other Resource
implementations, this is a descriptor for an
already opened resource - therefore returning
true
from isOpen()
. Do not
use it if you need to keep the resource descriptor somewhere, or if you
need to read a stream multiple times.
The ResourceLoader
interface is meant
to be implemented by objects that can return (i.e. load)
Resource
instances.
public interface ResourceLoader { Resource getResource(String location); }
All application contexts implement the
ResourceLoader
interface, and therefore all
application contexts may be used to obtain
Resource
instances.
When you call getResource()
on a specific
application context, and the location path specified doesn't have a
specific prefix, you will get back a
Resource
type that is appropriate to that
particular application context. For example, assume the following snippet
of code was executed against a
ClassPathXmlApplicationContext
instance:
Resource template = ctx.getResource("some/resource/path/myTemplate.txt);
What would be returned would be a
ClassPathResource
; if the same method was executed
against a FileSystemXmlApplicationContext
instance,
you'd get back a FileSystemResource
. For a
WebApplicationContext
, you'd get back a
ServletContextResource
, and so on.
As such, you can load resources in a fashion appropriate to the particular application context.
On the other hand, you may also force
ClassPathResource
to be used, regardless of the
application context type, by specifying the special
classpath:
prefix:
Resource template = ctx.getResource("classpath:some/resource/path/myTemplate.txt);
Similarly, one can force a UrlResource
to be
used by specifying any of the standard java.net.URL
prefixes:
Resource template = ctx.getResource("file:/some/resource/path/myTemplate.txt);
Resource template = ctx.getResource("http://myhost.com/resource/path/myTemplate.txt);
The following table summarizes the strategy for converting
String
s to
Resource
s:
Table 4.1. Resource strings
Prefix | Example | Explanation |
---|---|---|
classpath: | | Loaded from the classpath. |
file: | | Loaded as a |
http: | | Loaded as a
|
(none) | | Depends on the underlying
|
[a] But see also the section entitled Section 4.7.3, “ |
The ResourceLoaderAware
interface is
a special marker interface, identifying objects that expect to be provided
with a ResourceLoader
reference.
public interface ResourceLoaderAware { void setResourceLoader(ResourceLoader resourceLoader); }
When a class implements
ResourceLoaderAware
and is deployed into an
application context (as a Spring-managed bean), it is recognized as
ResourceLoaderAware
by the application
context. The application context will then invoke the
setResourceLoader(ResourceLoader)
, supplying
itself as the argument (remember, all application contexts in Spring
implement the ResourceLoader
interface).
Of course, since an
ApplicationContext
is a
ResourceLoader
, the bean could also
implement the ApplicationContextAware
interface and use the supplied application context directly to load
resources, but in general, it's better to use the specialized
ResourceLoader
interface if that's all
that's needed. The code would just be coupled to the resource loading
interface, which can be considered a utility interface, and not the whole
Spring ApplicationContext
interface.
If the bean itself is going to determine and supply the resource
path through some sort of dynamic process, it probably makes sense for the
bean to use the ResourceLoader
interface to
load resources. Consider as an example the loading of a template of some
sort, where the specific resource that is needed depends on the role of
the user. If the resources are static, it makes sense to eliminate the use
of the ResourceLoader
interface completely,
and just have the bean expose the Resource
properties it needs, and expect that they will be injected into it.
What makes it trivial to then inject these properties, is that all
application contexts register and use a special JavaBeans
PropertyEditor
which can convert
String
paths to
Resource
objects. So if
myBean
has a template property of type
Resource
, it can be configured with a
simple string for that resource, as follows:
<bean id="myBean" class="..."> <property name="template" value="some/resource/path/myTemplate.txt"/> </bean>
Note that the resource path has no prefix, so because the
application context itself is going to be used as the
ResourceLoader
, the resource itself will be
loaded via a ClassPathResource
,
FileSystemResource
, or
ServletContextResource
(as appropriate)
depending on the exact type of the context.
If there is a need to force a specific
Resource
type to be used, then a prefix may
be used. The following two examples show how to force a
ClassPathResource
and a
UrlResource
(the latter being used to access a
filesystem file).
<property name="template" value="classpath:some/resource/path/myTemplate.txt">
<property name="template" value="file:/some/resource/path/myTemplate.txt"/>
An application context constructor (for a specific application context type) generally takes a string or array of strings as the location path(s) of the resource(s) such as XML files that make up the definition of the context.
When such a location path doesn't have a prefix, the specific
Resource
type built from that path and
used to load the bean definitions, depends on and is appropriate to the
specific application context. For example, if you create a
ClassPathXmlApplicationContext
as follows:
ApplicationContext ctx = new ClassPathXmlApplicationContext("conf/appContext.xml");
The bean definitions will be loaded from the classpath, as a
ClassPathResource
will be
used. But if you create a
FileSystemXmlApplicationContext
as
follows:
ApplicationContext ctx = new FileSystemXmlApplicationContext("conf/appContext.xml");
The bean definition will be loaded from a filesystem location, in this case relative to the current working directory.
Note that the use of the special classpath prefix or a standard
URL prefix on the location path will override the default type of
Resource
created to load the definition.
So this FileSystemXmlApplicationContext
...
ApplicationContext ctx = new FileSystemXmlApplicationContext("classpath:conf/appContext.xml");
... will actually load it's bean definitions from the classpath.
However, it is still a FileSystemXmlApplicationContext
. If it is
subsequently used as a ResourceLoader
,
any unprefixed paths will still be treated as filesystem paths.
The ClassPathXmlApplicationContext
exposes a number of constructors to enable convenient instantiation.
The basic idea is that one supplies merely a string array containing
just the filenames of the XML files themselves (without the leading
path information), and one also supplies a
Class
; the
ClassPathXmlApplicationContext
will derive the
path information from the supplied class.
An example will hopefully make this clear. Consider a directory layout that looks like this:
com/ foo/ services.xml daos.xml MessengerService.class
A ClassPathXmlApplicationContext
instance
composed of the beans defined in the 'services.xml'
and 'daos.xml'
could be instantiated like
so...
ApplicationContext ctx = new ClassPathXmlApplicationContext( new String[] {"services.xml", "daos.xml"}, MessengerService.class);
Please do consult the Javadocs for the
ClassPathXmlApplicationContext
class for
details of the various constructors.
The resource paths in application context constructor values may
be a simple path (as shown above) which has a one-to-one mapping to a
target Resource, or alternately may contain the special "classpath*:"
prefix and/or internal Ant-style regular expressions (matched using
Spring's PathMatcher
utility). Both of the latter
are effectively wildcards
One use for this mechanism is when doing component-style
application assembly. All components can 'publish' context definition
fragments to a well-known location path, and when the final application
context is created using the same path prefixed via
classpath*:
, all component fragments will be picked
up automatically.
Note that this wildcarding is specific to use of resource paths in
application context constructors (or when using the
PathMatcher
utility class hierarchy directly),
and is resolved at construction time. It has nothing to do with the
Resource
type itself. It's not possible
to use the classpath*:
prefix to construct an actual
Resource
, as a resource points to just
one resource at a time.
When the path location contains an Ant-style pattern, for example:
/WEB-INF/*-context.xml com/mycompany/**/applicationContext.xml file:C:/some/path/*-context.xml classpath:com/mycompany/**/applicationContext.xml
... the resolver follows a more complex but defined procedure to
try to resolve the wildcard. It produces a Resource for the path up to
the last non-wildcard segment and obtains a URL from it. If this URL
is not a "jar:" URL or container-specific variant (e.g.
"zip:
" in WebLogic, "wsjar
" in
WebSphere, etc.), then a java.io.File
is
obtained from it, and used to resolve the wildcard by walking the
filesystem. In the case of a jar URL, the resolver either gets a
java.net.JarURLConnection
from it, or manually
parse the jar URL, and then traverse the contents of the jar file, to
resolve the wildcards.
If the specified path is already a file URL (either
explicitly, or implicitly because the base
ResourceLoader
is a
filesystem one, then wildcarding is guaranteed to work in a
completely portable fashion.
If the specified path is a classpath location, then the
resolver must obtain the last non-wildcard path segment URL via a
Classloader.getResource()
call. Since this
is just a node of the path (not the file at the end) it is actually
undefined (in the ClassLoader
Javadocs)
exactly what sort of a URL is returned in this case. In practice, it
is always a java.io.File
representing the
directory, where the classpath resource resolves to a filesystem
location, or a jar URL of some sort, where the classpath resource
resolves to a jar location. Still, there is a portability concern on
this operation.
If a jar URL is obtained for the last non-wildcard segment,
the resolver must be able to get a
java.net.JarURLConnection
from it, or
manually parse the jar URL, to be able to walk the contents of the
jar, and resolve the wildcard. This will work in most environments,
but will fail in others, and it is strongly recommended that the
wildcard resolution of resources coming from jars be thoroughly
tested in your specific environment before you rely on it.
When constructing an XML-based application context, a location
string may use the special classpath*:
prefix:
ApplicationContext ctx = new ClassPathXmlApplicationContext("classpath*:conf/appContext.xml");
This special prefix specifies that all classpath resources that
match the given name must be obtained (internally, this essentially
happens via a ClassLoader.getResources(...)
call), and then merged to form the final application context
definition.
![]() | Classpath*: portability |
---|---|
The wildcard classpath relies on the |
The "classpath*:
" prefix can also be combined
with a PathMatcher
pattern in the rest of the location path, for
example "classpath*:META-INF/*-beans.xml
". In this
case, the resolution strategy is fairly simple: a
ClassLoader.getResources() call is used on the last non-wildcard path
segment to get all the matching resources in the class loader
hierarchy, and then off each resource the same PathMatcher resoltion
strategy described above is used for the wildcard subpath.
Please note that "classpath*:
" when
combined with Ant-style patterns will only work reliably with at least
one root directory before the pattern starts, unless the actual target
files reside in the file system. This means that a pattern like
"classpath*:*.xml
" will not retrieve files from the
root of jar files but rather only from the root of expanded
directories. This originates from a limitation in the JDK's
ClassLoader.getResources()
method which only
returns file system locations for a passed-in empty string (indicating
potential roots to search).
Ant-style patterns with "classpath:
"
resources are not guaranteed to find matching resources if the root
package to search is available in multiple class path locations. This
is because a resource such as
com/mycompany/package1/service-context.xml
may be in only one location, but when a path such as
classpath:com/mycompany/**/service-context.xml
is used to try to resolve it, the resolver will work off the (first) URL
returned by getResource("com/mycompany")
;. If
this base package node exists in multiple classloader locations, the
actual end resource may not be underneath. Therefore, preferably, use
"classpath*:
" with the same Ant-style pattern in
such a case, which will search all class path locations that contain
the root package.
A FileSystemResource
that is not attached
to a FileSystemApplicationContext
(that is, a
FileSystemApplicationContext
is not the actual
ResourceLoader
) will treat absolute vs.
relative paths as you would expect. Relative paths are relative to the
current working directory, while absolute paths are relative to the root
of the filesystem.
For backwards compatibility (historical) reasons however, this
changes when the FileSystemApplicationContext
is
the ResourceLoader
. The
FileSystemApplicationContext
simply forces all
attached FileSystemResource
instances to treat
all location paths as relative, whether they start with a leading slash
or not. In practice, this means the following are equivalent:
ApplicationContext ctx = new FileSystemXmlApplicationContext("conf/context.xml");
ApplicationContext ctx = new FileSystemXmlApplicationContext("/conf/context.xml");
As are the following: (Even though it would make sense for them to be different, as one case is relative and the other absolute.)
FileSystemXmlApplicationContext ctx = ...; ctx.getResource("some/resource/path/myTemplate.txt");
FileSystemXmlApplicationContext ctx = ...; ctx.getResource("/some/resource/path/myTemplate.txt");
In practice, if true absolute filesystem paths are needed, it is
better to forgo the use of absolute paths with
FileSystemResource
/
FileSystemXmlApplicationContext
, and just force
the use of a UrlResource
, by using the
file:
URL prefix.
// actual context type doesn't matter, the Resource
will always be UrlResource
ctx.getResource("file:/some/resource/path/myTemplate.txt");
// force this FileSystemXmlApplicationContext to load it's definition via a UrlResource
ApplicationContext ctx =
new FileSystemXmlApplicationContext("file:/conf/context.xml");
There are pros and cons for considering validation as business logic,
and Spring offers a design for validation (and data binding) that
does not exclude either one of them. Specifically validation should not be
tied to the web tier, should be easy to localize and it should be
possible to plug in any validator available. Considering the above, Spring
has come up with a Validator
interface that
is both basic and eminently usable in every layer of an application.
Data binding is useful for allowing user input to be dynamically
bound to the domain model of an application (or whatever objects you use
to process user input). Spring provides the so-called
DataBinder
to do exactly that. The
Validator
and the
DataBinder
make up the validation
package,
which is primarily used in but not limited to the MVC framework.
The BeanWrapper
is a fundamental concept in the
Spring Framework and is used in a lot of places. However, you probably
will not ever have the need to use the BeanWrapper
directly. Because this
is reference documentation however, we felt that some explanation might be
in order. We're explaining the BeanWrapper
in this chapter since if you were
going to use it at all, you would probably do so when trying to bind
data to objects, which is strongly related to the BeanWrapper
.
Spring uses PropertyEditors all over the place. The concept of a
PropertyEditor
is part of the JavaBeans specification. Just as the
BeanWrapper
, it's best to explain the use of PropertyEditors in this
chapter as well, since it's closely related to the BeanWrapper
and the
DataBinder
.
Spring's features a Validator
interface that you can
use to validate objects. The Validator
interface works using
an Errors
object so that while validating, validators can report
validation failures to the Errors
object.
Let's consider a small data object:
public class Person {
private String name;
private int age;
// the usual getters and setters...
}
We're going to provide validation behavior for the Person
class by implementing the following two methods of the
org.springframework.validation.Validator
interface:
supports(Class)
- Can this
Validator
validate instances of the supplied
Class
?
validate(Object, org.springframework.validation.Errors)
-
validates the given object and in case of validation errors, registers
those with the given Errors
object
Implementing a Validator
is fairly straightforward,
especially when you know of the ValidationUtils
helper class
that the Spring Framework also provides.
public class PersonValidator implements Validator {
/**
* This Validator
validates just Person
instances
*/
public boolean supports(Class clazz) {
return Person.class.equals(clazz);
}
public void validate(Object obj, Errors e) {
ValidationUtils.rejectIfEmpty(e, "name", "name.empty");
Person p = (Person) obj;
if (p.getAge() < 0) {
e.rejectValue("age", "negativevalue");
} else if (p.getAge() > 110) {
e.rejectValue("age", "too.darn.old");
}
}
}
As you can see, the static
rejectIfEmpty(..)
method on the ValidationUtils
class is used to reject the
'name'
property if it is null
or the empty string.
Have a look at the Javadoc for the ValidationUtils
class to see
what functionality it provides besides the example shown previously.
While it is certainly possible to implement a single
Validator
class to validate each of the nested objects
in a rich object, it may be better to encapsulate the validation logic for each nested
class of object in its own Validator
implementation. A
simple example of a 'rich' object would be a
Customer
that is composed of two String
properties (a first and second name) and a complex Address
object.
Address
objects may be used independently of
Customer
objects, and so a distinct
AddressValidator
has been implemented. If you want your
CustomerValidator
to reuse the logic contained within the
AddressValidator
class without recourse to copy-n-paste you can
dependency-inject or instantiate an AddressValidator
within your
CustomerValidator
, and use it like so:
public class CustomerValidator implements Validator {
private final Validator addressValidator;
public CustomerValidator(Validator addressValidator) {
if (addressValidator == null) {
throw new IllegalArgumentException("The supplied [Validator] is required and must not be null.");
}
if (!addressValidator.supports(Address.class)) {
throw new IllegalArgumentException(
"The supplied [Validator] must support the validation of [Address] instances.");
}
this.addressValidator = addressValidator;
}
/**
* This Validator
validates Customer
instances, and any subclasses of Customer
too
*/
public boolean supports(Class clazz) {
return Customer.class.isAssignableFrom(clazz);
}
public void validate(Object target, Errors errors) {
ValidationUtils.rejectIfEmptyOrWhitespace(errors, "firstName", "field.required");
ValidationUtils.rejectIfEmptyOrWhitespace(errors, "surname", "field.required");
Customer customer = (Customer) target;
try {
errors.pushNestedPath("address");
ValidationUtils.invokeValidator(this.addressValidator, customer.getAddress(), errors);
} finally {
errors.popNestedPath();
}
}
}
Validation errors are reported to the Errors
object passed to the validator. In case of Spring Web MVC you can use
<spring:bind/>
tag to inspect the error messages, but
of course you can also inspect the errors object yourself. More information about
the methods it offers can be found from the Javadoc.
We've talked about databinding and validation. Outputting messages corresponding to
validation errors is the last thing we need to discuss. In the example we've shown
above, we rejected the name
and the age
field.
If we're going to output the error messages by using a MessageSource
,
we will do so using the error code we've given when rejecting the field ('name' and 'age'
in this case). When you call (either directly, or indirectly, using for example the
ValidationUtils
class) rejectValue
or one of
the other reject
methods from the Errors
interface, the underlying implementation will not only register the code you've
passed in, but also a number of additional error codes. What error codes it registers
is determined by the MessageCodesResolver
that is used.
By default, the DefaultMessageCodesResolver
is used, which for example
not only registers a message with the code you gave, but also messages that include the
field name you passed to the reject method. So in case you reject a field using
rejectValue("age", "too.darn.old")
, apart from the
too.darn.old
code, Spring will also register
too.darn.old.age
and too.darn.old.age.int
(so the first will include the field name and the second will include the type of the
field); this is done as a convenience to aid developers in targeting error
messages and suchlike.
More information on the MessageCodesResolver
and the default
strategy can be found online with the Javadocs for
MessageCodesResolver
and
DefaultMessageCodesResolver
respectively.
The org.springframework.beans
package adheres to
the JavaBeans standard provided by Sun. A JavaBean is simply a class with
a default no-argument constructor, which follows a naming conventions
where a property named bingoMadness
has a setter
setBingoMadness(..)
and a getter getBingoMadness()
.
For more information about JavaBeans and the specification, please refer
to Sun's website
(java.sun.com/products/javabeans).
One quite important concept of the beans package is the
BeanWrapper
interface and its corresponding
implementation (BeanWrapperImpl
). As quoted from the
Javadoc, the BeanWrapper
offers functionality to set and get property
values (individually or in bulk), get property descriptors, and to query
properties to determine if they are readable or writable. Also, the
BeanWrapper
offers support for nested properties, enabling the setting of
properties on sub-properties to an unlimited depth. Then, the BeanWrapper
supports the ability to add standard JavaBeans
PropertyChangeListeners
and
VetoableChangeListeners
, without the need for
supporting code in the target class. Last but not least, the BeanWrapper
provides support for the setting of indexed properties. The BeanWrapper
usually isn't used by application code directly, but by the
DataBinder
and the
BeanFactory
.
The way the BeanWrapper
works is partly indicated by its name:
it wraps a bean to perform actions on that bean, like
setting and retrieving properties.
Setting and getting properties is done using the
setPropertyValue(s)
and
getPropertyValue(s)
methods that both come with a
couple of overloaded variants. They're all described in more detail in
the Javadoc Spring comes with. What's important to know is that there
are a couple of conventions for indicating properties of an object. A
couple of examples:
Table 5.1. Examples of properties
Expression | Explanation |
---|---|
name | Indicates the property name
corresponding to the methods getName() or
isName() and
setName(..) |
account.name | Indicates the nested property name
of the property account corresponding e.g.
to the methods getAccount().setName() or
getAccount().getName() |
account[2] | Indicates the third element of the
indexed property account . Indexed
properties can be of type array ,
list or other naturally
ordered collection |
account[COMPANYNAME] | Indicates the value of the map entry indexed by the key
COMPANYNAME of the Map property
account |
Below you'll find some examples of working with the BeanWrapper
to
get and set properties.
(This next section is not vitally important to you if you're not
planning to work with the BeanWrapper
directly. If you're
just using the DataBinder
and the
BeanFactory
and their out-of-the-box implementation, you
should skip ahead to the section about
PropertyEditors
.)
Consider the following two classes:
public class Company { private String name; private Employee managingDirector; public String getName() { return this.name; } public void setName(String name) { this.name = name; } public Employee getManagingDirector() { return this.managingDirector; } public void setManagingDirector(Employee managingDirector) { this.managingDirector = managingDirector; } }
public class Employee { private String name; private float salary; public String getName() { return this.name; } public void setName(String name) { this.name = name; } public float getSalary() { return salary; } public void setSalary(float salary) { this.salary = salary; } }
The following code snippets show some examples of how to retrieve
and manipulate some of the properties of instantiated
Companies
and Employees
:
BeanWrapper company = BeanWrapperImpl(new Company()); // setting the company name.. company.setPropertyValue("name", "Some Company Inc."); // ... can also be done like this: PropertyValue value = new PropertyValue("name", "Some Company Inc."); company.setPropertyValue(value); // ok, let's create the director and tie it to the company: BeanWrapper jim = BeanWrapperImpl(new Employee()); jim.setPropertyValue("name", "Jim Stravinsky"); company.setPropertyValue("managingDirector", jim.getWrappedInstance()); // retrieving the salary of the managingDirector through the company Float salary = (Float) company.getPropertyValue("managingDirector.salary");
Spring heavily uses the concept of
PropertyEditors
. Sometimes it might be handy to be
able to represent properties in a different way than the object itself.
For example, a date can be represented in a human readable way, while
we're still able to convert the human readable form back to the original
date (or even better: convert any date entered in a human readable form,
back to Date
objects). This behavior can be achieved by
registering custom editors, of type
java.beans.PropertyEditor
. Registering custom editors
on a BeanWrapper
or alternately in a specific IoC container as
mentioned in the previous chapter, gives it the knowledge of how to
convert properties to the desired type. Read more about PropertyEditors
in the Javadoc of the java.beans
package provided by
Sun.
A couple of examples where property editing is used in Spring
setting properties on beans is done
using PropertyEditors
. When mentioning
java.lang.String
as the value of a property of
some bean you're declaring in XML file, Spring will (if the setter
of the corresponding property has a Class
-parameter) use the
ClassEditor
to try to resolve the parameter to
a Class
object
parsing HTTP request parameters in
Spring's MVC framework is done using all kinds of PropertyEditors
that you can manually bind in all subclasses of the
CommandController
Spring has a number of built-in PropertyEditors
to make life easy.
Each of those is listed below and they are all located in the
org.springframework.beans.propertyeditors
package.
Most, but not all (as indicated below), are registered by default by
BeanWrapperImpl
. Where the property editor is configurable in some
fashion, you can of course still register your own variant to override
the default one:
Table 5.2. Built-in PropertyEditors
Class | Explanation |
---|---|
ByteArrayPropertyEditor | Editor for byte arrays. Strings will simply be
converted to their corresponding byte representations.
Registered by default by BeanWrapperImpl . |
ClassEditor | Parses Strings representing classes to actual classes
and the other way around. When a class is not found, an
IllegalArgumentException is thrown. Registered by default by
BeanWrapperImpl . |
CustomBooleanEditor | Customizable property editor for Boolean properties.
Registered by default by BeanWrapperImpl , but, can be
overridden by registering custom instance of it as custom
editor. |
CustomCollectionEditor | Property editor for Collections, converting any source
Collection to a given target Collection type. |
CustomDateEditor | Customizable property editor for java.util.Date, supporting a custom DateFormat. NOT registered by default. Must be user registered as needed with appropriate format. |
CustomNumberEditor | Customizable property editor for any Number subclass
like Integer , Long ,
Float , Double . Registered
by default by BeanWrapperImpl , but can be
overridden by registering custom instance of it as a custom editor. |
FileEditor | Capable of resolving Strings to
java.io.File objects. Registered by default by
BeanWrapperImpl . |
InputStreamEditor | One-way property editor, capable of taking a text
string and producing (via an intermediate ResourceEditor and
Resource ) an
InputStream , so InputStream
properties may be directly set as Strings. Note that the default usage
will not close the InputStream for
you! Registered by default by BeanWrapperImpl . |
LocaleEditor | Capable of resolving Strings to
Locale objects and vice versa (the String
format is [language]_[country]_[variant], which is the same
thing the toString() method of Locale provides). Registered by
default by BeanWrapperImpl . |
PatternEditor | Capable of resolving Strings to JDK 1.5
Pattern objects and vice versa. |
PropertiesEditor | Capable of converting Strings (formatted using the
format as defined in the Javadoc for the java.lang.Properties
class) to Properties objects. Registered by
default by BeanWrapperImpl . |
StringTrimmerEditor | Property editor that trims Strings. Optionally allows
transforming an empty string into a null value. NOT
registered by default; must be user registered as needed. |
URLEditor | Capable of resolving a String representation of a URL
to an actual URL object. Registered by
default by BeanWrapperImpl . |
Spring uses the java.beans.PropertyEditorManager
to set
the search path for property editors that might be needed. The search path also includes
sun.bean.editors
, which includes
PropertyEditor
implementations for types such as
Font
, Color
, and most of the primitive types.
Note also that the standard JavaBeans infrastructure will automatically discover
PropertyEditor
classes (without you having to register them
explicitly) if they are in the same package as the class they handle, and have the same name
as that class, with 'Editor'
appended; for example, one could have the
following class and package structure, which would be sufficient for the
FooEditor
class to be recognized and used as the
PropertyEditor
for Foo
-typed
properties.
com
chank
pop
Foo
FooEditor // the PropertyEditor
for the Foo
class
Note that you can also use the standard BeanInfo
JavaBeans
mechanism here as well (described
in not-amazing-detail here).
Find below an example of using the BeanInfo
mechanism for
explicitly registering one or more PropertyEditor
instances
with the properties of an associated class.
com
chank
pop
Foo
FooBeanInfo // the BeanInfo
for the Foo
class
Here is the Java source code for the referenced FooBeanInfo
class. This
would associate a CustomNumberEditor
with the age
property of the Foo
class.
public class FooBeanInfo extends SimpleBeanInfo { public PropertyDescriptor[] getPropertyDescriptors() { try { final PropertyEditor numberPE = new CustomNumberEditor(Integer.class, true); PropertyDescriptor ageDescriptor = new PropertyDescriptor("age", Foo.class) { public PropertyEditor createPropertyEditor(Object bean) { return numberPE; }; }; return new PropertyDescriptor[] { ageDescriptor }; } catch (IntrospectionException ex) { throw new Error(ex.toString()); } } }
When setting bean properties as a string value, a Spring IoC container
ultimately uses standard JavaBeans PropertyEditors
to convert these
Strings to the complex type of the property. Spring pre-registers a number
of custom PropertyEditors
(for example, to convert a classname expressed
as a string into a real Class
object). Additionally, Java's standard
JavaBeans PropertyEditor
lookup mechanism allows a
PropertyEditor
for a class simply to be named appropriately and
placed in the same package as the class it provides support for, to be found automatically.
If there is a need to register other custom PropertyEditors
, there
are several mechanisms available. The most manual approach, which is not normally convenient or
recommended, is to simply use the registerCustomEditor()
method of the
ConfigurableBeanFactory
interface, assuming you have a
BeanFactory
reference. The more convenient mechanism is to use
a special bean factory post-processor called CustomEditorConfigurer
.
Although bean factory post-processors can be used semi-manually with
BeanFactory
implementations, this one has a nested property
setup, so it is strongly recommended that it is used
with the ApplicationContext
, where it may be deployed in similar fashion
to any other bean, and automatically detected and applied.
Note that all bean factories and application contexts automatically
use a number of built-in property editors, through their use of something
called a BeanWrapper
to handle property conversions.
The standard property editors that the BeanWrapper
registers
are listed in the previous section. Additionally,
ApplicationContexts
also override or add an additional number of editors
to handle resource lookups in a manner appropriate to the specific application context type.
Standard JavaBeans PropertyEditor
instances are used to convert property values expressed as strings to
the actual complex type of the property.
CustomEditorConfigurer
, a bean factory post-processor,
may be used to conveniently add support for additional
PropertyEditor
instances to an
ApplicationContext
.
Consider a user class ExoticType
, and another
class DependsOnExoticType
which needs
ExoticType
set as a property:
package example; public class ExoticType { private String name; public ExoticType(String name) { this.name = name; } } public class DependsOnExoticType { private ExoticType type; public void setType(ExoticType type) { this.type = type; } }
When things are properly set up, we want to be able to assign the type property
as a string, which a PropertyEditor
will behind the scenes
convert into a real ExoticType
object:
<bean id="sample" class="example.DependsOnExoticType"> <property name="type" value="aNameForExoticType"/> </bean>
The PropertyEditor
implementation could look similar to this:
// converts string representation to ExoticType
object
package example;
public class ExoticTypeEditor extends PropertyEditorSupport {
private String format;
public void setFormat(String format) {
this.format = format;
}
public void setAsText(String text) {
if (format != null && format.equals("upperCase")) {
text = text.toUpperCase();
}
ExoticType type = new ExoticType(text);
setValue(type);
}
}
Finally, we use CustomEditorConfigurer
to
register the new PropertyEditor
with the ApplicationContext
, which will
then be able to use it as needed:
<bean id="customEditorConfigurer" class="org.springframework.beans.factory.config.CustomEditorConfigurer"> <property name="customEditors"> <map> <entry key="example.ExoticType"> <bean class="example.ExoticTypeEditor"> <property name="format" value="upperCase"/> </bean> </entry> </map> </property> </bean>
Aspect-Oriented Programming (AOP) complements Object-Oriented Programming (OOP) by providing another way of thinking about program structure. In addition to classes, AOP gives you aspects. Aspects enable modularization of concerns such as transaction management that cut across multiple types and objects. (Such concerns are often termed crosscutting concerns.)
One of the key components of Spring is the AOP framework. While the Spring IoC container does not depend on AOP, meaning you don't need to use AOP if you don't want to, AOP complements Spring IoC to provide a very capable middleware solution.
AOP is used in the Spring Framework:
To provide declarative enterprise services, especially as a replacement for EJB declarative services. The most important such service is declarative transaction management, which builds on the Spring Framework's transaction abstraction.
To allow users to implement custom aspects, complementing their use of OOP with AOP.
If you are interested only in generic declarative services or other pre-packaged declarative middleware services such as pooling, you don't need to work directly with Spring AOP, and can skip most of this chapter.
Let us begin by defining some central AOP concepts. These terms are not Spring-specific. Unfortunately, AOP terminology is not particularly intuitive; however, it would be even more confusing if Spring used its own terminology.
Aspect: A modularization of a concern
that cuts across multiple objects. Transaction management is a good example
of a crosscutting concern in J2EE applications. In Spring AOP, aspects are
implemented using regular classes (the schema-based approach) or regular
classes annotated with the @Aspect
annotation
(@AspectJ
style).
Join point: A point during the execution of
a program, such as the execution of a method or the handling of
an exception. In Spring AOP, a join point always represents
a method execution. Join point information is available in advice
bodies by declaring a parameter of type org.aspectj.lang.JoinPoint
.
Advice: Action taken by an aspect at a particular join point. Different types of advice include "around," "before" and "after" advice. Advice types are discussed below. Many AOP frameworks, including Spring, model an advice as an interceptor, maintaining a chain of interceptors "around" the join point.
Pointcut: A predicate that matches join points. Advice is associated with a pointcut expression and runs at any join point matched by the pointcut (for example, the execution of a method with a certain name). The concept of join points as matched by pointcut expressions is central to AOP: Spring uses the AspectJ pointcut language by default.
Introduction: (Also known as an
inter-type declaration). Declaring additional methods or fields on
behalf of a type. Spring AOP allows you to introduce new interfaces
(and a corresponding implementation) to
any proxied object. For example, you could use an introduction to
make a bean implement an IsModified
interface, to simplify caching.
Target object: Object being advised by one or more aspects. Also referred to as the advised object. Since Spring AOP is implemented using runtime proxies, this object will always be a proxied object.
AOP proxy: An object created by the AOP framework in order to implement the aspect contracts (advise method executions and so on). In the Spring Framework, an AOP proxy will be a JDK dynamic proxy or a CGLIB proxy. Proxy creation is transparent to users of the schema-based and @AspectJ styles of aspect declaration introduced in Spring 2.0.
Weaving: Linking aspects with other application types or objects to create an advised object. This can be done at compile time (using the AspectJ compiler, for example), load time, or at runtime. Spring AOP, like other pure Java AOP frameworks, performs weaving at runtime.
Types of advice:
Before advice: Advice that executes before a join point, but which does not have the ability to prevent execution flow proceeding to the join point (unless it throws an exception).
After returning advice: Advice to be executed after a join point completes normally: for example, if a method returns without throwing an exception.
After throwing advice: Advice to be executed if a method exits by throwing an exception.
After (finally) advice: Advice to be executed regardless of the means by which a join point exits (normal or exceptional return).
Around advice: Advice that surrounds a join point such as a method invocation. This is the most powerful kind of advice. Around advice can perform custom behavior before and after the method invocation. It is also responsible for choosing whether to proceed to the join point or to shortcut the advised method execution by returning its own return value or throwing an exception.
Around advice is the most general kind of advice. Since Spring AOP,
like AspectJ, provides a full range of advice types, we recommend that you
use the least powerful advice type that can implement the required behavior.
For example, if you need only to update a cache with the return value of a
method, you are better off implementing an after returning advice than an
around advice, although an around advice can accomplish the same thing. Using
the most specific advice type provides a simpler programming model with less
potential for errors. For example, you do not need to invoke the
proceed()
method on the JoinPoint
used for around advice, and hence cannot fail to invoke it.
In Spring 2.0, all advice parameters are statically typed, so that you
work with advice parameters of the appropriate type (the type of the return
value from a method execution for example) rather than
Object
arrays.
The concept of join points, matched by pointcuts, is the key to AOP which distinguishes it from older technologies offering only interception. Pointcuts enable advice to be targeted independently of the Object-Oriented hierarchy. For example, an around advice providing declarative transaction management can be applied to a set of methods spanning multiple objects (such as all business operations in the service layer).
Spring AOP is implemented in pure Java. There is no need for a special compilation process. Spring AOP does not need to control the class loader hierarchy, and is thus suitable for use in a J2EE web container or application server.
Spring AOP currently supports only method execution join points (advising the execution of methods on Spring beans). Field interception is not implemented, although support for field interception could be added without breaking the core Spring AOP APIs. If you need to advise field access and update join points, consider a language such as AspectJ.
Spring AOP's approach to AOP differs from that of most other AOP frameworks. The aim is not to provide the most complete AOP implementation (although Spring AOP is quite capable); it is rather to provide a close integration between AOP implementation and Spring IoC to help solve common problems in enterprise applications.
Thus, for example, the Spring Framework's AOP functionality is normally used in conjunction with the Spring IoC container. Aspects are configured using normal bean definition syntax (although this allows powerful "autoproxying" capabilities): this is a crucial difference from other AOP implementations. There are some things you cannot do easily or efficiently with Spring AOP, such as advise very fine-grained objects: AspectJ is the best choice in such cases. However, our experience is that Spring AOP provides an excellent solution to most problems in J2EE applications that are amenable to AOP.
Spring AOP will never strive to compete with AspectJ to provide a comprehensive AOP solution. We believe that both proxy-based frameworks like Spring AOP and full-blown frameworks such as AspectJ are valuable, and that they are complementary, rather than in competition. Spring 2.0 seamlessly integrates Spring AOP and IoC with AspectJ, to enable all uses of AOP to be catered for within a consistent Spring-based application architecture. This integration does not affect the Spring AOP API or the AOP Alliance API: Spring AOP remains backward-compatible. See the following chapter for a discussion of the Spring AOP APIs.
![]() | Note |
---|---|
One of the central tenets of the Spring Framework is that of non-invasiveness; this is the idea that you should not be forced to introduce framework-specific classes and interfaces into your business/domain model. However, in some places the Spring Framework does give you the option to introduce Spring Framework-specific dependencies into your codebase: the rationale in giving you such options is because in certain scenarios it might be just plain easier to read or code some specific piece of functionality in such a way. The Spring Framework (almost) always offers you the choice though: you have the freedom to make an informed decision as to which option best suits your particular use case or scenario. One such choice that is relevant to this chapter is that of which AOP framework (and which AOP style) to choose. You have the choice of AspectJ and/or Spring AOP, and you also have the choice of either the @AspectJ annotation-style approach or the Spring XML configuration-style approach. The fact that this chapter chooses to introduce the @AspectJ-style approach first should not be taken as an indication that the Spring team favors the @AspectJ annotation-style approach over the Spring XML configuration-style. See the section entitled Section 6.4, “Choosing which AOP declaration style to use” for a fuller discussion of the whys and wherefores of each style. |
Spring AOP defaults to using standard J2SE dynamic proxies for AOP proxies. This enables any interface (or set of interfaces) to be proxied.
Spring AOP can also use CGLIB proxies. This is necessary to proxy classes, rather than interfaces. CGLIB is used by default if a business object does not implement an interface. As it is good practice to program to interfaces rather than classes, business classes normally will implement one or more business interfaces. It is possible to force the use of CGLIB, in those (hopefully rare) cases where you need to advise a method that is not declared on an interface, or where you need to pass a proxied object to a method as a concrete type.
It is important to grasp the fact that Spring AOP is proxy-based. See the section entitled Section 6.6.1, “Understanding AOP proxies” for a thorough examination of exactly what this implementation detail actually means.
@AspectJ refers to a style of declaring aspects as regular Java classes annotated with Java 5 annotations. The @AspectJ style was introduced by the AspectJ project as part of the AspectJ 5 release. Spring 2.0 interprets the same annotations as AspectJ 5, using a library supplied by AspectJ for pointcut parsing and matching. The AOP runtime is still pure Spring AOP though, and there is no dependency on the AspectJ compiler or weaver.
Using the AspectJ compiler and weaver enables use of the full AspectJ language,
and is discussed in Section 6.8, “Using AspectJ with Spring applications”.
To use @AspectJ aspects in a Spring configuration you need to enable Spring support for configuring Spring AOP based on @AspectJ aspects, and autoproxying beans based on whether or not they are advised by those aspects. By autoproxying we mean that if Spring determines that a bean is advised by one or more aspects, it will automatically generate a proxy for that bean to intercept method invocations and ensure that advice is executed as needed.
The @AspectJ support is enabled by including the following element inside your spring configuration:
<aop:aspectj-autoproxy/>
This assumes that you are using schema support as described in
Appendix A, XML Schema-based configuration. See Section A.2.6, “The aop
schema”
for how to import the tags in the aop namespace.
If you are using the DTD, it is still possible to enable @AspectJ support by adding the following definition to your application context:
<bean class="org.springframework.aop.aspectj.annotation.AnnotationAwareAspectJAutoProxyCreator" />
You will also need two AspectJ libraries on the classpath of your application:
aspectjweaver.jar
and
aspectjrt.jar
. These libraries
are available in the 'lib'
directory of an AspectJ installation (version
1.5.1 or later required), or in the 'lib/aspectj'
directory of the
Spring-with-dependencies distribution.
With the @AspectJ support enabled, any bean defined in your application context
with a class that is an @AspectJ aspect (has the @Aspect
annotation) will be automatically detected by Spring and used to configure Spring AOP.
The following example shows the minimal definition required for a not-very-useful
aspect:
A regular bean definition in the application context, pointing to a bean class
that has the @Aspect
annotation:
<bean id="myAspect" class="org.xyz.NotVeryUsefulAspect">
<!-- configure properties of aspect here as normal -->
</bean>
And the NotVeryUsefulAspect
class definition, annotated with
org.aspectj.lang.annotation.Aspect
annotation;
package org.xyz; import org.aspectj.lang.annotation.Aspect; @Aspect public class NotVeryUsefulAspect { }
Aspects (classes annotated with @Aspect
) may have
methods and fields just like any other class. They may also contain pointcut, advice,
and introduction (inter-type) declarations.
![]() | Advising aspects |
---|---|
In Spring AOP, it is not possible to have aspects themselves be the target of advice from other aspects. The @Aspect annotation on a class marks it as an aspect, and hence excludes it from auto-proxying. |
Recall that pointcuts determine join points of interest, and thus enable us
to control when advice executes. Spring AOP only supports method execution
join points for Spring beans, so you can think of a pointcut as matching
the execution of methods on Spring beans. A pointcut declaration has two parts: a
signature comprising a name and any parameters, and a pointcut expression
that determines exactly which method executions we are interested in.
In the @AspectJ annotation-style of AOP, a pointcut signature is provided by a regular method
definition, and the pointcut expression is indicated using the
@Pointcut
annotation (the method serving as the pointcut
signature must have a void
return
type).
An example will help make this distinction between a pointcut signature and a
pointcut expression clear. The following example defines a pointcut named
'anyOldTransfer'
that will match the execution of any method named
'transfer'
:
@Pointcut("execution(* transfer(..))")// the pointcut expression private void anyOldTransfer() {}// the pointcut signature
The pointcut expression that forms the value of the @Pointcut
annotation is a regular AspectJ 5 pointcut expression. For a full discussion of AspectJ's
pointcut language, see the
AspectJ Programming Guide
(and for Java 5 based extensions, the
AspectJ 5 Developers Notebook)
or one of the books on AspectJ such as “Eclipse AspectJ” by Colyer et. al.
or “AspectJ in Action” by Ramnivas Laddad.
Spring AOP supports the following AspectJ pointcut designators for use in pointcut expressions:
execution - for matching method execution join points, this is the primary pointcut designator you will use when working with Spring AOP
within - limits matching to join points within certain types (simply the execution of a method declared within a matching type when using Spring AOP)
this - limits matching to join points (the execution of methods when using Spring AOP) where the bean reference (Spring AOP proxy) is an instance of the given type
target - limits matching to join points (the execution of methods when using Spring AOP) where the target object (application object being proxied) is an instance of the given type
args - limits matching to join points (the execution of methods when using Spring AOP) where the arguments are instances of the given types
@target
- limits matching
to join points (the execution of methods when using Spring AOP) where the
class of the executing object has an annotation of the given type
@args
- limits matching to
join points (the execution of methods when using Spring AOP) where the
runtime type of the actual arguments passed have annotations of the given type(s)
@within
- limits matching
to join points within types that have the given annotation (the execution of methods
declared in types with the given annotation when using Spring AOP)
@annotation - limits matching to join points where the subject of the join point (method being executed in Spring AOP) has the given annotation
Because Spring AOP limits matching to only method execution join points, the discussion of the pointcut designators above gives a narrower definition than you will find in the AspectJ programming guide. In addition, AspectJ itself has type-based semantics and at an execution join point both 'this' and 'target' refer to the same object - the object executing the method. Spring AOP is a proxy based system and differentiates between the proxy object itself (bound to 'this') and the target object behind the proxy (bound to 'target').
Pointcut expressions can be combined using '&&', '||' and '!'. It is also possible
to refer to pointcut expressions by name. The following example shows three pointcut
expressions: anyPublicOperation
(which matches if a method execution
join point represents the execution of any public method); inTrading
(which matches
if a method execution is in the trading module), and tradingOperation
(which matches if a method execution represents any public method in the trading module).
@Pointcut("execution(public * *(..))") private void anyPublicOperation() {} @Pointcut("within(com.xyz.someapp.trading..*") private void inTrading() {} @Pointcut("anyPublicOperation() && inTrading()") private void tradingOperation() {}
It is a best practice to build more complex pointcut expressions out of smaller named components as shown above. When referring to pointcuts by name, normal Java visibility rules apply (you can see private pointcuts in the same type, protected pointcuts in the hierarchy, public pointcuts anywhere and so on). Visibility does not affect pointcut matching.
When working with enterprise applications, you often want to refer to modules of the application and particular sets of operations from within several aspects. We recommend defining a "SystemArchitecture" aspect that captures common pointcut expressions for this purpose. A typical such aspect would look as follows:
package com.xyz.someapp; import org.aspectj.lang.annotation.Aspect; import org.aspectj.lang.annotation.Pointcut; @Aspect public class SystemArchitecture { /** * A join point is in the web layer if the method is defined * in a type in the com.xyz.someapp.web package or any sub-package * under that. */ @Pointcut("within(com.xyz.someapp.web..*)") public void inWebLayer() {} /** * A join point is in the service layer if the method is defined * in a type in the com.xyz.someapp.service package or any sub-package * under that. */ @Pointcut("within(com.xyz.someapp.service..*)") public void inServiceLayer() {} /** * A join point is in the data access layer if the method is defined * in a type in the com.xyz.someapp.dao package or any sub-package * under that. */ @Pointcut("within(com.xyz.someapp.dao..*)") public void inDataAccessLayer() {} /** * A business service is the execution of any method defined on a service * interface. This definition assumes that interfaces are placed in the * "service" package, and that implementation types are in sub-packages. * * If you group service interfaces by functional area (for example, * in packages com.xyz.someapp.abc.service and com.xyz.def.service) then * the pointcut expression "execution(* com.xyz.someapp..service.*.*(..))" * could be used instead. */ @Pointcut("execution(* com.xyz.someapp.service.*.*(..))") public void businessService() {} /** * A data access operation is the execution of any method defined on a * dao interface. This definition assumes that interfaces are placed in the * "dao" package, and that implementation types are in sub-packages. */ @Pointcut("execution(* com.xyz.someapp.dao.*.*(..))") public void dataAccessOperation() {} }
The pointcuts defined in such an aspect can be referred to anywhere that you need a pointcut expression. For example, to make the service layer transactional, you could write:
<aop:config> <aop:advisor pointcut="com.xyz.someapp.SystemArchitecture.businessService()" advice-ref="tx-advice"/> </aop:config> <tx:advice id="tx-advice"> <tx:attributes> <tx:method name="*" propagation="REQUIRED"/> </tx:attributes> </tx:advice>
The <aop:config>
and <aop:advisor>
tags are discussed in the section entitled Section 6.3, “Schema-based AOP support”. The transaction
tags are discussed in the chapter entitled Chapter 9, Transaction management.
Spring AOP users are likely to use the execution
pointcut
designator the most often. The format of an execution expression is:
execution(modifiers-pattern? ret-type-pattern declaring-type-pattern? name-pattern(param-pattern) throws-pattern?)
All parts except the returning type pattern (ret-type-pattern in the snippet above),
name pattern, and parameters pattern are optional.
The returning type pattern determines what the return type of the method must be in order
for a join point to be matched. Most frequently you will use *
as the
returning type pattern, which matches any return type. A fully-qualified type name will
match only when the method returns the given type. The name pattern matches the method name.
You can use the *
wildcard as all or part of a name pattern. The
parameters pattern is slightly more complex: ()
matches a method that takes
no parameters, whereas (..)
matches any number of parameters (zero or more).
The pattern (*)
matches a method taking one parameter of any type,
(*,String)
matches a method taking two parameters, the first can be of
any type, the second must be a String. Consult the
Language Semantics section of the AspectJ Programming Guide for more
information.
Some examples of common pointcut expressions are given below.
the execution of any public method:
execution(public * *(..))
the execution of any method with a name beginning with "set":
execution(* set*(..))
the execution of any method defined by the AccountService
interface:
execution(* com.xyz.service.AccountService.*(..))
the execution of any method defined in the service package:
execution(* com.xyz.service.*.*(..))
the execution of any method defined in the service package or a sub-package:
execution(* com.xyz.service..*.*(..))
any join point (method execution only in Spring AOP) within the service package:
within(com.xyz.service.*)
any join point (method execution only in Spring AOP) within the service package or a sub-package:
within(com.xyz.service..*)
any join point (method execution only in Spring AOP) where the proxy implements the AccountService
interface:
this(com.xyz.service.AccountService)
'this' is more commonly used in a binding form :- see the following section on advice for how to make the proxy object available in the advice body.
any join point (method execution only in Spring AOP) where the target object implements the AccountService
interface:
target(com.xyz.service.AccountService)
'target' is more commonly used in a binding form :- see the following section on advice for how to make the target object available in the advice body.
any join point (method execution only in Spring AOP) which takes a single parameter, and where the argument passed at runtime is Serializable
:
args(java.io.Serializable)
'args' is more commonly used in a binding form :- see the following section on advice for how to make the method arguments available in the advice body.
Note that the pointcut given in this example is different to
execution(* *(java.io.Serializable))
: the args version matches if
the argument passed at runtime is Serializable, the execution version matches if the
method signature declares a single parameter of type Serializable
.
any join point (method execution only in Spring AOP) where the target object has an @Transactional
annotation:
@target(org.springframework.transaction.annotation.Transactional)
'@target' can also be used in a binding form :- see the following section on advice for how to make the annotation object available in the advice body.
any join point (method execution only in Spring AOP) where the declared type of the target object has an @Transactional
annotation:
@within(org.springframework.transaction.annotation.Transactional)
'@within' can also be used in a binding form :- see the following section on advice for how to make the annotation object available in the advice body.
any join point (method execution only in Spring AOP) where the executing method has an @Transactional
annotation:
@annotation(org.springframework.transaction.annotation.Transactional)
'@annotation' can also be used in a binding form :- see the following section on advice for how to make the annotation object available in the advice body.
any join point (method execution only in Spring AOP) which takes a single parameter, and where the
runtime type of the argument passed has the @Classified
annotation:
@args(com.xyz.security.Classified)
'@args' can also be used in a binding form :- see the following section on advice for how to make the annotation object(s) available in the advice body.
Advice is associated with a pointcut expression, and runs before, after, or around method executions matched by the pointcut. The pointcut expression may be either a simple reference to a named pointcut, or a pointcut expression declared in place.
Before advice is declared in an aspect using the @Before
annotation:
import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.Before;
@Aspect
public class BeforeExample {
@Before("com.xyz.myapp.SystemArchitecture.dataAccessOperation()")
public void doAccessCheck() {
// ...
}
}
If using an in-place pointcut expression we could rewrite the above example as:
import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.Before;
@Aspect
public class BeforeExample {
@Before("execution(* com.xyz.myapp.dao.*.*(..))")
public void doAccessCheck() {
// ...
}
}
After returning advice runs when a matched method execution returns
normally. It is declared using the @AfterReturning
annotation:
import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.AfterReturning;
@Aspect
public class AfterReturningExample {
@AfterReturning("com.xyz.myapp.SystemArchitecture.dataAccessOperation()")
public void doAccessCheck() {
// ...
}
}
Note: it is of course possible to have multiple advice declarations, and other members as well, all inside the same aspect. We're just showing a single advice declaration in these examples to focus on the issue under discussion at the time.
Sometimes you need access in the advice body to the actual value that was returned. You
can use the form of @AfterReturning
that binds the return
value for this:
import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.AfterReturning;
@Aspect
public class AfterReturningExample {
@AfterReturning(
pointcut="com.xyz.myapp.SystemArchitecture.dataAccessOperation()",
returning="retVal")
public void doAccessCheck(Object retVal) {
// ...
}
}
The name used in the returning
attribute must correspond to
the name of a parameter in the advice method. When a method execution returns, the
return value will be passed to the advice method as the corresponding argument value.
A returning
clause also restricts matching to only those method
executions that return a value of the specified type (Object
in this case, which will match any return value).
Please note that it is not possible to return a totally different reference when using after-returning advice.
After throwing advice runs when a matched method execution exits by throwing
an exception. It is declared using the @AfterThrowing
annotation:
import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.AfterThrowing;
@Aspect
public class AfterThrowingExample {
@AfterThrowing("com.xyz.myapp.SystemArchitecture.dataAccessOperation()")
public void doRecoveryActions() {
// ...
}
}
Often you want the advice to run only when exceptions of a given type are
thrown, and you also often need access to the thrown exception in the advice
body. Use the throwing
attribute to both restrict matching
(if desired, use Throwable
as the exception type
otherwise) and bind the thrown exception to an advice parameter.
import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.AfterThrowing;
@Aspect
public class AfterThrowingExample {
@AfterThrowing(
pointcut="com.xyz.myapp.SystemArchitecture.dataAccessOperation()",
throwing="ex")
public void doRecoveryActions(DataAccessException ex) {
// ...
}
}
The name used in the throwing
attribute must correspond to
the name of a parameter in the advice method. When a method execution exits by
throwing an exception, the
exception will be passed to the advice method as the corresponding argument value.
A throwing
clause also restricts matching to only those method
executions that throw an exception of the specified type
(DataAccessException
in this case).
After (finally) advice runs however a matched method execution exits.
It is declared using the @After
annotation. After
advice must be prepared to handle both normal and exception return conditions.
It is typically used for releasing resources, etc.
import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.After;
@Aspect
public class AfterFinallyExample {
@After("com.xyz.myapp.SystemArchitecture.dataAccessOperation()")
public void doReleaseLock() {
// ...
}
}
The final kind of advice is around advice. Around advice runs "around" a matched method execution. It has the opportunity to do work both before and after the method executes, and to determine when, how, and even if, the method actually gets to execute at all. Around advice is often used if you need to share state before and after a method execution in a thread-safe manner (starting and stopping a timer for example). Always use the least powerful form of advice that meets your requirements (i.e. don't use around advice if simple before advice would do).
Around advice is declared using the @Around
annotation. The first parameter
of the advice method must be of type ProceedingJoinPoint
. Within the body of the
advice, calling proceed()
on the ProceedingJoinPoint
causes the
underlying method to execute. The proceed
method may also be
called passing in an Object[]
- the values in the array will be used as the arguments
to the method execution when it proceeds.
The behavior of proceed when called with an Object[]
is a little different
than the behavior of proceed for around advice compiled by the AspectJ compiler. For
around advice written using the traditional AspectJ language, the number of arguments
passed to proceed must match the number of arguments passed to the around advice (not the
number of arguments taken by the underlying join point), and the value passed to proceed
in a given argument position supplants the original value at the join point for the entity
the value was bound to. (Don't worry if this doesn't make sense right now!) The
approach taken by Spring is simpler and a better match to its proxy-based, execution only
semantics. You only need to be aware of this difference if you compiling @AspectJ aspects
written for Spring and using proceed with arguments with the AspectJ compiler and weaver.
There is a way to write such aspects that is 100% compatible across both Spring AOP and
AspectJ, and this is discussed in the following section on advice parameters.
import org.aspectj.lang.annotation.Aspect; import org.aspectj.lang.annotation.Around; import org.aspectj.lang.ProceedingJoinPoint; @Aspect public class AroundExample { @Around("com.xyz.myapp.SystemArchitecture.businessService()") public Object doBasicProfiling(ProceedingJoinPoint pjp) throws Throwable { // start stopwatch Object retVal = pjp.proceed(); // stop stopwatch return retVal; } }
The value returned by the around advice will be the return value seen by the caller of the method. A simple caching aspect for example could return a value from a cache if it has one, and invoke proceed() if it does not. Note that proceed may be invoked once, many times, or not at all within the body of the around advice, all of these are quite legal.
Spring 2.0 offers fully typed advice - meaning that you declare the parameters
you need in the advice signature (as we saw for the returning and throwing examples
above) rather than work with Object[]
arrays all the time.
We'll see how to make argument and other contextual values available to the advice
body in a moment. First let's take a look at how to write generic advice that can find
out about the method the advice is currently advising.
Any advice method may declare as its first parameter, a parameter of
type org.aspectj.lang.JoinPoint
(please note that
around advice is required to declare a first parameter of type
ProceedingJoinPoint
, which is a subclass of
JoinPoint
. The JoinPoint
interface provides a number of useful methods such as getArgs()
(returns the method arguments), getThis()
(returns the proxy
object), getTarget()
(returns the target object),
getSignature()
(returns a description of the method that is
being advised) and toString()
(prints a useful description of
the method being advised). Please do consult the Javadocs for full details.
We've already seen how to bind the returned value or exception value (using
after returning and after throwing advice). To make argument values available to
the advice body, you can use the binding form of args
. If
a parameter name is used in place of a type name in an args expression, then the
value of the corresponding argument will be passed as the parameter value when the
advice is invoked. An example should make this clearer. Suppose you want to advise
the execution of dao operations that take an Account object as the first parameter,
and you need access to the account in the advice body. You could write the following:
@Before("com.xyz.myapp.SystemArchitecture.dataAccessOperation() &&" +
"args(account,..)")
public void validateAccount(Account account) {
// ...
}
The args(account,..)
part of the pointcut expression serves
two purposes: firstly, it restricts matching to only those method executions where
the method takes at least one parameter, and the argument passed to that parameter
is an instance of Account
; secondly, it makes the actual
Account
object available to the advice via the
account
parameter.
Another way of writing this is to declare a pointcut that "provides" the
Account
object value when it matches a join point, and then
just refer to the named pointcut from the advice. This would look as follows:
@Pointcut("com.xyz.myapp.SystemArchitecture.dataAccessOperation() &&" +
"args(account,..)")
private void accountDataAccessOperation(Account account) {}
@Before("accountDataAccessOperation(account)")
public void validateAccount(Account account) {
// ...
}
The interested reader is once more referred to the AspectJ programming guide for more details.
The proxy object (this
), target object (target
),
and annotations (@within, @target, @annotation, @args
) can all
be bound in a similar fashion. The following example shows how you could match
the execution of methods annotated with an @Auditable
annotation, and extract the audit code.
First the definition of the @Auditable
annotation:
@Retention(RetentionPolicy.RUNTIME) @Target(ElementType.METHOD) public @interface Auditable { AuditCode value(); }
And then the advice that matches the execution of @Auditable
methods:
@Before("com.xyz.lib.Pointcuts.anyPublicMethod() && " +
"@annotation(auditable)")
public void audit(Auditable auditable) {
AuditCode code = auditable.value();
// ...
}
The parameter binding in advice invocations relies on matching names used in pointcut expressions to declared parameter names in (advice and pointcut) method signatures. Parameter names are not available through Java reflection, so Spring AOP uses the following strategies to determine parameter names:
If the parameter names have been specified by the user explicitly, then the specified parameter names are used: both the advice and the pointcut annotations have an optional "argNames" attribute which can be used to specify the argument names of the annotated method - these argument names are available at runtime. For example:
@Before(
value="com.xyz.lib.Pointcuts.anyPublicMethod() && @annotation(auditable)",
argNames="auditable")
public void audit(Auditable auditable) {
AuditCode code = auditable.value();
// ...
}
If an @AspectJ aspect has been compiled by the AspectJ compiler (ajc) then
there is no need to add the argNames
attribute as the compiler will do this
automatically.
Using the 'argNames'
attribute is a little clumsy, so if the
'argNames'
attribute has
not been specified, then Spring AOP will look at the debug information for the
class and try to determine the parameter names from the local variable table. This
information will be present as long as the classes have been compiled with debug
information ('-g:vars'
at a minimum). The consequences of
compiling with this flag on are: (1) your code will be slightly easier to understand
(reverse engineer), (2) the class file sizes will be very slightly bigger (typically
inconsequential), (3) the optimization to remove unused local variables will not be
applied by your compiler. In other words, you should encounter no difficulties building
with this flag on.
If the code has been compiled without the necessary debug information, then Spring AOP
will attempt to deduce the pairing of binding variables to parameters (for example,
if only one variable is bound in the pointcut expression, and the advice method only
takes one parameter, the pairing is obvious!). If the binding of variables is ambiguous
given the available information, then an AmbiguousBindingException
will be thrown.
If all of the above strategies fail then an
IllegalArgumentException
will be thrown.
We remarked earlier that we would describe how to write a proceed call with arguments that works consistently across Spring AOP and AspectJ. The solution is simply to ensure that the advice signature binds each of the method parameters in order. For example:
@Around("execution(List<Account> find*(..)) &&" + "com.xyz.myapp.SystemArchitecture.inDataAccessLayer() && " + "args(accountHolderNamePattern)") public Object preProcessQueryPattern(ProceedingJoinPoint pjp, String accountHolderNamePattern) throws Throwable { String newPattern = preProcess(accountHolderNamePattern); return pjp.proceed(new Object[] {newPattern}); }
In many cases you will be doing this binding anyway (as in the example above).
What happens when multiple pieces of advice all want to run at the same join point? Spring AOP follows the same precedence rules as AspectJ to determine the order of advice execution. The highest precedence advice runs first "on the way in" (so given two pieces of before advice, the one with highest precedence runs first). "On the way out" from a join point, the highest precedence advice runs last (so given two pieces of after advice, the one with the highest precedence will run second). For advice defined within the same aspect, precedence is established by declaration order. Given the aspect:
@Aspect public class AspectWithMultipleAdviceDeclarations { @Pointcut("execution(* foo(..))") public void fooExecution() {} @Before("fooExecution()") public void doBeforeOne() { // ... } @Before("fooExecution()") public void doBeforeTwo() { // ... } @AfterReturning("fooExecution()") public void doAfterOne() { // ... } @AfterReturning("fooExecution()") public void doAfterTwo() { // ... } }
then for any execution of a method named foo, the doBeforeOne
,
doBeforeTwo
, doAfterOne
, and doAfterTwo
advice methods all need to run. The precedence rules are such that the advice will execute
in declaration order. In this case the execution trace would be:
doBeforeOne doBeforeTwo foo doAfterOne doAfterTwo
In other words doBeforeOne has precedence over doBeforeTwo, because it was defined before doBeforeTwo, and doAfterTwo has precedence over doAfterOne because it was defined after doAfterOne. It's easiest just to remember that advice runs in declaration order ;) - see the AspectJ Programming Guide for full details.
When two pieces of advice defined in different aspects both need
to run at the same join point, unless you specify otherwise the order of execution is
undefined. You can control the order of execution by specifying precedence. This is done in
the normal Spring way by either implementing the org.springframework.core.Ordered
interface in the aspect class or annotating it with the Order
annotation.
Given two aspects, the aspect returning the lower value from
Ordered.getValue()
(or the annotation value) has the higher precedence.
Introductions (known as inter-type declarations in AspectJ) enable an aspect to declare that advised objects implement a given interface, and to provide an implementation of that interface on behalf of those objects.
An introduction is made using the @DeclareParents
annotation. This annotation is
used to declare that matching types have a new parent (hence the name). For example, given
an interface UsageTracked
, and an implementation of that interface
DefaultUsageTracked
, the following
aspect declares that all implementors of service interfaces also implement the
UsageTracked
interface.
(In order to expose statistics via JMX for example.)
@Aspect public class UsageTracking { @DeclareParents(value="com.xzy.myapp.service.*+", defaultImpl=DefaultUsageTracked.class) public static UsageTracked mixin; @Before("com.xyz.myapp.SystemArchitecture.businessService() &&" + "this(usageTracked)") public void recordUsage(UsageTracked usageTracked) { usageTracked.incrementUseCount(); } }
The interface to be implemented is determined by the type of the annotated field. The
value
attribute of the @DeclareParents
annotation
is an AspectJ type pattern :- any bean of a matching type will implement the UsageTracked
interface. Note that in the before advice of the above example, service
beans can be directly used as implementations of the UsageTracked
interface. If accessing
a bean programmatically you would write the following:
UsageTracked usageTracked = (UsageTracked) context.getBean("myService");
(This is an advanced topic, so if you are just starting out with AOP you can safely skip it until later.)
By default there will be a single instance of each aspect within the application
context. AspectJ calls this the singleton instantiation model. It is possible to define
aspects with alternate lifecycles :- Spring supports AspectJ's perthis
and pertarget
instantiation models (percflow, percflowbelow,
and pertypewithin
are not currently supported).
A "perthis" aspect is declared by specifying a perthis
clause
in the @Aspect
annotation. Let's look at an example, and then
we'll explain how it works.
@Aspect("perthis(com.xyz.myapp.SystemArchitecture.businessService())")
public class MyAspect {
private int someState;
@Before(com.xyz.myapp.SystemArchitecture.businessService())
public void recordServiceUsage() {
// ...
}
}
The effect of the 'perthis'
clause is that one aspect
instance will be created for each unique service object executing a business
service (each unique object bound to 'this' at join points matched by the pointcut expression).
The aspect instance is created
the first time that a method is invoked on the service object. The aspect goes out
of scope when the service object goes out of scope. Before the aspect instance is created,
none of the advice within it executes. As soon as the aspect instance has been created,
the advice declared within it will execute at matched join points, but only when the service
object is the one this aspect is associated with. See the AspectJ programming guide for
more information on per-clauses.
The 'pertarget'
instantiation model works in exactly the same way as perthis, but
creates one aspect instance for each unique target object at matched join points.
Now that you have seen how all the constituent parts work, let's put them together to do something useful!
The execution of business services can sometimes fail
due to concurrency issues (for example, deadlock loser).
If the operation is retried, it is quite likely to succeed next time round. For
business services where it is appropriate to retry in such conditions (idempotent
operations that don't need to go back to the user for conflict resolution), we'd like
to transparently retry the operation to avoid the client seeing a
PessimisticLockingFailureException
. This is a requirement that clearly cuts across
multiple services in the service layer, and hence is ideal for implementing via an aspect.
Because we want to retry the operation, we will need to use around advice so that we can call proceed multiple times. Here's how the basic aspect implementation looks:
@Aspect public class ConcurrentOperationExecutor implements Ordered { private static final int DEFAULT_MAX_RETRIES = 2; private int maxRetries = DEFAULT_MAX_RETRIES; private int order = 1; public void setMaxRetries(int maxRetries) { this.maxRetries = maxRetries; } public int getOrder() { return this.order; } public void setOrder(int order) { this.order = order; } @Around("com.xyz.myapp.SystemArchitecture.businessService()") public Object doConcurrentOperation(ProceedingJoinPoint pjp) throws Throwable { int numAttempts = 0; PessimisticLockingFailureException lockFailureException; do { numAttempts++; try { return pjp.proceed(); } catch(PessimisticLockingFailureException ex) { lockFailureException = ex; } } while(numAttempts <= this.maxRetries); throw lockFailureException; } }
Note that the aspect implements the Ordered
interface so we can set the precedence
of the aspect higher than the transaction advice (we want a fresh transaction
each time we retry). The maxRetries
and order
properties will both be configured by Spring. The main action happens in the
doConcurrentOperation
around advice. Notice that for the moment
we're applying the retry logic to all businessService()s
. We
try to proceed, and if we fail with an PessimisticLockingFailureException
we simply try again unless we have exhausted all of our retry attempts.
The corresponding Spring configuration is:
<aop:aspectj-autoproxy/> <bean id="concurrentOperationExecutor" class="com.xyz.myapp.service.impl.ConcurrentOperationExecutor"> <property name="maxRetries" value="3"/> <property name="order" value="100"/> </bean>
To refine the aspect so that it only retries idempotent operations, we might
define an Idempotent
annotation:
@Retention(RetentionPolicy.RUNTIME)
public @interface Idempotent {
// marker annotation
}
and use the annotation to annotate the implementation of service operations.
The change to the aspect to only retry idempotent operations simply involves
refining the pointcut expression so that only @Idempotent
operations match:
@Around("com.xyz.myapp.SystemArchitecture.businessService() && " + "@annotation(com.xyz.myapp.service.Idempotent)") public Object doConcurrentOperation(ProceedingJoinPoint pjp) throws Throwable { ... }
If you are unable to use Java 5, or simply prefer an XML-based format, then Spring 2.0 also offers support for defining aspects using the new "aop" namespace tags. The exact same pointcut expressions and advice kinds are supported as when using the @AspectJ style, hence in this section we will focus on the new syntax and refer the reader to the discussion in the previous section (Section 6.2, “@AspectJ support”) for an understanding of writing pointcut expressions and the binding of advice parameters.
To use the aop namespace tags described in this section, you need to import
the spring-aop schema as described in
Appendix A, XML Schema-based configuration. See Section A.2.6, “The aop
schema”
for how to import the tags in the aop namespace.
Within your Spring configurations, all aspect and advisor elements must be
placed within an <aop:config>
element (you can have more
than one <aop:config>
element in an application context
configuration). An <aop:config>
element can contain
pointcut, advisor, and aspect elements (note these must be declared in that order).
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The |
Using the schema support, an aspect is simply a regular Java object defined as a bean in your Spring application context. The state and behavior is captured in the fields and methods of the object, and the pointcut and advice information is captured in the XML.
An aspect is declared using the <aop:aspect> element, and the backing bean
is referenced using the ref
attribute:
<aop:config> <aop:aspect id="myAspect" ref="aBean"> ... </aop:aspect> </aop:config> <bean id="aBean" class="..."> ... </bean>
The bean backing the aspect ("aBean
" in this case) can of course be
configured and dependency injected just like any other Spring bean.
A pointcut can be declared inside an aspect, in which case it is visible only within that aspect. A pointcut can also be declared directly inside an <aop:config> element, enabling the pointcut definition to be shared across several aspects and advisors.
A pointcut representing the execution of any business service in the service layer could be defined as follows:
<aop:config> <aop:pointcut id="businessService" expression="execution(* com.xyz.myapp.service.*.*(..))"/> </aop:config>
Note that the pointcut expression itself is using the same AspectJ pointcut expression language as described in Section 6.2, “@AspectJ support”. If you are using the schema based declaration style with Java 5, you can refer to named pointcuts defined in types (@Aspects) within the pointcut expression, but this feature is not available on JDK 1.4 and below (it relies on the Java 5 specific AspectJ reflection APIs). On JDK 1.5 therefore, another way of defining the above pointcut would be:
<aop:config> <aop:pointcut id="businessService" expression="com.xyz.myapp.SystemArchitecture.businessService()"/> </aop:config>
Assuming you have a SystemArchitecture
aspect as described in
Section 6.2.3.3, “Sharing common pointcut definitions”.
Declaring a pointcut inside an aspect is very similar to declaring a top-level pointcut:
<aop:config> <aop:aspect id="myAspect" ref="aBean"> <aop:pointcut id="businessService" expression="execution(* com.xyz.myapp.service.*.*(..))"/> ... </aop:aspect> </aop:config>
When combining pointcut sub-expressions, '&&' is awkward within an XML document, and so the keywords 'and', 'or' and 'not' can be used in place of '&&', '||' and '!' respectively.
Note that pointcuts defined in this way are referred to by their XML id, and cannot define pointcut parameters. The named pointcut support in the schema based definition style is thus more limited than that offered by the @AspectJ style.
The same five advice kinds are supported as for the @AspectJ style, and they have exactly the same semantics.
Before advice runs before a matched method execution.
It is declared inside an <aop:aspect>
using
the <aop:before> element.
<aop:aspect id="beforeExample" ref="aBean"> <aop:before pointcut-ref="dataAccessOperation" method="doAccessCheck"/> ... </aop:aspect>
Here dataAccessOperation
is the id of a pointcut defined at
the top (<aop:config>
) level. To define the pointcut
inline instead, replace the pointcut-ref
attribute with a
pointcut
attribute:
<aop:aspect id="beforeExample" ref="aBean"> <aop:before pointcut="execution(* com.xyz.myapp.dao.*.*(..))" method="doAccessCheck"/> ... </aop:aspect>
As we noted in the discussion of the @AspectJ style, using named pointcuts can significantly improve the readability of your code.
The method attribute identifies a method (doAccessCheck
) that
provides the body of the advice. This method must be defined for the bean referenced
by the aspect element containing the advice. Before a data access operation is
executed (a method execution join point matched by the pointcut expression), the
"doAccessCheck" method on the aspect bean will be invoked.
After returning advice runs when a matched method execution completes normally.
It is declared inside an <aop:aspect>
in
the same way as before advice. For example:
<aop:aspect id="afterReturningExample" ref="aBean"> <aop:after-returning pointcut-ref="dataAccessOperation" method="doAccessCheck"/> ... </aop:aspect>
Just as in the @AspectJ style, it is possible to get hold of the return value within the advice body. Use the returning attribute to specify the name of the parameter to which the return value should be passed:
<aop:aspect id="afterReturningExample" ref="aBean"> <aop:after-returning pointcut-ref="dataAccessOperation" returning="retVal" method="doAccessCheck"/> ... </aop:aspect>
The doAccessCheck method must declare a parameter named retVal
. The
type of this parameter constrains matching in the same way as described for @AfterReturning.
For example, the method signature may be declared as:
public void doAccessCheck(Object retVal) {...
After throwing advice executes when a matched method execution exits by throwing
an exception. It is declared inside an <aop:aspect>
using
the after-throwing element:
<aop:aspect id="afterThrowingExample" ref="aBean"> <aop:after-throwing pointcut-ref="dataAccessOperation" method="doRecoveryActions"/> ... </aop:aspect>
Just as in the @AspectJ style, it is possible to get hold of the thrown exception within the advice body. Use the throwing attribute to specify the name of the parameter to which the exception should be passed:
<aop:aspect id="afterThrowingExample" ref="aBean"> <aop:after-throwing pointcut-ref="dataAccessOperation" throwing="dataAccessEx" method="doRecoveryActions"/> ... </aop:aspect>
The doRecoveryActions method must declare a parameter named dataAccessEx
.
The type of this parameter constrains matching in the same way as described for
@AfterThrowing.
For example, the method signature may be declared as:
public void doRecoveryActions(DataAccessException dataAccessEx) {...
After (finally) advice runs however a matched method execution exits. It is declared
using the after
element:
<aop:aspect id="afterFinallyExample" ref="aBean"> <aop:after pointcut-ref="dataAccessOperation" method="doReleaseLock"/> ... </aop:aspect>
The final kind of advice is around advice. Around advice runs "around" a matched method execution. It has the opportunity to do work both before and after the method executes, and to determine when, how, and even if, the method actually gets to execute at all. Around advice is often used if you need to share state before and after a method execution in a thread-safe manner (starting and stopping a timer for example). Always use the least powerful form of advice that meets your requirements; don't use around advice if simple before advice would do.
Around advice is declared using the aop:around
element.
The first parameter of the advice method must be of type
ProceedingJoinPoint
. Within the body of the
advice, calling proceed()
on the
ProceedingJoinPoint
causes the
underlying method to execute. The proceed
method may also be
calling passing in an Object[]
- the values in the array will be used as the arguments
to the method execution when it proceeds. See Section 6.2.4.5, “Around advice”
for notes on calling proceed with an Object[]
.
<aop:aspect id="aroundExample" ref="aBean"> <aop:around pointcut-ref="businessService" method="doBasicProfiling"/> ... </aop:aspect>
The implementation of the doBasicProfiling
advice would be
exactly the same as in the @AspectJ example (minus the annotation of course):
public Object doBasicProfiling(ProceedingJoinPoint pjp) throws Throwable { // start stopwatch Object retVal = pjp.proceed(); // stop stopwatch return retVal; }
The schema based declaration style supports fully typed advice in the same way as described for the @AspectJ support - by matching pointcut parameters by name against advice method parameters. See Section 6.2.4.6, “Advice parameters” for details.
If you wish to explicity specify argument names for the advice methods (not
relying on either of the detection strategies previously described) then this is
done using the arg-names
attribute of the advice element. For
example:
<aop:before pointcut="com.xyz.lib.Pointcuts.anyPublicMethod() and @annotation(auditable)" method="audit" arg-names="auditable"/>
The arg-names
attribute accepts a comma-delimited list of
parameter names.
Find below a slightly more involved example of the XSD-based approach that illustrates some around advice used in conjunction with a number of strongly typed parameters.
package x.y.service; public interface FooService { Foo getFoo(String fooName, int age); } public class DefaultFooService implements FooService { public Foo getFoo(String name, int age) { return new Foo(name, age); } }
Next up is the aspect. Notice the fact that the
profile(..)
method accepts a number of strongly-typed parameters,
the first of which happens to be the join point used to proceed with the method call: the
presence of this parameter is an indication that the profile(..)
is to be used as around
advice:
package x.y; import org.aspectj.lang.ProceedingJoinPoint; import org.springframework.util.StopWatch; public class SimpleProfiler { public Object profile(ProceedingJoinPoint call, String name, int age) throws Throwable { StopWatch clock = new StopWatch( "Profiling for '" + name + "' and '" + age + "'"); try { clock.start(call.toShortString()); return call.proceed(); } finally { clock.stop(); System.out.println(clock.prettyPrint()); } } }
Finally, here is the XML configuration that is required to effect the execution of the above advice for a particular joinpoint:
<beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:aop="http://www.springframework.org/schema/aop" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.0.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-2.0.xsd"> <!-- this is the object that will be proxied by Spring's AOP infrastructure --> <bean id="fooService" class="x.y.service.DefaultFooService"/> <!-- this is the actual advice itself --> <bean id="profiler" class="x.y.SimpleProfiler"/> <aop:config> <aop:aspect ref="profiler"> <aop:pointcut id="theExecutionOfSomeFooServiceMethod" expression="execution(* x.y.service.FooService.getFoo(String,int)) and args(name, age)"/> <aop:around pointcut-ref="theExecutionOfSomeFooServiceMethod" method="profile"/> </aop:aspect> </aop:config> </beans>
If we had the following driver script, we would get output something like this on standard output:
import org.springframework.beans.factory.BeanFactory; import org.springframework.context.support.ClassPathXmlApplicationContext; import x.y.service.FooService; public final class Boot { public static void main(final String[] args) throws Exception { BeanFactory ctx = new ClassPathXmlApplicationContext("x/y/plain.xml"); FooService foo = (FooService) ctx.getBean("fooService"); foo.getFoo("Pengo", 12); } }
StopWatch 'Profiling for 'Pengo' and '12'': running time (millis) = 0 ----------------------------------------- ms % Task name ----------------------------------------- 00000 ? execution(getFoo)
When multiple advice needs to execute at the same join point (executing
method) the ordering rules are as described in Section 6.2.4.7, “Advice ordering”.
The precedence between aspects is determined by either adding the Order
annotation to the bean backing the aspect or by having the bean implement the
Ordered
interface.
Introductions (known as inter-type declarations in AspectJ) enable an aspect to declare that advised objects implement a given interface, and to provide an implementation of that interface on behalf of those objects.
An introduction is made using the aop:declare-parents
element
inside an aop:aspect
This element is
used to declare that matching types have a new parent (hence the name). For example, given
an interface UsageTracked
, and an implementation of that
interface DefaultUsageTracked
, the following
aspect declares that all implementors of service interfaces also implement the
UsageTracked
interface. (In order to expose statistics
via JMX for example.)
<aop:aspect id="usageTrackerAspect" ref="usageTracking"> <aop:declare-parents types-matching="com.xzy.myapp.service.*+", implement-interface="UsageTracked" default-impl="com.xyz.myapp.service.tracking.DefaultUsageTracked"/> <aop:before pointcut="com.xyz.myapp.SystemArchitecture.businessService() and this(usageTracked)" method="recordUsage"/> </aop:aspect>
The class backing the usageTracking
bean would contain the
method:
public void recordUsage(UsageTracked usageTracked) { usageTracked.incrementUseCount(); }
The interface to be implemented is determined by implement-interface
attribute. The value of the
types-matching
attribute is an AspectJ type pattern :-
any bean of a matching type will implement the UsageTracked
interface. Note that in the before advice of the above example, service
beans can be directly used as implementations of the UsageTracked
interface. If accessing
a bean programmatically you would write the following:
UsageTracked usageTracked = (UsageTracked) context.getBean("myService");
The only supported instantiation model for schema-defined aspects is the singleton model. Other instantiation models may be supported in future releases.
The concept of "advisors" is brought forward from the AOP support defined in Spring 1.2 and does not have a direct equivalent in AspectJ. An advisor is like a small self-contained aspect that has a single piece of advice. The advice itself is represented by a bean, and must implement one of the advice interfaces described in Section 7.3.2, “Advice types in Spring”. Advisors can take advantage of AspectJ pointcut expressions though.
Spring 2.0 supports the advisor concept with the <aop:advisor>
element. You will most commonly see it used in conjunction with transactional advice,
which also has its own namespace support in Spring 2.0. Here's how it looks:
<aop:config> <aop:pointcut id="businessService" expression="execution(* com.xyz.myapp.service.*.*(..))"/> <aop:advisor pointcut-ref="businessService" advice-ref="tx-advice"/> </aop:config> <tx:advice id="tx-advice"> <tx:attributes> <tx:method name="*" propagation="REQUIRED"/> </tx:attributes> </tx:advice>
As well as the pointcut-ref
attribute used in the above
example, you can also use the pointcut
attribute to define a
pointcut expression inline.
To define the precedence of an advisor so that the advice can participate
in ordering, use the order
attribute to define the
Ordered
value of the advisor.
Let's see how the concurrent locking failure retry example from Section 6.2.7, “Example” looks when rewritten using the schema support.
The execution of business services can sometimes fail
due to concurrency issues (for example, deadlock loser).
If the operation is retried, it is quite likely it will succeed next time round. For
business services where it is appropriate to retry in such conditions (idempotent
operations that don't need to go back to the user for conflict resolution), we'd like
to transparently retry the operation to avoid the client seeing a
PessimisticLockingFailureException
. This is a requirement that clearly cuts across
multiple services in the service layer, and hence is ideal for implementing via an aspect.
Because we want to retry the operation, we'll need to use around advice so that we can call proceed multiple times. Here's how the basic aspect implementation looks (it's just a regular Java class using the schema support):
public class ConcurrentOperationExecutor implements Ordered { private static final int DEFAULT_MAX_RETRIES = 2; private int maxRetries = DEFAULT_MAX_RETRIES; private int order = 1; public void setMaxRetries(int maxRetries) { this.maxRetries = maxRetries; } public int getOrder() { return this.order; } public void setOrder(int order) { this.order = order; } public Object doConcurrentOperation(ProceedingJoinPoint pjp) throws Throwable { int numAttempts = 0; PessimisticLockingFailureException lockFailureException; do { numAttempts++; try { return pjp.proceed(); } catch(PessimisticLockingFailureException ex) { lockFailureException = ex; } } while(numAttempts <= this.maxRetries); throw lockFailureException; } }
Note that the aspect implements the Ordered
interface so we can set the precedence
of the aspect higher than the transaction advice (we want a fresh transaction
each time we retry). The maxRetries
and order
properties will both be configured by Spring. The main action happens in the
doConcurrentOperation
around advice method. We
try to proceed, and if we fail with a PessimisticLockingFailureException
we simply try again unless we have exhausted all of our retry attempts.
This class is identical to the one used in the @AspectJ example, but with the annotations removed.
The corresponding Spring configuration is:
<aop:config> <aop:aspect id="concurrentOperationRetry" ref="concurrentOperationExecutor"> <aop:pointcut id="idempotentOperation" expression="execution(* com.xyz.myapp.service.*.*(..))"/> <aop:around pointcut-ref="idempotentOperation" method="doConcurrentOperation"/> </aop:aspect> </aop:config> <bean id="concurrentOperationExecutor" class="com.xyz.myapp.service.impl.ConcurrentOperationExecutor"> <property name="maxRetries" value="3"/> <property name="order" value="100"/> </bean>
Notice that for the time being we assume that all business services
are idempotent. If this is not the case we can refine the aspect so that it only
retries genuinely idempotent operations, by introducing an
Idempotent
annotation:
@Retention(RetentionPolicy.RUNTIME)
public @interface Idempotent {
// marker annotation
}
and using the annotation to annotate the implementation of service operations.
The change to the aspect to only retry idempotent operations simply involves
refining the pointcut expression so that only @Idempotent
operations match:
<aop:pointcut id="idempotentOperation" expression="execution(* com.xyz.myapp.service.*.*(..)) and @annotation(com.xyz.myapp.service.Idempotent)"/>
Once you have decided that an aspect is the best approach for implementing a given requirement, how do you decide between using Spring AOP or AspectJ, and between the Aspect language (code) style, @AspectJ annotation style, and the XML style? These decisions are influenced by a number of factors including application requirements, development tools, and team familiarity with AOP.
Use the simplest thing that can work. Spring AOP is simpler than using full AspectJ as there is no requirement to introduce the AspectJ compiler / weaver into your development and build processes. If you only need to advise the execution of operations on Spring beans, then Spring AOP is the right choice. If you need to advise domain objects, or any other object not managed by the Spring container, then you will need to use AspectJ. You will also need to use AspectJ if you wish to advise join points other than simple method executions (for example, call join points, field get or set join points, and so on).
When using AspectJ, you have the choice of the AspectJ language syntax (also known as the "code style") or the @AspectJ annotation style. If aspects play a large role in your design, and you are able to use the AspectJ Development Tools (AJDT) in Eclipse, then the AspectJ language syntax is the preferred option: it is cleaner and simpler because the language was purposefully designed for writing aspects. If you are not using Eclipse, or have only a few aspects that do not play a major role in your application, then you may want to consider using the @AspectJ style and sticking with a regular Java compilation in your IDE, and adding an aspect weaving (linking) phase to your build scripts.
The XML style will be most familiar to existing Spring users. It can be used with any JDK level (referring to named pointcuts from within pointcut expressions does still require Java 5 though) and is backed by genuine POJOs. When using AOP as a tool to configure enterprise services (a good test is whether you consider the pointcut expression to be a part of your configuration you might want to change independently) then XML can be a good choice. With the XML style it is arguably clearer from your configuration what aspects are present in the system.
The XML style has two disadvantages. Firstly it does not fully encapsulate the implementation of the requirement it addresses in a single place. The DRY principle says that there should be a single, unambiguous, authoritative representation of any piece of knowledge within a system. When using the XML style, the knowledge of how a requirement is implemented is split across the declaration of the backing bean class, and the XML in the configuration file. When using the @AspectJ style there is a single module - the aspect - in which this information is encapsulated. Secondly, the XML style is more limited in what in can express than the @AspectJ style: only the "singleton" aspect instantiation model is supported, and it is not possible to combine named pointcuts declared in XML. For example, in the @AspectJ style we can write something like:
@Pointcut(execution(* get*())) public void propertyAccess() {} @Pointcut(execution(org.xyz.Account+ *(..)) public void operationReturningAnAccount() {} @Pointcut(propertyAccess() && operationReturningAnAccount()) public void accountPropertyAccess() {}
In the XML style I certainly can declare the first two pointcuts:
<aop:pointcut id="propertyAccess" expression="execution(* get*())"/> <aop:pointcut id="operationReturningAnAccount" expression="execution(org.xyz.Account+ *(..))"/>
The downside of the XML approach becomes evident in this case because I
cannot define the 'accountPropertyAccess
' pointcut by
combining these definitions.
The @AspectJ style supports additional instantiation models, and richer pointcut composition. It has the advantage of keeping the aspect as a modular unit. It also has the advantage the @AspectJ aspects can be understood both by Spring AOP and by AspectJ - so if you later decide you need the capabilities of AspectJ to implement additional requirements then it is very easy to migrate to an AspectJ based approach.
So much for the pros and cons of each style then: which is best? If you are not using Java5 (or above) then clearly the XML-style is the best because it is the only option available to you. If you are using Java5+, then you really will have to come to your own decision as to which style suits you best. In the experience of the Spring team, we advocate the use of the @AspectJ style whenever there are aspects that do more than simple "configuration" of enterprise services. If you are writing, have written, or have access to an aspect that is not part of the business contract of a particular class (such as a tracing aspect), then the XML-style is better.
It is perfectly possible to mix @AspectJ style aspects using the autoproxying
support, schema-defined <aop:aspect>
aspects,
<aop:advisor>
declared advisors and even proxies and
interceptors defined using the Spring 1.2 style in the same configuration. All of
these are implemented using the same underlying support mechanism and will co-exist
without any difficulty.
Spring AOP uses either JDK dynamic proxies or CGLIB to create the proxy for a given target object. (JDK dynamic proxies are preferred whenever you have a choice).
If the target object to be proxied implements at least one interface then a JDK dynamic proxy will be used. All of the interfaces implemented by the target type will be proxied. If the target object does not implement any interfaces then a CGLIB proxy will be created.
If you want to force the use of CGLIB proxying (for example, to proxy every method defined for the target object, not just those implemented by its interfaces) you can do so. However, there are some issues to consider:
final
methods cannot be advised, as they cannot
be overriden.
You will need the CGLIB 2 binaries on your classpath, whereas dynamic proxies are available with the JDK. Spring will automatically warn you when it needs CGLIB but it isn't available on the classpath.
The constructor of your proxied object will be called twice. This is a natural consequence of the CGLIB proxy model whereby a subclass is generated for each proxied object. For each proxied instance, two objects are created: the actual proxied object and an instance of the subclass that implements the advice. This behavior does not show when using JDK proxies. Usually, calling the constructor of the proxied type twice, is not a huge problem, as there are usually only assignments taking place and no real logic is (and probably should be) implemented in the constructor.
To force the use of CGLIB proxies set the value of the proxy-target-class
attribute of the <aop:config>
element to true:
<aop:config proxy-target-class="true"> <!-- other beans defined here... --> </aop:config>
To force CGLIB proxying when using the @AspectJ autoproxy support, set the
'proxy-target-class'
attribute of the
<aop:aspectj-autoproxy>
element to true
:
<aop:aspectj-autoproxy proxy-target-class="true"/>
Spring AOP is proxy-based. It is vitally important that you grasp the semantics of what that last statement actually means before you write your own aspects or use any of the Spring AOP-based aspects supplied with the Spring Framework.
Consider first the scenario where you have a plain-vanilla, un-proxied, nothing-special-about-it, straight object reference, as illustrated by the following code snippet.
public class SimplePojo implements Pojo { public void foo() { // this is a direct method call on the 'this' reference this.bar(); } public void bar() { // some logic... } }
If you invoke a method on an object reference, the method is invoked directly on that object reference, as can be seen below.
public class Main {
public static void main(String[] args) {
Pojo pojo = new SimplePojo();
// this is a direct method call on the 'pojo' reference
pojo.foo();
}
}
Things change slightly when the reference that client code has is a proxy. Consider the following diagram and code snippet.
public class Main {
public static void main(String[] args) {
ProxyFactory factory = new ProxyFactory(new SimplePojo());
factory.addInterface(Pojo.class);
factory.addAdvice(new RetryAdvice());
Pojo pojo = (Pojo) factory.getProxy();
// this is a method call on the proxy!
pojo.foo();
}
}
The key thing to understand here is that the client code inside the
main(..)
of the Main
class
has a reference to the proxy. This means that method calls on that object
reference will be calls on the proxy, and as such the proxy will be able to delegate to
all of the interceptors (advice) that are relevant to that particular method call.
However, once the call has finally reached the target object, the SimplePojo
reference in this case, any method calls that it may make on itself, such as
this.bar()
or this.foo()
, are going to be
invoked against the this
reference, and not
the proxy. This has important implications. It means that self-invocation is not
going to result in the advice associated with a method invocation getting a chance to execute.
Okay, so what is to be done about this? The best approach (the term best is used loosely here) is to refactor your code such that the self-invocation does not happen. For sure, this does entail some work on your part, but it is the best, least-invasive approach. The next approach is absolutely horrendous, and I am almost reticent to point it out precisely because it is so horrendous. You can (choke!) totally tie the logic within your class to Spring AOP by doing this:
public class SimplePojo implements Pojo { public void foo() { // this works, but... gah! ((Pojo) AopContext.currentProxy()).bar(); } public void bar() { // some logic... } }
This totally couples your code to Spring AOP, and it makes the class itself aware of the fact that it is being used in an AOP context, which flies in the face of AOP. It also requires some additional configuration when the proxy is being created:
public class Main { public static void main(String[] args) { ProxyFactory factory = new ProxyFactory(new SimplePojo()); factory.adddInterface(Pojo.class); factory.addAdvice(new RetryAdvice()); factory.setExposeProxy(true); Pojo pojo = (Pojo) factory.getProxy(); // this is a method call on the proxy! pojo.foo(); } }
Finally, it must be noted that AspectJ does not have this self-invocation issue because it is not a proxy-based AOP framework.
In addition to declaring aspects in your configuration using either
<aop:config>
or <aop:aspectj-autoproxy>
,
it is also possible programmatically to create proxies that advise target objects. For the
full details of Spring's AOP API, see the next chapter. Here we want to focus on the ability
to automatically create proxies using @AspectJ aspects.
The class org.springframework.aop.aspectj.annotation.AspectJProxyFactory
can be used to create a proxy for a target object that is advised by one or more
@AspectJ aspects. Basic usage for this class is very simple, as illustrated below. See
the Javadocs for full information.
// create a factory that can generate a proxy for the given target object AspectJProxyFactory factory = new AspectJProxyFactory(targetObject); // add an aspect, the class must be an @AspectJ aspect // you can call this as many times as you need with different aspects factory.addAspect(SecurityManager.class); // you can also add existing aspect instances, the type of the object supplied must be an @AspectJ aspect factory.addAspect(usageTracker); // now get the proxy object... MyInterfaceType proxy = factory.getProxy();
Everything we've covered so far in this chapter is pure Spring AOP. In this section, we're going to look at how you can use the AspectJ compiler/weaver instead of or in addition to Spring AOP if your needs go beyond the facilities offered by Spring AOP alone.
Spring ships with a small AspectJ aspect library (it's available standalone in
your distribution as spring-aspects.jar
, you'll need to add
this to your classpath to use the aspects in it). Section 6.8.1, “Using AspectJ to dependency inject domain objects with Spring”
and Section 6.8.2, “Other Spring aspects for AspectJ” discuss the content of this library and how you
can use it. Section 6.8.3, “Configuring AspectJ aspects using Spring IoC” discusses how to dependency inject AspectJ
aspects that are woven using the AspectJ compiler. Finally,
Section 6.8.4, “Using AspectJ Load-time weaving (LTW) with Spring applications” provides an introduction to load-time weaving for Spring applications
using AspectJ.
The Spring container instantiates and configures beans defined in your
application context. It is also possible to ask a bean factory to configure
a pre-existing object given the name of a bean definition
containing the configuration to be applied. The spring-aspects.jar
contains an annotation-driven aspect that exploits this capability to allow dependency-injection
of any object. The support is intended to be used for objects created
outside of the control of any container. Domain objects
often fall into this category: they may be created programmatically using the
new
operator, or by an ORM tool as a result of a database query.
The @Configurable
annotation marks a class as eligible for
Spring-driven configuration. In the simplest case it can be used just as a
marker annotation:
package com.xyz.myapp.domain; import org.springframework.beans.factory.annotation.Configurable; @Configurable public class Account { ... }
When used as a marker interface in this way, Spring will configure
new instances of the annotated type (Account
in this case) using a
prototypical bean definition with the same name as the fully-qualified
type name (com.xyz.myapp.domain.Account
). Since the
default name for a bean is the fully-qualified name of its type, a
convenient way to declare the prototype definition is simply to omit the
id attribute:
<bean class="com.xyz.myapp.domain.Account" scope="prototype"> <property name="fundsTransferService" ref="fundsTransferService"/> ... </bean>
If you want to explicitly specify the name of the prototype bean definition to use, you can do so directly in the annotation:
package com.xyz.myapp.domain; import org.springframework.beans.factory.annotation.Configurable; @Configurable("account") public class Account { ... }
Spring will now look for a bean definition named "account" and
use that as a prototypical definition to configure new Account
instances.
You can also use autowiring to avoid having to specify a prototypical
bean definition at all. To have Spring apply autowiring use the
autowire property of the @Configurable
annotation: specify either
@Configurable(autowire=Autowire.BY_TYPE)
or
@Configurable(autowire=Autowire.BY_NAME
for autowiring
by type or by name respectively.
Finally you can enable Spring dependency checking for the object references
in the newly created and configured object by using the dependencyCheck
attribute (for example:
@Configurable(autowire=Autowire.BY_NAME,dependencyCheck=true)
).
If this attribute is set to true, then Spring will validate after configuration that all properties
(that are not primitives or collections) have been set.
Using the annotation on its own does nothing of course. It's the
AnnotationBeanConfigurerAspect
in spring-aspects.jar
that
acts on the presence of the annotation. In essence the aspect says "after
returning from the initialization of a new object of a type with the
@Configurable
annotation, configure the newly created object using Spring
in accordance with the properties of the annotation". For this to work the
annotated types must be woven with the AspectJ weaver - you can either
use a build-time ant or maven task to do this (see for example the
AspectJ
Development Environment Guide) or load-time weaving (see Section 6.8.4, “Using AspectJ Load-time weaving (LTW) with Spring applications”).
The AnnotationBeanConfigurerAspect
itself needs
configuring by Spring (in order to obtain a reference to the bean factory that
is to be used to configure new objects). The Spring AOP namespace defines a
convenient tag for doing this. Simply include the following in your
application context configuration:
<aop:spring-configured/>
If you are using the DTD instead of schema, the equivalent definition is:
<bean class="org.springframework.beans.factory.aspectj.AnnotationBeanConfigurerAspect" factory-method="aspectOf"/>
Instances of @Configurable objects created before the aspect has been configured will result in a warning being issued to the log and no configuration of the object taking place. An example might be a bean in the Spring configuration that creates domain objects when it is initialized by Spring. In this case you can use the "depends-on" bean attribute to manually specify that the bean depends on the configuration aspect.
<bean id="myService" class="com.xzy.myapp.service.MyService" depends-on="org.springframework.beans.factory.aspectj.AnnotationBeanConfigurerAspect"> ... </bean>
One of the goals of the @Configurable
support is to enable independent
unit testing of domain objects without the difficulties associated with
hard-coded lookups. If @Configurable
types have not been woven by AspectJ
then the annotation has no affect during unit testing, and you can simply
set mock or stub property references in the object under test and proceed
as normal. If @Configurable
types have been woven by
AspectJ then you can still unit test outside of the container as normal,
but you will see a warning message each time that you construct an
@Configurable
object indicating that it has not been configured by Spring.
The AnnotationBeanConfigurerAspect
used to implement
the @Configurable
support is an AspectJ singleton aspect. The scope of a
singleton aspect is the same as the scope of static
members, that is to say there is
one aspect instance per classloader that defines the type. This means that if
you define multiple application contexts within the same classloader hierarchy
you need to consider where to define the <aop:spring-configured/>
bean and where to place
spring-aspects.jar
on the classpath.
Consider a typical Spring web-app configuration with a shared parent application
context defining common business services and everything needed to support
them, and one child application context per servlet containing definitions
particular to that servlet. All of these contexts will co-exist within the
same classloader hierarchy, and so the AnnotationBeanConfigurerAspect
can only hold a reference to one of them. In this case we recommend
defining the <aop:spring-configured/>
bean in the
shared (parent) application context: this defines the services that you are
likely to want to inject into domain objects. A consequence is that you cannot
configure domain objects with references to beans defined in the child
(servlet-specific) contexts using the @Configurable mechanism
(probably not something you want to do anyway!).
When deploying multiple web-apps within the same container, ensure that each
web-application loads the types in spring-aspects.jar
using its own classloader
(for example, by placing spring-aspects.jar
in 'WEB-INF/lib'
). If spring-aspects.jar
is only added to the container wide classpath (and hence loaded by the shared
parent classloader), all web applications will share the same aspect instance
which is probably not what you want.
In addition to the @Configurable
support,
spring-aspects.jar
contains an
AspectJ aspect that can be used to drive Spring's transaction management for
types and methods annotated with the @Transactional
annotation.
This is primarily intended for users who want to use Spring's transaction support outside of the Spring container.
The aspect that interprets @Transactional
annotations is the
AnnotationTransactionAspect
. When using this
aspect, you must annotate the implementation class
(and/or methods within that class), not the interface
(if any) that the class implements. AspectJ follows Java's rule that annotations on
interfaces are not inherited.
A @Transactional
annotation on a class specifies the default transaction
semantics for the execution of any public operation in the class.
A @Transactional
annotation on a method within the class overrides the
default transaction semantics given by the class annotation (if present).
Methods with public, protected, and default visibility may all be annotated.
Annotating protected and default visibility methods directly is the only way
to get transaction demarcation for the execution of such operations.
For AspectJ programmers that want to use the Spring configuration and
transaction management support but don't want to (or can't) use annotations,
spring-aspects.jar
also contains abstract aspects you can extend to provide
your own pointcut definitions. See the Javadocs for
AbstractBeanConfigurerAspect
and
AbstractTransactionAspect
for more information. As an example,
the following excerpt shows how you could write an aspect to configure
all instances of objects defined in the domain model
using prototypical bean definitions that match the fully-qualified class
names:
public aspect DomainObjectConfiguration extends AbstractBeanConfigurerAspect {
public DomainObjectConfiguration() {
setBeanWiringInfoResolver(new ClassNameBeanWiringInfoResolver());
}
// the creation of a new bean (any object in the domain model)
protected pointcut beanCreation(Object beanInstance) :
initialization(new(..)) &&
SystemArchitecture.inDomainModel() &&
this(beanInstance);
}
When using AspectJ aspects with Spring applications, it's natural to want to configure such aspects using Spring. The AspectJ runtime itself is responsible for aspect creation, and the means of configuring the AspectJ created aspects via Spring depends on the AspectJ instantiation model (per-clause) used by the aspect.
The majority of AspectJ aspects are
singleton aspects. Configuration of these aspects is very
easy, simply create a bean definition referencing the aspect type as normal, and
include the bean attribute 'factory-method="aspectOf"'
. This
ensures that Spring obtains the aspect instance by asking AspectJ for it rather
than trying to create an instance itself. For example:
<bean id="profiler" class="com.xyz.profiler.Profiler" factory-method="aspectOf"> <property name="profilingStrategy" ref="jamonProfilingStrategy"/> </bean>
For non-singleton aspects, the easiest way to configure them is to create
prototypical bean definitions and annotate use the @Configurable support from
spring-aspects.jar
to configure the aspect instances once they have bean created
by the AspectJ runtime.
If you have some @AspectJ aspects that you want to weave with AspectJ
(for example, using load-time weaving for domain model types) and other @AspectJ
aspects that you want to use with Spring AOP, and these aspects are all configured
using Spring, then you'll need to tell the Spring AOP @AspectJ autoproxying support
which subset of the @AspectJ aspects defined in the configuration should be used
for autoproxying. You can do this by using one or more <include/>
elements inside the <aop:aspectj-autoproxy/>
declaration.
Each include element specifies a name pattern, and only beans with names matched
by at least one of the patterns will be used for Spring AOP autoproxy configuration:
<aop:aspectj-autoproxy> <include name="thisBean"/> <include name="thatBean"/> </aop:aspectj-autoproxy>
Load-time weaving (or LTW) refers to the process of weaving AspectJ aspects with an application's class files as they are loaded into the VM. For full details on configuring load-time weaving with AspectJ, see the LTW section of the AspectJ Development Environment Guide . We will focus here on the essentials of configuring load-time weaving for Spring applications running on Java 5.
Load-time weaving is controlled by defining a file 'aop.xml
' in
the META-INF directory. AspectJ automatically looks for all 'META-INF/aop.xml' files
visible on the classpath and configures itself based on the aggregation of their
content.
A basic META-INF/aop.xml for your application should look like this:
<!DOCTYPE aspectj PUBLIC "-//AspectJ//DTD//EN" "http://www.eclipse.org/aspectj/dtd/aspectj.dtd"> <aspectj> <weaver> <include within="com.xyz.myapp..*"/> </weaver> </aspectj>
The <include/>
element tells AspectJ which set of types
should be included in the weaving process. Use the package prefix for your
application followed by "..*" (meaning '... and any type defined in a subpackage
of this') as a good default. Using the include element is important as otherwise
AspectJ will look at every type loaded in support of your application (including
all the Spring library classes and many more besides). Normally you don't want
to weave these types and don't want to pay the overhead of AspectJ attempting
to match against them.
To get informational messages in your log file regarding the activity of the load-time weaver, add the following options to the weaver element:
<!DOCTYPE aspectj PUBLIC "-//AspectJ//DTD//EN" "http://www.eclipse.org/aspectj/dtd/aspectj.dtd"> <aspectj> <weaver options="-showWeaveInfo -XmessageHandlerClass:org.springframework.aop.aspectj.AspectJWeaverMessageHandler"> <include within="com.xyz.myapp..*"/> </weaver> </aspectj>
Finally, to control exactly which aspects are used, you can use the
aspects
element. By default all defined aspects are used for
weaving (spring-aspects.jar
contains a META-INF/aop.xml file that defines the
configuration and transaction aspects). If you were using spring-aspects.jar
, but
only want the configuration support and not the transaction support you could
specify this as follows:
<!DOCTYPE aspectj PUBLIC "-//AspectJ//DTD//EN" "http://www.eclipse.org/aspectj/dtd/aspectj.dtd"> <aspectj> <weaver options="-showWeaveInfo -XmessageHandlerClass:org.springframework.aop.aspectj.AspectJWeaverMessageHandler"> <include within="com.xyz.myapp..*"/> </weaver> <aspects> <include within="org.springframework.beans.factory.aspectj.AnnotationBeanConfigurerAspect"/> </aspects> </aspectj>
On the Java 5 platform, load-time weaving is enabled by specifying the following VM argument when launching the Java virtual machine:
-javaagent:<path-to-ajlibs>/aspectjweaver.jar
More information on AspectJ can be found at the AspectJ home page.
The book Eclipse AspectJ by Adrian Colyer et. al. (Addison-Wesley, 2005) provides a comprehensive introduction and reference for the AspectJ language.
The excellent AspectJ in Action by Ramnivas Laddad (Manning, 2003) comes highly recommended as an introduction to AOP; the focus of the book is on AspectJ, but a lot of general AOP themes are explored in some depth.
The previous chapter described the Spring 2.0 support for AOP using @AspectJ and schema-based aspect definitions. In this chapter we discuss the lower-level Spring AOP APIs and the AOP support used in Spring 1.2 applications. For new applications, we recommend the use of the Spring 2.0 AOP support described in the previous chapter, but when working with existing applications, or when reading books and articles, you may come across Spring 1.2 style examples. Spring 2.0 is fully backwards compatible with Spring 1.2 and everything described in this chapter is fully supported in Spring 2.0.
Let's look at how Spring handles the crucial pointcut concept.
Spring's pointcut model enables pointcut reuse independent of advice types. It's possible to target different advice using the same pointcut.
The org.springframework.aop.Pointcut
interface
is the central interface, used to target advices to particular classes
and methods. The complete interface is shown below:
public interface Pointcut { ClassFilter getClassFilter(); MethodMatcher getMethodMatcher(); }
Splitting the Pointcut
interface into two parts
allows reuse of class and method matching parts, and fine-grained
composition operations (such as performing a "union" with another method
matcher).
The ClassFilter
interface is used to restrict
the pointcut to a given set of target classes. If the
matches()
method always returns true, all target
classes will be matched:
public interface ClassFilter { boolean matches(Class clazz); }
The MethodMatcher
interface is normally more
important. The complete interface is shown below:
public interface MethodMatcher { boolean matches(Method m, Class targetClass); boolean isRuntime(); boolean matches(Method m, Class targetClass, Object[] args); }
The matches(Method, Class)
method is used to
test whether this pointcut will ever match a given method on a target
class. This evaluation can be performed when an AOP proxy is created, to
avoid the need for a test on every method invocation. If the 2-argument
matches method returns true for a given method, and the
isRuntime()
method for the MethodMatcher returns
true, the 3-argument matches method will be invoked on every method
invocation. This enables a pointcut to look at the arguments passed to
the method invocation immediately before the target advice is to
execute.
Most MethodMatchers are static, meaning that their
isRuntime()
method returns false. In this case, the
3-argument matches method will never be invoked.
![]() | Tip |
---|---|
If possible, try to make pointcuts static, allowing the AOP framework to cache the results of pointcut evaluation when an AOP proxy is created. |
Spring supports operations on pointcuts: notably, union and intersection.
Union means the methods that either pointcut matches.
Intersection means the methods that both pointcuts match.
Union is usually more useful.
Pointcuts can be composed using the static methods in the org.springframework.aop.support.Pointcuts class, or using the ComposablePointcut class in the same package. However, using AspectJ pointcut expressions is usually a simpler approach.
Since 2.0, the most important type of pointcut used by Spring is
org.springframework.aop.aspectj.AspectJExpressionPointcut
.
This is a pointcut that uses an AspectJ supplied library to parse an AspectJ
pointcut expression string.
See the previous chapter for a discussion of supported AspectJ pointcut primitives.
Spring provides several convenient pointcut implementations. Some can be used out of the box; others are intended to be subclassed in application-specific pointcuts.
Static pointcuts are based on method and target class, and cannot take into account the method's arguments. Static pointcuts are sufficient - and best - for most usages. It's possible for Spring to evaluate a static pointcut only once, when a method is first invoked: after that, there is no need to evaluate the pointcut again with each method invocation.
Let's consider some static pointcut implementations included with Spring.
One obvious way to specify static pointcuts is regular
expressions. Several AOP frameworks besides Spring make this
possible.
org.springframework.aop.support.Perl5RegexpMethodPointcut
is a generic regular expression pointcut, using Perl 5 regular
expression syntax. The Perl5RegexpMethodPointcut
class depends on Jakarta ORO for regular expression matching. Spring
also provides the JdkRegexpMethodPointcut
class
that uses the regular expression support in JDK 1.4+.
Using the Perl5RegexpMethodPointcut
class,
you can provide a list of pattern Strings. If any of these is a
match, the pointcut will evaluate to true. (So the result is
effectively the union of these pointcuts.)
The usage is shown below:
<bean id="settersAndAbsquatulatePointcut" class="org.springframework.aop.support.Perl5RegexpMethodPointcut"> <property name="patterns"> <list> <value>.*set.*</value> <value>.*absquatulate</value> </list> </property> </bean>
Spring provides a convenience class,
RegexpMethodPointcutAdvisor
, that allows us to
also reference an Advice (remember that an Advice can be an
interceptor, before advice, throws advice etc.). Behind the scenes,
Spring will use the JdkRegexpMethodPointcut
on
J2SE 1.4 or above, and will fall back to
Perl5RegexpMethodPointcut
on older VMs. The use
of Perl5RegexpMethodPointcut
can be forced by
setting the perl5
property to true. Using
RegexpMethodPointcutAdvisor
simplifies wiring, as
the one bean encapsulates both pointcut and advice, as shown
below:
<bean id="settersAndAbsquatulateAdvisor" class="org.springframework.aop.support.RegexpMethodPointcutAdvisor"> <property name="advice"> <ref local="beanNameOfAopAllianceInterceptor"/> </property> <property name="patterns"> <list> <value>.*set.*</value> <value>.*absquatulate</value> </list> </property> </bean>
RegexpMethodPointcutAdvisor can be used with any Advice type.
Dynamic pointcuts are costlier to evaluate than static pointcuts. They take into account method arguments, as well as static information. This means that they must be evaluated with every method invocation; the result cannot be cached, as arguments will vary.
The main example is the control flow
pointcut.
Spring control flow pointcuts are conceptually similar to
AspectJ cflow pointcuts, although less
powerful. (There is currently no way to specify that a pointcut
executes below a join point matched by another pointcut.)
A control flow pointcut matches
the current call stack. For example, it might fire if the join point
was invoked by a method in the com.mycompany.web
package, or by the SomeCaller
class. Control flow
pointcuts are specified using the
org.springframework.aop.support.ControlFlowPointcut
class.
![]() | Note |
---|---|
Control flow pointcuts are significantly more expensive to evaluate at runtime than even other dynamic pointcuts. In Java 1.4, the cost is about 5 times that of other dynamic pointcuts; in Java 1.3 more than 10. |
Spring provides useful pointcut superclasses to help you to implement your own pointcuts.
Because static pointcuts are most useful, you'll probably subclass StaticMethodMatcherPointcut, as shown below. This requires implementing just one abstract method (although it's possible to override other methods to customize behavior):
class TestStaticPointcut extends StaticMethodMatcherPointcut { public boolean matches(Method m, Class targetClass) { // return true if custom criteria match } }
There are also superclasses for dynamic pointcuts.
You can use custom pointcuts with any advice type in Spring 1.0 RC2 and above.
Because pointcuts in Spring AOP are Java classes, rather than language features (as in AspectJ) it's possible to declare custom pointcuts, whether static or dynamic. Custom pointcuts in Spring can be arbitrarily complex. However, using the AspectJ pointcut expression language is recommended if possible.
![]() | Note |
---|---|
Later versions of Spring may offer support for "semantic pointcuts" as offered by JAC: for example, "all methods that change instance variables in the target object." |
Let's now look at how Spring AOP handles advice.
Each advice is a Spring bean. An advice instance can be shared across all advised objects, or unique to each advised object. This corresponds to per-class or per-instance advice.
Per-class advice is used most often. It is appropriate for generic advice such as transaction advisors. These do not depend on the state of the proxied object or add new state; they merely act on the method and arguments.
Per-instance advice is appropriate for introductions, to support mixins. In this case, the advice adds state to the proxied object.
It's possible to use a mix of shared and per-instance advice in the same AOP proxy.
Spring provides several advice types out of the box, and is extensible to support arbitrary advice types. Let us look at the basic concepts and standard advice types.
The most fundamental advice type in Spring is interception around advice.
Spring is compliant with the AOP Alliance interface for around advice using method interception. MethodInterceptors implementing around advice should implement the following interface:
public interface MethodInterceptor extends Interceptor { Object invoke(MethodInvocation invocation) throws Throwable; }
The MethodInvocation
argument to the
invoke()
method exposes the method being invoked;
the target join point; the AOP proxy; and the arguments to the method.
The invoke()
method should return the
invocation's result: the return value of the join point.
A simple MethodInterceptor
implementation
looks as follows:
public class DebugInterceptor implements MethodInterceptor { public Object invoke(MethodInvocation invocation) throws Throwable { System.out.println("Before: invocation=[" + invocation + "]"); Object rval = invocation.proceed(); System.out.println("Invocation returned"); return rval; } }
Note the call to the MethodInvocation's
proceed()
method. This proceeds down the
interceptor chain towards the join point. Most interceptors will invoke
this method, and return its return value. However, a
MethodInterceptor, like any around advice, can return a different
value or throw an exception rather than invoke the proceed method.
However, you don't want to do this without good reason!
![]() | Note |
---|---|
MethodInterceptors offer interoperability with other AOP Alliance-compliant AOP implementations. The other advice types discussed in the remainder of this section implement common AOP concepts, but in a Spring-specific way. While there is an advantage in using the most specific advice type, stick with MethodInterceptor around advice if you are likely to want to run the aspect in another AOP framework. Note that pointcuts are not currently interoperable between frameworks, and the AOP Alliance does not currently define pointcut interfaces. |
A simpler advice type is a before
advice. This does not need a
MethodInvocation
object, since it will only be
called before entering the method.
The main advantage of a before advice is that there is no need
to invoke the proceed()
method, and therefore no
possibility of inadvertently failing to proceed down the interceptor
chain.
The MethodBeforeAdvice
interface is shown
below. (Spring's API design would allow for field before advice,
although the usual objects apply to field interception and it's
unlikely that Spring will ever implement it).
public interface MethodBeforeAdvice extends BeforeAdvice { void before(Method m, Object[] args, Object target) throws Throwable; }
Note the return type is void
. Before
advice can insert custom behavior before the join point executes, but
cannot change the return value. If a before advice throws an
exception, this will abort further execution of the interceptor chain.
The exception will propagate back up the interceptor chain. If it is
unchecked, or on the signature of the invoked method, it will be
passed directly to the client; otherwise it will be wrapped in an
unchecked exception by the AOP proxy.
An example of a before advice in Spring, which counts all method invocations:
public class CountingBeforeAdvice implements MethodBeforeAdvice { private int count; public void before(Method m, Object[] args, Object target) throws Throwable { ++count; } public int getCount() { return count; } }
![]() | Tip |
---|---|
Before advice can be used with any pointcut. |
Throws advice is invoked after
the return of the join point if the join point threw an exception.
Spring offers typed throws advice. Note that this means that the
org.springframework.aop.ThrowsAdvice
interface does
not contain any methods: it is a tag interface identifying that the
given object implements one or more typed throws advice methods. These
should be in the form of:
afterThrowing([Method, args, target], subclassOfThrowable)
Only the last argument is required. The method signatures may have either one or four arguments, depending on whether the advice method is interested in the method and arguments. The following classes are examples of throws advice.
The advice below is invoked if a RemoteException
is thrown (including subclasses):
public class RemoteThrowsAdvice implements ThrowsAdvice {
public void afterThrowing(RemoteException ex) throws Throwable {
// Do something with remote exception
}
}
The following advice is invoked if a
ServletException
is thrown. Unlike the above
advice, it declares 4 arguments, so that it has access to the invoked
method, method arguments and target object:
public class ServletThrowsAdviceWithArguments implements ThrowsAdvice {
public void afterThrowing(Method m, Object[] args, Object target, ServletException ex) {
// Do something with all arguments
}
}
The final example illustrates how these two methods could be
used in a single class, which handles both
RemoteException
and
ServletException
. Any number of throws advice
methods can be combined in a single class.
public static class CombinedThrowsAdvice implements ThrowsAdvice { public void afterThrowing(RemoteException ex) throws Throwable { // Do something with remote exception } public void afterThrowing(Method m, Object[] args, Object target, ServletException ex) { // Do something with all arguments } }
![]() | Tip |
---|---|
Throws advice can be used with any pointcut. |
An after returning advice in Spring must implement the org.springframework.aop.AfterReturningAdvice interface, shown below:
public interface AfterReturningAdvice extends Advice { void afterReturning(Object returnValue, Method m, Object[] args, Object target) throws Throwable; }
An after returning advice has access to the return value (which it cannot modify), invoked method, methods arguments and target.
The following after returning advice counts all successful method invocations that have not thrown exceptions:
public class CountingAfterReturningAdvice implements AfterReturningAdvice { private int count; public void afterReturning(Object returnValue, Method m, Object[] args, Object target) throws Throwable { ++count; } public int getCount() { return count; } }
This advice doesn't change the execution path. If it throws an exception, this will be thrown up the interceptor chain instead of the return value.
![]() | Tip |
---|---|
After returning advice can be used with any pointcut. |
Spring treats introduction advice as a special kind of interception advice.
Introduction requires an IntroductionAdvisor
,
and an IntroductionInterceptor
, implementing the
following interface:
public interface IntroductionInterceptor extends MethodInterceptor { boolean implementsInterface(Class intf); }
The invoke()
method inherited from the AOP
Alliance MethodInterceptor
interface must implement
the introduction: that is, if the invoked method is on an introduced
interface, the introduction interceptor is responsible for handling
the method call - it cannot invoke proceed()
.
Introduction advice cannot be used with any pointcut, as it
applies only at class, rather than method, level. You can only use
introduction advice with the IntroductionAdvisor
,
which has the following methods:
public interface IntroductionAdvisor extends Advisor, IntroductionInfo { ClassFilter getClassFilter(); void validateInterfaces() throws IllegalArgumentException; } public interface IntroductionInfo { Class[] getInterfaces(); }
There is no MethodMatcher
, and hence no
Pointcut
, associated with introduction advice. Only
class filtering is logical.
The getInterfaces()
method returns the
interfaces introduced by this advisor.
validateInterfaces()
method is used internally to see whether or not the introduced interfaces can be implemented by the configured
IntroductionInterceptor
.
Let's look at a simple example from the Spring test suite. Let's suppose we want to introduce the following interface to one or more objects:
public interface Lockable { void lock(); void unlock(); boolean locked(); }
This illustrates a mixin. We
want to be able to cast advised objects to Lockable, whatever their
type, and call lock and unlock methods. If we call the lock() method,
we want all setter methods to throw a
LockedException
. Thus we can add an aspect that
provides the ability to make objects immutable, without them having
any knowledge of it: a good example of AOP.
Firstly, we'll need an
IntroductionInterceptor
that does the heavy
lifting. In this case, we extend the
org.springframework.aop.support.DelegatingIntroductionInterceptor
convenience class. We could implement IntroductionInterceptor
directly, but using
DelegatingIntroductionInterceptor
is best for most
cases.
The DelegatingIntroductionInterceptor
is
designed to delegate an introduction to an actual implementation of
the introduced interface(s), concealing the use of interception to do
so. The delegate can be set to any object using a constructor
argument; the default delegate (when the no-arg constructor is used)
is this. Thus in the example below, the delegate is the
LockMixin
subclass of
DelegatingIntroductionInterceptor
. Given a delegate
(by default itself), a
DelegatingIntroductionInterceptor
instance looks
for all interfaces implemented by the delegate (other than
IntroductionInterceptor), and will support introductions against any
of them. It's possible for subclasses such as
LockMixin
to call the
suppressInterface(Class intf)
method to suppress
interfaces that should not be exposed. However, no matter how many
interfaces an IntroductionInterceptor
is prepared
to support, the IntroductionAdvisor
used will
control which interfaces are actually exposed. An introduced interface
will conceal any implementation of the same interface by the
target.
Thus LockMixin subclasses
DelegatingIntroductionInterceptor
and implements
Lockable itself. The superclass automatically picks up that Lockable
can be supported for introduction, so we don't need to specify that.
We could introduce any number of interfaces in this way.
Note the use of the locked
instance variable.
This effectively adds additional state to that held in the target
object.
public class LockMixin extends DelegatingIntroductionInterceptor implements Lockable { private boolean locked; public void lock() { this.locked = true; } public void unlock() { this.locked = false; } public boolean locked() { return this.locked; } public Object invoke(MethodInvocation invocation) throws Throwable { if (locked() && invocation.getMethod().getName().indexOf("set") == 0) throw new LockedException(); return super.invoke(invocation); } }
Often it isn't necessary to override the invoke()
method: the
DelegatingIntroductionInterceptor
implementation - which calls the delegate method if the method is
introduced, otherwise proceeds towards the join point - is usually
sufficient. In the present case, we need to add a check: no setter
method can be invoked if in locked mode.
The introduction advisor required is simple. All it needs to do
is hold a distinct LockMixin
instance, and specify
the introduced interfaces - in this case, just
Lockable
. A more complex example might take a
reference to the introduction interceptor (which would be defined as a
prototype): in this case, there's no configuration relevant for a
LockMixin
, so we simply create it using
new
.
public class LockMixinAdvisor extends DefaultIntroductionAdvisor { public LockMixinAdvisor() { super(new LockMixin(), Lockable.class); } }
We can apply this advisor very simply: it requires no
configuration. (However, it is necessary: It's
impossible to use an IntroductionInterceptor
without an IntroductionAdvisor.) As usual with
introductions, the advisor must be per-instance, as it is stateful. We
need a different instance of LockMixinAdvisor
, and
hence LockMixin
, for each advised object. The
advisor comprises part of the advised object's state.
We can apply this advisor programmatically, using the
Advised.addAdvisor()
method, or (the recommended
way) in XML configuration, like any other advisor. All proxy creation
choices discussed below, including "auto proxy creators," correctly
handle introductions and stateful mixins.
In Spring, an Advisor is an aspect that contains just a single advice object associated with a pointcut expression.
Apart from the special case of introductions, any advisor can be
used with any advice.
org.springframework.aop.support.DefaultPointcutAdvisor
is the most commonly used advisor class. For example, it can be used with
a MethodInterceptor
, BeforeAdvice
or
ThrowsAdvice
.
It is possible to mix advisor and advice types in Spring in the same AOP proxy. For example, you could use a interception around advice, throws advice and before advice in one proxy configuration: Spring will automatically create the necessary interceptor chain.
If you're using the Spring IoC container (an ApplicationContext or BeanFactory) for your business objects - and you should be! - you will want to use one of Spring's AOP FactoryBeans. (Remember that a factory bean introduces a layer of indirection, enabling it to create objects of a different type.)
![]() | Note |
---|---|
The Spring 2.0 AOP support also uses factory beans under the covers. |
The basic way to create an AOP proxy in Spring is to use the org.springframework.aop.framework.ProxyFactoryBean. This gives complete control over the pointcuts and advice that will apply, and their ordering. However, there are simpler options that are preferable if you don't need such control.
The ProxyFactoryBean
, like other Spring
FactoryBean
implementations, introduces a level of
indirection. If you define a ProxyFactoryBean
with
name foo
, what objects referencing
foo
see is not the
ProxyFactoryBean
instance itself, but an object
created by the ProxyFactoryBean
's implementation of
the getObject()
method. This method will create an
AOP proxy wrapping a target object.
One of the most important benefits of using a
ProxyFactoryBean
or another IoC-aware class to create
AOP proxies, is that it means that advices and pointcuts can also be
managed by IoC. This is a powerful feature, enabling certain approaches
that are hard to achieve with other AOP frameworks. For example, an
advice may itself reference application objects (besides the target,
which should be available in any AOP framework), benefiting from all the
pluggability provided by Dependency Injection.
In common with most FactoryBean
implementations
provided with Spring, the ProxyFactoryBean
class is
itself a JavaBean. Its properties are used to:
Specify the target you want to proxy.
Specify whether to use CGLIB (see below and also the section entitled Section 7.5.3, “JDK- and CGLIB-based proxies”).
Some key properties are inherited from
org.springframework.aop.framework.ProxyConfig
(the
superclass for all AOP proxy factories in Spring). These key properties include:
proxyTargetClass
: true
if the
target class is to be proxied, rather than the target class' interfaces.
If this property value is set to true
, then CGLIB proxies
will be created (but see also below the section entitled
Section 7.5.3, “JDK- and CGLIB-based proxies”).
optimize
: controls whether or not aggressive
optimizations are applied to proxies created via CGLIB.
One should not blithely use this setting unless one fully understands
how the relevant AOP proxy handles optimization. This is currently used only
for CGLIB proxies; it has no effect with JDK dynamic proxies.
frozen
: if a proxy configuration is frozen
,
then changes to the configuration are no longer allowed. This is useful both as
a slight optimization and for those cases when you don't want callers to be able
to manipulate the proxy (via the Advised
interface)
after the proxy has been created. The default value of this property is
false
, so changes such as adding additional advice are allowed.
exposeProxy
: determines whether or not the current
proxy should be exposed in a ThreadLocal
so that
it can be accessed by the target. If a target needs to obtain
the proxy and the exposeProxy
property is set to
true
, the target can use the
AopContext.currentProxy()
method.
aopProxyFactory
: the implementation of
AopProxyFactory
to use. Offers a way of
customizing whether to use dynamic proxies, CGLIB or any other proxy
strategy. The default implementation will choose dynamic proxies or
CGLIB appropriately. There should be no need to use this property;
it is intended to allow the addition of new proxy types in Spring 1.1.
Other properties specific to ProxyFactoryBean
include:
proxyInterfaces
: array of String interface
names. If this isn't supplied, a CGLIB proxy for the target class
will be used (but see also below the section entitled
Section 7.5.3, “JDK- and CGLIB-based proxies”).
interceptorNames
: String array of
Advisor
, interceptor or other advice
names to apply. Ordering is significant, on a first come-first served
basis. That is to say that the first interceptor in the list
will be the first to be able to intercept the invocation.
The names are bean names in the current factory, including
bean names from ancestor factories. You can't mention bean
references here since doing so would result in the
ProxyFactoryBean
ignoring the singleton
setting of the advice.
You can append an interceptor name with an asterisk
(*
). This will result in the application of all
advisor beans with names starting with the part before the asterisk
to be applied. An example of using this feature can be found in
Section 7.5.6, “Using 'global' advisors”.
singleton: whether or not the factory should return a single
object, no matter how often the getObject()
method is called. Several FactoryBean
implementations offer such a method. The default value is
true
. If you want to use stateful advice -
for example, for stateful mixins - use prototype advices along
with a singleton value of false
.
This section serves as the definitive documentation on how the
ProxyFactoryBean
chooses to create one of
either a JDK- and CGLIB-based proxy for a particular target object
(that is to be proxied).
![]() | Note |
---|---|
The behavior of the |
If the class of a target object that is to be proxied (hereafter simply
referred to as the target class) doesn't implement any interfaces, then
a CGLIB-based proxy will be created. This is the easiest scenario, because
JDK proxies are interface based, and no interfaces means JDK proxying
isn't even possible. One simply plugs in the target bean, and specifies the
list of interceptors via the interceptorNames
property.
Note that a CGLIB-based proxy will be created even if the
proxyTargetClass
property of the
ProxyFactoryBean
has been set to false
.
(Obviously this makes no sense, and is best removed from the bean
definition because it is at best redundant, and at worst confusing.)
If the target class implements one (or more) interfaces, then the type of
proxy that is created depends on the configuration of the
ProxyFactoryBean
.
If the proxyTargetClass
property of the
ProxyFactoryBean
has been set to true
,
then a CGLIB-based proxy will be created. This makes sense, and is in
keeping with the principle of least surprise. Even if the
proxyInterfaces
property of the
ProxyFactoryBean
has been set to one or more
fully qualified interface names, the fact that the
proxyTargetClass
property is set to
true
will cause
CGLIB-based proxying to be in effect.
If the proxyInterfaces
property of the
ProxyFactoryBean
has been set to one or more
fully qualified interface names, then a JDK-based proxy will be created.
The created proxy will implement all of the interfaces that were specified
in the proxyInterfaces
property; if the target class
happens to implement a whole lot more interfaces than those specified in
the proxyInterfaces
property, that is all well and
good but those additional interfaces will not be implemented by the
returned proxy.
If the proxyInterfaces
property of the
ProxyFactoryBean
has not been
set, but the target class does implement one (or more)
interfaces, then the ProxyFactoryBean
will auto-detect
the fact that the target class does actually implement at least one interface,
and a JDK-based proxy will be created. The interfaces that are actually
proxied will be all of the interfaces that the target
class implements; in effect, this is the same as simply supplying a list
of each and every interface that the target class implements to the
proxyInterfaces
property. However, it is significantly less
work, and less prone to typos.
Let's look at a simple example of ProxyFactoryBean
in action. This example involves:
A target bean that will be proxied. This is the "personTarget" bean definition in the example below.
An Advisor and an Interceptor used to provide advice.
An AOP proxy bean definition specifying the target object (the personTarget bean) and the interfaces to proxy, along with the advices to apply.
<bean id="personTarget" class="com.mycompany.PersonImpl"> <property name="name"><value>Tony</value></property> <property name="age"><value>51</value></property> </bean> <bean id="myAdvisor" class="com.mycompany.MyAdvisor"> <property name="someProperty"><value>Custom string property value</value></property> </bean> <bean id="debugInterceptor" class="org.springframework.aop.interceptor.DebugInterceptor"> </bean> <bean id="person" class="org.springframework.aop.framework.ProxyFactoryBean"> <property name="proxyInterfaces"><value>com.mycompany.Person</value></property> <property name="target"><ref local="personTarget"/></property> <property name="interceptorNames"> <list> <value>myAdvisor</value> <value>debugInterceptor</value> </list> </property> </bean>
Note that the interceptorNames
property takes a
list of String: the bean names of the interceptor or advisors in the
current factory. Advisors, interceptors, before, after returning and
throws advice objects can be used. The ordering of advisors is
significant.
![]() | Note |
---|---|
You might be wondering why the list doesn't hold bean references. The reason for this is that if the ProxyFactoryBean's singleton property is set to false, it must be able to return independent proxy instances. If any of the advisors is itself a prototype, an independent instance would need to be returned, so it's necessary to be able to obtain an instance of the prototype from the factory; holding a reference isn't sufficient. |
The "person" bean definition above can be used in place of a Person implementation, as follows:
Person person = (Person) factory.getBean("person");
Other beans in the same IoC context can express a strongly typed dependency on it, as with an ordinary Java object:
<bean id="personUser" class="com.mycompany.PersonUser"> <property name="person"><ref local="person" /></property> </bean>
The PersonUser
class in this example would
expose a property of type Person. As far as it's concerned, the AOP
proxy can be used transparently in place of a "real" person
implementation. However, its class would be a dynamic proxy class. It
would be possible to cast it to the Advised
interface
(discussed below).
It's possible to conceal the distinction between target and proxy
using an anonymous inner bean, as follows. Only the
ProxyFactoryBean
definition is different; the advice
is included only for completeness:
<bean id="myAdvisor" class="com.mycompany.MyAdvisor"> <property name="someProperty"><value>Custom string property value</value></property> </bean> <bean id="debugInterceptor" class="org.springframework.aop.interceptor.DebugInterceptor"/> <bean id="person" class="org.springframework.aop.framework.ProxyFactoryBean"> <property name="proxyInterfaces"><value>com.mycompany.Person</value></property> <!-- Use inner bean, not local reference to target --> <property name="target"> <bean class="com.mycompany.PersonImpl"> <property name="name"><value>Tony</value></property> <property name="age"><value>51</value></property> </bean> </property> <property name="interceptorNames"> <list> <value>myAdvisor</value> <value>debugInterceptor</value> </list> </property> </bean>
This has the advantage that there's only one object of type
Person
: useful if we want to prevent users of the
application context from obtaining a reference to the un-advised object, or
need to avoid any ambiguity with Spring IoC
autowiring. There's also arguably an advantage in
that the ProxyFactoryBean definition is self-contained. However, there
are times when being able to obtain the un-advised target from the
factory might actually be an advantage: for
example, in certain test scenarios.
What if you need to proxy a class, rather than one or more interfaces?
Imagine that in our example above, there was no
Person
interface: we needed to advise a class called
Person
that didn't implement any business interface.
In this case, you can configure Spring to use CGLIB proxying, rather
than dynamic proxies. Simply set the proxyTargetClass
property on the ProxyFactoryBean above to true. While it's best to
program to interfaces, rather than classes, the ability to advise
classes that don't implement interfaces can be useful when working with
legacy code. (In general, Spring isn't prescriptive. While it makes it
easy to apply good practices, it avoids forcing a particular
approach.)
If you want to, you can force the use of CGLIB in any case, even if you do have interfaces.
CGLIB proxying works by generating a subclass of the target class at runtime. Spring configures this generated subclass to delegate method calls to the original target: the subclass is used to implement the Decorator pattern, weaving in the advice.
CGLIB proxying should generally be transparent to users. However, there are some issues to consider:
Final
methods can't be advised, as they
can't be overridden.
You'll need the CGLIB 2 binaries on your classpath; dynamic proxies are available with the JDK.
There's little performance difference between CGLIB proxying and dynamic proxies. As of Spring 1.0, dynamic proxies are slightly faster. However, this may change in the future. Performance should not be a decisive consideration in this case.
By appending an asterisk to an interceptor name, all advisors with bean names matching the part before the asterisk, will be added to the advisor chain. This can come in handy if you need to add a standard set of 'global' advisors:
<bean id="proxy" class="org.springframework.aop.framework.ProxyFactoryBean"> <property name="target" ref="service"/> <property name="interceptorNames"> <list> <value>global*</value> </list> </property> </bean> <bean id="global_debug" class="org.springframework.aop.interceptor.DebugInterceptor"/> <bean id="global_performance" class="org.springframework.aop.interceptor.PerformanceMonitorInterceptor"/>
Especially when defining transactional proxies, you may end up with many similar proxy definitions. The use of parent and child bean definitions, along with inner bean definitions, can result in much cleaner and more concise proxy definitions.
First a parent, template, bean definition is created for the proxy:
<bean id="txProxyTemplate" abstract="true" class="org.springframework.transaction.interceptor.TransactionProxyFactoryBean"> <property name="transactionManager" ref="transactionManager"/> <property name="transactionAttributes"> <props> <prop key="*">PROPAGATION_REQUIRED</prop> </props> </property> </bean>
This will never be instantiated itself, so may actually be incomplete. Then each proxy which needs to be created is just a child bean definition, which wraps the target of the proxy as an inner bean definition, since the target will never be used on its own anyway.
<bean id="myService" parent="txProxyTemplate"> <property name="target"> <bean class="org.springframework.samples.MyServiceImpl"> </bean> </property> </bean>
It is of course possible to override properties from the parent template, such as in this case, the transaction propagation settings:
<bean id="mySpecialService" parent="txProxyTemplate"> <property name="target"> <bean class="org.springframework.samples.MySpecialServiceImpl"> </bean> </property> <property name="transactionAttributes"> <props> <prop key="get*">PROPAGATION_REQUIRED,readOnly</prop> <prop key="find*">PROPAGATION_REQUIRED,readOnly</prop> <prop key="load*">PROPAGATION_REQUIRED,readOnly</prop> <prop key="store*">PROPAGATION_REQUIRED</prop> </props> </property> </bean>
Note that in the example above, we have explicitly marked the parent bean definition as abstract by using the abstract attribute, as described previously, so that it may not actually ever be instantiated. Application contexts (but not simple bean factories) will by default pre-instantiate all singletons. It is therefore important (at least for singleton beans) that if you have a (parent) bean definition which you intend to use only as a template, and this definition specifies a class, you must make sure to set the abstract attribute to true, otherwise the application context will actually try to pre-instantiate it.
It's easy to create AOP proxies programmatically using Spring. This enables you to use Spring AOP without dependency on Spring IoC.
The following listing shows creation of a proxy for a target object, with one interceptor and one advisor. The interfaces implemented by the target object will automatically be proxied:
ProxyFactory factory = new ProxyFactory(myBusinessInterfaceImpl); factory.addInterceptor(myMethodInterceptor); factory.addAdvisor(myAdvisor); MyBusinessInterface tb = (MyBusinessInterface) factory.getProxy();
The first step is to construct an object of type
org.springframework.aop.framework.ProxyFactory
. You can
create this with a target object, as in the above example, or specify the
interfaces to be proxied in an alternate constructor.
You can add interceptors or advisors, and manipulate them for the life of the ProxyFactory. If you add an IntroductionInterceptionAroundAdvisor you can cause the proxy to implement additional interfaces.
There are also convenience methods on ProxyFactory (inherited from
AdvisedSupport
) which allow you to add other advice types
such as before and throws advice. AdvisedSupport is the superclass of both
ProxyFactory and ProxyFactoryBean.
![]() | Tip |
---|---|
Integrating AOP proxy creation with the IoC framework is best practice in most applications. We recommend that you externalize configuration from Java code with AOP, as in general. |
However you create AOP proxies, you can manipulate them using the
org.springframework.aop.framework.Advised
interface.
Any AOP proxy can be cast to this interface, whichever other interfaces it
implements. This interface includes the following methods:
Advisor[] getAdvisors(); void addAdvice(Advice advice) throws AopConfigException; void addAdvice(int pos, Advice advice) throws AopConfigException; void addAdvisor(Advisor advisor) throws AopConfigException; void addAdvisor(int pos, Advisor advisor) throws AopConfigException; int indexOf(Advisor advisor); boolean removeAdvisor(Advisor advisor) throws AopConfigException; void removeAdvisor(int index) throws AopConfigException; boolean replaceAdvisor(Advisor a, Advisor b) throws AopConfigException; boolean isFrozen();
The getAdvisors()
method will return an Advisor
for every advisor, interceptor or other advice type that has been added to
the factory. If you added an Advisor, the returned advisor at this index
will be the object that you added. If you added an interceptor or other
advice type, Spring will have wrapped this in an advisor with a pointcut
that always returns true. Thus if you added a
MethodInterceptor
, the advisor returned for this index
will be an DefaultPointcutAdvisor
returning your
MethodInterceptor
and a pointcut that matches all
classes and methods.
The addAdvisor()
methods can be used to add any
Advisor. Usually the advisor holding pointcut and advice will be the
generic DefaultPointcutAdvisor
, which can be used with
any advice or pointcut (but not for introductions).
By default, it's possible to add or remove advisors or interceptors even once a proxy has been created. The only restriction is that it's impossible to add or remove an introduction advisor, as existing proxies from the factory will not show the interface change. (You can obtain a new proxy from the factory to avoid this problem.)
A simple example of casting an AOP proxy to the
Advised
interface and examining and manipulating its
advice:
Advised advised = (Advised) myObject; Advisor[] advisors = advised.getAdvisors(); int oldAdvisorCount = advisors.length; System.out.println(oldAdvisorCount + " advisors"); // Add an advice like an interceptor without a pointcut // Will match all proxied methods // Can use for interceptors, before, after returning or throws advice advised.addAdvice(new DebugInterceptor()); // Add selective advice using a pointcut advised.addAdvisor(new DefaultPointcutAdvisor(mySpecialPointcut, myAdvice)); assertEquals("Added two advisors", oldAdvisorCount + 2, advised.getAdvisors().length);
![]() | Note |
---|---|
It's questionable whether it's advisable (no pun intended) to modify advice on a business object in production, although there are no doubt legitimate usage cases. However, it can be very useful in development: for example, in tests. I have sometimes found it very useful to be able to add test code in the form of an interceptor or other advice, getting inside a method invocation I want to test. (For example, the advice can get inside a transaction created for that method: for example, to run SQL to check that a database was correctly updated, before marking the transaction for roll back.) |
Depending on how you created the proxy, you can usually set a
frozen
flag, in which case the
Advised
isFrozen()
method will
return true, and any attempts to modify advice through addition or removal
will result in an AopConfigException
. The ability to
freeze the state of an advised object is useful in some cases, for
example, to prevent calling code removing a security interceptor. It may
also be used in Spring 1.1 to allow aggressive optimization if runtime
advice modification is known not to be required.
So far we've considered explicit creation of AOP proxies using a
ProxyFactoryBean
or similar factory bean.
Spring also allows us to use "autoproxy" bean definitions, which can automatically proxy selected bean definitions. This is built on Spring "bean post processor" infrastructure, which enables modification of any bean definition as the container loads.
In this model, you set up some special bean definitions in your XML
bean definition file to configure the auto proxy infrastructure. This
allows you just to declare the targets eligible for autoproxying: you
don't need to use ProxyFactoryBean
.
There are two ways to do this:
Using an autoproxy creator that refers to specific beans in the current context.
A special case of autoproxy creation that deserves to be considered separately; autoproxy creation driven by source-level metadata attributes.
The org.springframework.aop.framework.autoproxy
package provides the following standard autoproxy creators.
The BeanNameAutoProxyCreator automatically creates AOP proxies for beans with names matching literal values or wildcards.
<bean class="org.springframework.aop.framework.autoproxy.BeanNameAutoProxyCreator"> <property name="beanNames"><value>jdk*,onlyJdk</value></property> <property name="interceptorNames"> <list> <value>myInterceptor</value> </list> </property> </bean>
As with ProxyFactoryBean
, there is an
interceptorNames
property rather than a list of interceptors, to allow
correct behavior for prototype advisors. Named "interceptors" can be
advisors or any advice type.
As with auto proxying in general, the main point of using
BeanNameAutoProxyCreator
is to apply the same
configuration consistently to multiple objects, with minimal
volume of configuration. It is a popular choice for applying
declarative transactions to multiple objects.
Bean definitions whose names match, such as "jdkMyBean" and
"onlyJdk" in the above example, are plain old bean definitions with
the target class. An AOP proxy will be created automatically by the
BeanNameAutoProxyCreator
. The same advice will be
applied to all matching beans. Note that if advisors are used (rather
than the interceptor in the above example), the pointcuts may apply
differently to different beans.
A more general and extremely powerful auto proxy creator is
DefaultAdvisorAutoProxyCreator
. This will
automagically apply eligible advisors in the current context, without
the need to include specific bean names in the autoproxy advisor's
bean definition. It offers the same merit of consistent configuration
and avoidance of duplication as
BeanNameAutoProxyCreator
.
Using this mechanism involves:
Specifying a
DefaultAdvisorAutoProxyCreator
bean
definition.
Specifying any number of Advisors in the same or related contexts. Note that these must be Advisors, not just interceptors or other advices. This is necessary because there must be a pointcut to evaluate, to check the eligibility of each advice to candidate bean definitions.
The DefaultAdvisorAutoProxyCreator
will
automatically evaluate the pointcut contained in each advisor, to see
what (if any) advice it should apply to each business object (such as
"businessObject1" and "businessObject2" in the example).
This means that any number of advisors can be applied automatically to each business object. If no pointcut in any of the advisors matches any method in a business object, the object will not be proxied. As bean definitions are added for new business objects, they will automatically be proxied if necessary.
Autoproxying in general has the advantage of making it impossible for callers or dependencies to obtain an un-advised object. Calling getBean("businessObject1") on this ApplicationContext will return an AOP proxy, not the target business object. (The "inner bean" idiom shown earlier also offers this benefit.)
<bean class="org.springframework.aop.framework.autoproxy.DefaultAdvisorAutoProxyCreator"/> <bean class="org.springframework.transaction.interceptor.TransactionAttributeSourceAdvisor"> <property name="transactionInterceptor" ref="transactionInterceptor"/> </bean> <bean id="customAdvisor" class="com.mycompany.MyAdvisor"/> <bean id="businessObject1" class="com.mycompany.BusinessObject1"> <!-- Properties omitted --> </bean> <bean id="businessObject2" class="com.mycompany.BusinessObject2"/>
The DefaultAdvisorAutoProxyCreator
is very
useful if you want to apply the same advice consistently to many
business objects. Once the infrastructure definitions are in place,
you can simply add new business objects without including specific
proxy configuration. You can also drop in additional aspects very
easily - for example, tracing or performance monitoring aspects - with
minimal change to configuration.
The DefaultAdvisorAutoProxyCreator offers support for filtering
(using a naming convention so that only certain advisors are
evaluated, allowing use of multiple, differently configured,
AdvisorAutoProxyCreators in the same factory) and ordering. Advisors
can implement the org.springframework.core.Ordered
interface to ensure correct ordering if this is an issue. The
TransactionAttributeSourceAdvisor used in the above example has a
configurable order value; the default setting is unordered.
This is the superclass of DefaultAdvisorAutoProxyCreator. You
can create your own autoproxy creators by subclassing this class, in
the unlikely event that advisor definitions offer insufficient
customization to the behavior of the framework
DefaultAdvisorAutoProxyCreator
.
A particularly important type of autoproxying is driven by
metadata. This produces a similar programming model to .NET
ServicedComponents
. Instead of using XML deployment
descriptors as in EJB, configuration for transaction management and
other enterprise services is held in source-level attributes.
In this case, you use the
DefaultAdvisorAutoProxyCreator
, in combination with
Advisors that understand metadata attributes. The metadata specifics are
held in the pointcut part of the candidate advisors, rather than in the
autoproxy creation class itself.
This is really a special case of the
DefaultAdvisorAutoProxyCreator
, but deserves
consideration on its own. (The metadata-aware code is in the pointcuts
contained in the advisors, not the AOP framework itself.)
The /attributes
directory of the JPetStore
sample application shows the use of attribute-driven autoproxying. In
this case, there's no need to use the
TransactionProxyFactoryBean
. Simply defining
transactional attributes on business objects is sufficient, because of
the use of metadata-aware pointcuts. The bean definitions include the
following code, in /WEB-INF/declarativeServices.xml
.
Note that this is generic, and can be used outside the JPetStore:
<bean class="org.springframework.aop.framework.autoproxy.DefaultAdvisorAutoProxyCreator"/> <bean class="org.springframework.transaction.interceptor.TransactionAttributeSourceAdvisor"> <property name="transactionInterceptor" ref="transactionInterceptor"/> </bean> <bean id="transactionInterceptor" class="org.springframework.transaction.interceptor.TransactionInterceptor"> <property name="transactionManager" ref="transactionManager"/> <property name="transactionAttributeSource"> <bean class="org.springframework.transaction.interceptor.AttributesTransactionAttributeSource"> <property name="attributes" ref="attributes"/> </bean> </property> </bean> <bean id="attributes" class="org.springframework.metadata.commons.CommonsAttributes"/>
The DefaultAdvisorAutoProxyCreator
bean
definition (the name is not significant, hence it can even be omitted)
will pick up all eligible pointcuts in the current application context.
In this case, the "transactionAdvisor" bean definition, of type
TransactionAttributeSourceAdvisor
, will apply to
classes or methods carrying a transaction attribute. The
TransactionAttributeSourceAdvisor depends on a TransactionInterceptor,
via constructor dependency. The example resolves this via autowiring.
The AttributesTransactionAttributeSource
depends on
an implementation of the
org.springframework.metadata.Attributes
interface. In
this fragment, the "attributes" bean satisfies this, using the Jakarta
Commons Attributes API to obtain attribute information. (The application
code must have been compiled using the Commons Attributes compilation
task.)
The /annotation
directory of the JPetStore
sample application contains an analogous example for auto-proxying
driven by JDK 1.5+ annotations. The following configuration enables
automatic detection of Spring's Transactional
annotation, leading to implicit proxies for beans containing that
annotation:
<bean class="org.springframework.aop.framework.autoproxy.DefaultAdvisorAutoProxyCreator"/> <bean class="org.springframework.transaction.interceptor.TransactionAttributeSourceAdvisor"> <property name="transactionInterceptor" ref="transactionInterceptor"/> </bean> <bean id="transactionInterceptor" class="org.springframework.transaction.interceptor.TransactionInterceptor"> <property name="transactionManager" ref="transactionManager"/> <property name="transactionAttributeSource"> <bean class="org.springframework.transaction.annotation.AnnotationTransactionAttributeSource"/> </property> </bean>
The TransactionInterceptor
defined here depends
on a PlatformTransactionManager
definition, which is
not included in this generic file (although it could be) because it will
be specific to the application's transaction requirements (typically
JTA, as in this example, or Hibernate, JDO or JDBC):
<bean id="transactionManager" class="org.springframework.transaction.jta.JtaTransactionManager"/>
![]() | Tip |
---|---|
If you require only declarative transaction management, using these generic XML definitions will result in Spring automatically proxying all classes or methods with transaction attributes. You won't need to work directly with AOP, and the programming model is similar to that of .NET ServicedComponents. |
This mechanism is extensible. It's possible to do autoproxying based on custom attributes. You need to:
Define your custom attribute.
Specify an Advisor with the necessary advice, including a pointcut that is triggered by the presence of the custom attribute on a class or method. You may be able to use an existing advice, merely implementing a static pointcut that picks up the custom attribute.
It's possible for such advisors to be unique to each advised class
(for example, mixins): they simply need to be defined as prototype,
rather than singleton, bean definitions. For example, the
LockMixin
introduction interceptor from the Spring
test suite, shown above, could be used in conjunction with an
attribute-driven pointcut to target a mixin, as shown here. We use the
generic DefaultPointcutAdvisor
, configured using
JavaBean properties:
<bean id="lockMixin" class="org.springframework.aop.LockMixin" scope="prototype"/> <bean id="lockableAdvisor" class="org.springframework.aop.support.DefaultPointcutAdvisor" scope="prototype"> <property name="pointcut" ref="myAttributeAwarePointcut"/> <property name="advice" ref="lockMixin"/> </bean> <bean id="anyBean" class="anyclass" ...
If the attribute aware pointcut matches any methods in the
anyBean
or other bean definitions, the mixin will be
applied. Note that both lockMixin
and
lockableAdvisor
definitions are prototypes. The
myAttributeAwarePointcut
pointcut can be a singleton
definition, as it doesn't hold state for individual advised
objects.
Spring offers the concept of a TargetSource,
expressed in the org.springframework.aop.TargetSource
interface. This interface is responsible for returning the "target object"
implementing the join point. The TargetSource
implementation is asked for a target instance each time the AOP proxy
handles a method invocation.
Developers using Spring AOP don't normally need to work directly with TargetSources, but this provides a powerful means of supporting pooling, hot swappable and other sophisticated targets. For example, a pooling TargetSource can return a different target instance for each invocation, using a pool to manage instances.
If you do not specify a TargetSource, a default implementation is used that wraps a local object. The same target is returned for each invocation (as you would expect).
Let's look at the standard target sources provided with Spring, and how you can use them.
![]() | Tip |
---|---|
When using a custom target source, your target will usually need to be a prototype rather than a singleton bean definition. This allows Spring to create a new target instance when required. |
The
org.springframework.aop.target.HotSwappableTargetSource
exists to allow the target of an AOP proxy to be switched while allowing
callers to keep their references to it.
Changing the target source's target takes effect immediately. The
HotSwappableTargetSource
is threadsafe.
You can change the target via the swap()
method
on HotSwappableTargetSource as follows:
HotSwappableTargetSource swapper = (HotSwappableTargetSource) beanFactory.getBean("swapper"); Object oldTarget = swapper.swap(newTarget);
The XML definitions required look as follows:
<bean id="initialTarget" class="mycompany.OldTarget"/> <bean id="swapper" class="org.springframework.aop.target.HotSwappableTargetSource"> <constructor-arg ref="initialTarget"/> </bean> <bean id="swappable" class="org.springframework.aop.framework.ProxyFactoryBean"> <property name="targetSource" ref="swapper"/> </bean>
The above swap()
call changes the target of the
swappable bean. Clients who hold a reference to that bean will be
unaware of the change, but will immediately start hitting the new
target.
Although this example doesn't add any advice - and it's not
necessary to add advice to use a TargetSource
- of
course any TargetSource
can be used in conjunction
with arbitrary advice.
Using a pooling target source provides a similar programming model to stateless session EJBs, in which a pool of identical instances is maintained, with method invocations going to free objects in the pool.
A crucial difference between Spring pooling and SLSB pooling is that Spring pooling can be applied to any POJO. As with Spring in general, this service can be applied in a non-invasive way.
Spring provides out-of-the-box support for Jakarta Commons Pool
1.3, which provides a fairly efficient pooling implementation. You'll
need the commons-pool Jar on your application's classpath to use this
feature. It's also possible to subclass
org.springframework.aop.target.AbstractPoolingTargetSource
to support any other pooling API.
Sample configuration is shown below:
<bean id="businessObjectTarget" class="com.mycompany.MyBusinessObject" scope="prototype"> ... properties omitted </bean> <bean id="poolTargetSource" class="org.springframework.aop.target.CommonsPoolTargetSource"> <property name="targetBeanName" value="businessObjectTarget"/> <property name="maxSize" value="25"/> </bean> <bean id="businessObject" class="org.springframework.aop.framework.ProxyFactoryBean"> <property name="targetSource" ref="poolTargetSource"/> <property name="interceptorNames" value="myInterceptor"/> </bean>
Note that the target object - "businessObjectTarget" in the
example - must be a prototype. This allows the
PoolingTargetSource
implementation to create new
instances of the target to grow the pool as necessary. See the Javadoc
for AbstractPoolingTargetSource
and the concrete
subclass you wish to use for information about it's properties: maxSize
is the most basic, and always guaranteed to be present.
In this case, "myInterceptor" is the name of an interceptor that would need to be defined in the same IoC context. However, it isn't necessary to specify interceptors to use pooling. If you want only pooling, and no other advice, don't set the interceptorNames property at all.
It's possible to configure Spring so as to be able to cast any
pooled object to the
org.springframework.aop.target.PoolingConfig
interface, which exposes information about the configuration and current
size of the pool through an introduction. You'll need to define an
advisor like this:
<bean id="poolConfigAdvisor" class="org.springframework.beans.factory.config.MethodInvokingFactoryBean"> <property name="targetObject" ref="poolTargetSource"/> <property name="targetMethod" value="getPoolingConfigMixin"/> </bean>
This advisor is obtained by calling a convenience method on the
AbstractPoolingTargetSource
class, hence the use of
MethodInvokingFactoryBean. This advisor's name ("poolConfigAdvisor"
here) must be in the list of interceptors names in the ProxyFactoryBean
exposing the pooled object.
The cast will look as follows:
PoolingConfig conf = (PoolingConfig) beanFactory.getBean("businessObject"); System.out.println("Max pool size is " + conf.getMaxSize());
![]() | Note |
---|---|
Pooling stateless service objects is not usually necessary. We don't believe it should be the default choice, as most stateless objects are naturally thread safe, and instance pooling is problematic if resources are cached. |
Simpler pooling is available using autoproxying. It's possible to set the TargetSources used by any autoproxy creator.
Setting up a "prototype" target source is similar to a pooling TargetSource. In this case, a new instance of the target will be created on every method invocation. Although the cost of creating a new object isn't high in a modern JVM, the cost of wiring up the new object (satisfying its IoC dependencies) may be more expensive. Thus you shouldn't use this approach without very good reason.
To do this, you could modify the
poolTargetSource
definition shown above as follows.
(I've also changed the name, for clarity.)
<bean id="prototypeTargetSource" class="org.springframework.aop.target.PrototypeTargetSource"> <property name="targetBeanName" ref="businessObjectTarget"/> </bean>
There's only one property: the name of the target bean. Inheritance is used in the TargetSource implementations to ensure consistent naming. As with the pooling target source, the target bean must be a prototype bean definition.
ThreadLocal
target sources are useful if you need an object to be
created for each incoming request (per thread that is). The concept of a
ThreadLocal
provide a JDK-wide facility to
transparently store resource alongside a thread. Setting up a
ThreadLocalTargetSource
is pretty much the same as was explained for the
other types of target source:
<bean id="threadlocalTargetSource" class="org.springframework.aop.target.ThreadLocalTargetSource"> <property name="targetBeanName" value="businessObjectTarget"/> </bean>
![]() | Note |
---|---|
ThreadLocals come with serious issues (potentially
resulting in memory leaks) when incorrectly using them in a
multi-threaded and multi-classloader environments. One should always
consider wrapping a threadlocal in some other class and never directly
use the |
Spring AOP is designed to be extensible. While the interception implementation strategy is presently used internally, it is possible to support arbitrary advice types in addition to the out-of-the-box interception around advice, before, throws advice and after returning advice.
The org.springframework.aop.framework.adapter
package is an SPI package allowing support for new custom advice types to
be added without changing the core framework. The only constraint on a
custom Advice
type is that it must implement the
org.aopalliance.aop.Advice
tag interface.
Please refer to the
org.springframework.aop.framework.adapter
package's
Javadocs for further information.
Please refer to the Spring sample applications for further examples of Spring AOP:
The JPetStore's default configuration illustrates the use of the
TransactionProxyFactoryBean
for declarative transaction
management.
The /attributes
directory of the JPetStore
illustrates the use of attribute-driven declarative transaction management.
The Spring team considers developer testing to be an absolutely integral part of enterprise software development. A thorough treatment of testing in the enterprise is beyond the scope of this chapter; rather, the focus here is on the value add that the adoption of the IoC principle can bring to unit testing; and on the benefits that the Spring Framework provides in integration testing.
One of the main benefits of Dependency Injection is that your code
should really depend far less on the container than in traditional J2EE
development. The POJOs that comprise your application should be testable
in JUnit tests, with objects simply instantiated using the
new
operator, without Spring or any other
container. You can use mock objects (in conjunction with many
other valuable testing techniques) to test your code in isolation. If you
follow the architecture recommendations around Spring you will find that
the resulting clean layering and componentization of your codebase will
naturally faciliate easier unit testing. For example,
you will be able to test service layer objects by stubbing or mocking DAO
interfaces, without any need to access persistent data while running unit
tests.
True unit tests typically will run extremely quickly, as there is no runtime infrastructure to set up, whether application server, database, ORM tool, or whatever. Thus emphasizing true unit tests as part of your development methodology will boost your productivity. The upshot of this is that you do not need this section of the testing chapter to help you write effective unit tests for your IoC-based applications.
However, it is also important to be able to perform some integration testing without requiring deployment to your application server or connecting to other enterprise infrastructure. This will enable you to test things such as:
The correct wiring of your Spring IoC container contexts.
Data access using JDBC or an ORM tool. This would include such things such as the correctness of SQL statements / or Hibernate XML mapping files.
The Spring Framework provides first class support for integration
testing in the form of the classes that are packaged in the
spring-mock.jar
library.
Please note that these test classes are JUnit-specific.
The org.springframework.test
package provides
valuable JUnit TestCase
superclasses for integration
testing using a Spring container, while at the same time not being reliant
on an application server or other deployed environment. They will be slower
to run than unit tests, but much faster to run than the equivalent Cactus
tests or remote tests relying on deployment to an application server.
These superclasses provide the following functionality:
Spring IoC container caching between test case execution.
The pretty-much-transparent Dependency Injection of test fixture instances (this is nice).
Transaction management appropriate to integration testing (this is even nicer).
A number of Spring-specific inherited instance variables that are really useful when integration testing.
The org.springframework.test
package provides
support for consistent loading of Spring contexts, and caching of loaded
contexts. Support for the caching of loaded contexts is important,
because if you are working on a large project, startup time may become
an issue - not because of the overhead of Spring itself, but because the
objects instantiated by the Spring container will themselves take time
to instantiate. For example, a project with 50-100 Hibernate mapping
files might take 10-20 seconds to load the mapping files, and incurring
that cost before running every single test case in every single test
fixture will lead to slower overall test runs that could reduce productivity.
To address this issue, the
AbstractDependencyInjectionSpringContextTests
has an
abstract
protected
method that
subclasses must implement, to provide the location of contexts:
protected abstract String[] getConfigLocations();
Implementations of this method must provide an array containing the
resource locations of XML configuration metadata - typically on the
classpath - used to configure the application. This will be the same, or
nearly the same, as the list of configuration locations specified in
web.xml
or other deployment configuration.
By default, once loaded, the configuration fileset will be reused
for each test case. Thus the setup cost will be incurred only once (per
test fixture), and subsequent test execution will be much faster. In
the unlikely case that a test may 'dirty' the config location, requiring
reloading - for example, by changing a bean definition or the state of
an application object - you can call the
setDirty()
method on
AbstractDependencyInjectionSpringContextTests
to
cause the test fixture to reload the configurations and rebuild the
application context before executing the next test case.
When AbstractDependencyInjectionSpringContextTests
(and subclasses) load your application context, they can optionally
configure instances of your test classes by Setter Injection. All you
need to do is to define instance variables and the corresponding
setters. AbstractDependencyInjectionSpringContextTests
will automatically locate the corresponding object in the set of
configuration files specified in the getConfigLocations()
method.
Consider the scenario where we have a class,
HibernateTitleDao
, that performs data access
logic for say, the Title
domain object. We want
to write integration tests that test all of the following areas:
The Spring configuration; basically, is everything related
to the configuration of the
HibernateTitleDao
bean correct and present?
The Hibernate mapping file configuration; is everything mapped correctly and are the correct lazy-loading settings in place?
The logic of the HibernateTitleDao
;
does the configured instance of this class perform as anticipated?
Let's look at the test class itself (we will look at the configuration immediately afterwards).
public final class HibernateTitleDaoTests extends AbstractDependencyInjectionSpringContextTests { // this instance will be (automatically) dependency injected private HibernateTitleDao titleDao; // a setter method to enable DI of the 'titleDao' instance variable public void setTitleDao(HibernateTitleDao titleDao) { this.titleDao = titleDao; } public void testLoadTitle() throws Exception { Title title = this.titleDao.loadTitle(new Long(10)); assertNotNull(title); } // specifies the Spring configuration to load for this test fixture protected String[] getConfigLocations() { return new String[] { "classpath:com/foo/daos.xml" }; } }
The file referenced by the
getConfigLocations()
method
('classpath:com/foo/daos.xml'
) looks like this:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE beans PUBLIC "-//SPRING//DTD BEAN 2.0//EN"
"http://www.springframework.org/dtd/http://www.springframework.org/dtd/spring-beans-2.0.dtd">
<beans>
<!-- this bean will be injected into the HibernateTitleDaoTests
class -->
<bean id="titleDao" class="com.foo.dao.hibernate.HibernateTitleDao">
<property name="sessionFactory" ref="sessionFactory"/>
</bean>
<bean id="sessionFactory" class="org.springframework.orm.hibernate3.LocalSessionFactoryBean">
<!-- dependencies elided for clarity -->
</bean>
</beans>
The AbstractDependencyInjectionSpringContextTests
classes uses
autowire by type.
Thus if you have multiple bean definitions of the same type, you cannot
rely on this approach for those particular beans. In that case, you can
use the inherited applicationContext
instance variable,
and explicit lookup using (for example) an explicit call to
applicationContext.getBean("titleDao")
.
If you don't want dependency injection applied to your test cases,
simply don't declare any setters. Alternatively, you can extend the
AbstractSpringContextTests
- the root of the class
hierarchy in the org.springframework.test
package.
It merely contains convenience methods to load Spring contexts, and
performs no Dependency Injection of the test fixture.
If, for whatever reason, you don't fancy having setter
methods in your test fixtures, Spring can (in this one case)
inject dependencies into protected
fields.
Find below a reworking of the previous example to use field level
injection (the Spring XML configuration does not need to change,
merely the test fixture).
public final class HibernateTitleDaoTests extends AbstractDependencyInjectionSpringContextTests { public HibernateTitleDaoTests() { // switch on field level injection setPopulateProtectedVariables(true); } // this instance will be (automatically) dependency injected protected HibernateTitleDao titleDao; public void testLoadTitle() throws Exception { Title title = this.titleDao.loadTitle(new Long(10)); assertNotNull(title); } // specifies the Spring configuration to load for this test fixture protected String[] getConfigLocations() { return new String[] { "classpath:com/foo/daos.xml" }; } }
In the case of field injection, there is no autowiring going on:
the name of your protected
instances variable(s)
are used as the lookup bean name in the configured Spring container.
One common issue in tests that access a real database is their affect on the state of the persistence store. Even when you're using a development database, changes to the state may affect future tests. Also, many operations - such as inserting to or modifying persistent data - cannot be done (or verified) outside a transaction.
The
org.springframework.test.AbstractTransactionalDataSourceSpringContextTests
superclass (and subclasses) exist to meet this need. By default, they
create and roll back a transaction for each test. You simply write
code that can assume the existence of a transaction. If you call
transactionally proxied objects in your tests, they will behave
correctly, according to their transactional semantics.
AbstractTransactionalSpringContextTests
depends on a PlatformTransactionManager
bean being
defined in the application context. The name doesn't matter, due to the
use of autowire by type.
Typically you will extend the subclass,
AbstractTransactionalDataSourceSpringContextTests
.
This also requires that a DataSource
bean
definition - again, with any name - be present in the configurations. It
creates a JdbcTemplate
instance variable that is
useful for convenient querying, and provides handy methods to delete the
contents of selected tables (remember that the transaction will roll
back by default, so this is safe to do).
If you want a transaction to commit - unusual, but occasionally
useful when you want a particular test to populate the database - you can
call the setComplete()
method inherited from
AbstractTransactionalSpringContextTests
. This will
cause the transaction to commit instead of roll back.
There is also convenient ability to end a transaction before the
test case ends, through calling the endTransaction()
method.
This will roll back the transaction by default, and commit it only if
setComplete()
had previously been called. This
functionality is useful if you want to test the behavior of
'disconnected' data objects, such as Hibernate-mapped objects
that will be used in a web or remoting tier outside a transaction.
Often, lazy loading errors are discovered only through UI testing; if
you call endTransaction()
you can ensure correct
operation of the UI through your JUnit test suite.
When you extend the
AbstractTransactionalDataSourceSpringContextTests
class you will have access to the following protected
instance variables:
applicationContext
(a ConfigurableApplicationContext
):
inherited from the
AbstractDependencyInjectionSpringContextTests
superclass. Use this to perfom explicit bean lookup, or test the
state of the context as a whole.
jdbcTemplate
: inherited from
AbstractTransactionalDataSourceSpringContextTests
.
Useful for querying to confirm state. For example, you might query
before and after testing application code that creates an object and
persists it using an ORM tool, to verify that the data appears in
the database. (Spring will ensure that the query runs in the scope
of the same transaction.) You will need to tell your ORM tool to
'flush' its changes for this to work correctly, for example
using the flush()
method on Hibernate's
Session
interface.
Often you will provide an application-wide superclass for integration tests that provides further useful instance variables used in many tests.
If you are developing against Java5 or greater, there are
some additional annotations and support classes that you can use
in your testing. The AbstractAnnotationAwareTransactionalTests
class extends the AbstractTransactionalDataSourceSpringContextTests
makes the text fixtures that you write that inherit from it aware of a number
of (Spring-specific) annotations.
The Spring Framework provides a number of annotations to
help when writing integration tests. Please note that these annotations
must be used in conjunction with the
aforementioned
AbstractAnnotationAwareTransactionalTests
in
order for the presence of these annotations to have any effect.
@DirtiesContext
.
The presence
of this annotation on a text method indicates that the
underlying Spring container is 'dirtied' during the execution of
of the test method, and thus must be rebuilt after the test
method finishes execution (regardless of whether the test
passed or not). Has the same effect as a regular
setDirty()
invocation.
@DirtiesContext
public void testProcess() {
// some logic that results in the Spring container being dirtied
}
@ExpectedException
.
Indicates that the annotated test method is expected to throw an exception during execution. The type of the expected exception is provided in the annotation, and if an an instance of the exception is thrown during the test method execution then the test passes. Likewise if an instance of the exception is not thrown during the test method execution then the test fails.
@ExpectedException(SomeBusinessException.class)
public void testProcessRainyDayScenario() {
// some logic that results in an Exception
being thrown
}
@NotTransactional
.
Simply indicates that the annotated test method must not execute in a transactional context.
@NotTransactional
public void testProcess() {
// ...
}
@Repeat
Indicates that the annotated test method must be executed repeatedly. The number of times that the test method is to be executed is specified in the annotation.
@Repeat(10)
public void testProcessRepeatedly() {
// ...
}
The PetClinic sample application included with the Spring
distribution illustrates the use of these test superclasses. Most
test functionality is included in the
AbstractClinicTests
, for which a partial
listing is shown below:
public abstract class AbstractClinicTests extends AbstractTransactionalDataSourceSpringContextTests { protected Clinic clinic; public void setClinic(Clinic clinic) { this.clinic = clinic; } public void testGetVets() { Collection vets = this.clinic.getVets(); assertEquals('JDBC query must show the same number of vets', jdbcTemplate.queryForInt('SELECT COUNT(0) FROM VETS'), vets.size()); Vet v1 = (Vet) EntityUtils.getById(vets, Vet.class, 2); assertEquals('Leary', v1.getLastName()); assertEquals(1, v1.getNrOfSpecialties()); assertEquals('radiology', ((Specialty) v1.getSpecialties().get(0)).getName()); Vet v2 = (Vet) EntityUtils.getById(vets, Vet.class, 3); assertEquals('Douglas', v2.getLastName()); assertEquals(2, v2.getNrOfSpecialties()); assertEquals('dentistry', ((Specialty) v2.getSpecialties().get(0)).getName()); assertEquals('surgery', ((Specialty) v2.getSpecialties().get(1)).getName()); }
Notes:
This test case extends the
AbstractTransactionalDataSourceSpringContextTests
class, from which it inherits Dependency Injection and transactional
behavior.
The clinic
instance variable - the application object
being tested - is set by Dependency Injection through the
setClinic(..)
method.
The testGetVets()
method illustrates how the
inherited JdbcTemplate
variable can be used to verify
correct behavior of the application code being tested. This allows
for stronger tests, and lessens dependency on the exact test data.
For example, you can add additional rows in the database without
breaking tests.
Like many integration tests using a database, most of the
tests in AbstractClinicTests
depend on a minimum
amount of data already in the database before the test cases run.
You might, however, choose to populate the database in your test
cases also - again, within the one transaction.
The PetClinic application supports four data access
technologies - JDBC, Hibernate, TopLink, and JPA. Thus the
AbstractClinicTests
class does not itself specify the
context locations - this is deferred to subclasses,
that implement the necessary protected abstract method from
AbstractDependencyInjectionSpringContextTests
.
For example, the Hibernate implementation of the PetClinic tests contains the following implementation:
public final class HibernateClinicTests extends AbstractClinicTests { protected String[] getConfigLocations() { return new String[] { "/org/springframework/samples/petclinic/hibernate/applicationContext-hibernate.xml" }; } }
As the PetClinic is a very simple application, there is only one
Spring configuration file. Of course, more complex applications will
typically break their Spring configuration across multiple files.
Instead of being defined in a leaf class, config locations will
often be specified in a common base class for all application-specific
integration tests. This may also add useful instance
variables - populated by Dependency Injection, naturally - such as a
HibernateTemplate
, in the case of an application
using Hibernate.
As far as possible, you should have exactly the same Spring
configuration files in your integration tests as in the deployed
environment. One likely point of difference concerns database connection
pooling and transaction infrastructure. If you are deploying to a
full-blown application server, you will probably use its connection pool
(available through JNDI) and JTA implementation. Thus in production you
will use a JndiObjectFactoryBean
for the
DataSource
, and JtaTransactionManager
.
JNDI and JTA will not be available in out-of-container integration
tests, so you should use a combination like the Commons DBCP
BasicDataSource
and DataSourceTransactionManager
or HibernateTransactionManager
for them. You can
factor out this variant behavior into a single XML file, having the
choice between application server and 'local' configuration
separated from all other configuration, which will not vary between the
test and production environments.
This section contains links to further resources about testing in general.
The JUnit homepage. The Spring Framework's unit test suite is written using JUnit as the testing framework.
The EasyMock homepage. The Spring Framework uses EasyMock extensively in it's test suite.
The JMock homepage.
The DbUnit homepage.
The Grinder homepage (load testing framework).
This part of the reference documentation is concerned with the middle tier, and specifically the data access responsibilities of said tier.
Spring's comprehensive transaction management support is covered in some detail, followed by thorough coverage of the various middle tier data access frameworks and technologies that the Spring Framework integrates with.
One of the most compelling reasons to use the Spring Framework is the comprehensive transaction support. The Spring Framework provides a consistent abstraction for transaction management that delivers the following benefits:
Provides a consistent programming model across different transaction APIs such as JTA, JDBC, Hibernate, JPA, and JDO.
Supports declarative transaction management.
Provides a simpler API for programmatic transaction management than a number of complex transaction APIs such as JTA.
Integrates very well with Spring's various data access abstractions.
This chapter is divided up into a number of sections, each detailing one of the value-adds or technologies of the Spring Framework's transaction support. The chapter closes up with some discussion of best practices surrounding transaction management (for example, choosing between declarative and programmatic transaction management).
The first section, entitled Motivations, describes why one would want to use the Spring Framework's transaction abstraction as opposed to EJB CMT or driving transactions via a proprietary API such as Hibernate.
The second section, entitled
Key abstractions
outlines the core classes in the Spring Framework's transaction support,
as well as how to configure and obtain DataSource
instances from a variety of sources.
The third section, entitled Declarative transaction management, covers the Spring Framework's support for declarative transaction management.
The fourth section, entitled Programmatic transaction management, covers the Spring Framework's support for programmatic (that is, explicitly coded) transaction management.
Traditionally, J2EE developers have had two choices for transaction management: global or local transactions. Global transactions are managed by the application server, using the Java Transaction API (JTA). Local transactions are resource-specific: the most common example would be a transaction associated with a JDBC connection. This choice has profound implications. For instance, global transactions provide the ability to work with multiple transactional resources (typically relational databases and message queues). With local transactions, the application server is not involved in transaction management and cannot help ensure correctness across multiple resources. (It is worth noting that most applications use a single transaction resource.)
Global Transactions. Global transactions have a significant downside, in that code needs
to use JTA, and JTA is a cumbersome API to use (partly due to its exception
model). Furthermore, a JTA UserTransaction
normally needs to be sourced from JNDI: meaning that we need to use
both JNDI and JTA to use JTA.
Obviously all use of global transactions limits the reusability of application
code, as JTA is normally only available in an application server environment. Previously, the preferred way to use global transactions was via EJB
CMT (Container Managed Transaction):
CMT is a form of declarative transaction management
(as distinguished from programmatic transaction management).
EJB CMT removes the need for transaction-related JNDI lookups - although of course
the use of EJB itself necessitates the use of JNDI. It removes most of the need (although
not entirely) to write Java code to control transactions. The significant
downside is that CMT is tied to JTA and an application server
environment. Also, it is only available if one chooses to implement
business logic in EJBs, or at least behind a transactional EJB facade. The
negatives around EJB in general are so great that this is not an
attractive proposition, especially in the face of compelling alternatives for
declarative transaction management.
Local Transactions. Local transactions may be easier to use, but have significant disadvantages: they cannot work across multiple transactional resources. For example, code that manages transactions using a JDBC connection cannot run within a global JTA transaction. Another downside is that local transactions tend to be invasive to the programming model.
Spring resolves these problems. It enables application developers to use a consistent programming model in any environment. You write your code once, and it can benefit from different transaction management strategies in different environments. The Spring Framework provides both declarative and programmatic transaction management. Declarative transaction management is preferred by most users, and is recommended in most cases.
With programmatic transaction management, developers work with the Spring Framework transaction abstraction, which can run over any underlying transaction infrastructure. With the preferred declarative model, developers typically write little or no code related to transaction management, and hence don't depend on the Spring Framework's transaction API (or indeed on any other transaction API).
The key to the Spring transaction abstraction is the notion of a
transaction strategy. A transaction strategy is
defined by the
org.springframework.transaction.PlatformTransactionManager
interface, shown below:
public interface PlatformTransactionManager { TransactionStatus getTransaction(TransactionDefinition definition) throws TransactionException; void commit(TransactionStatus status) throws TransactionException; void rollback(TransactionStatus status) throws TransactionException; }
This is primarily an SPI interface, although it can be used
programmatically.
Note that in keeping with the Spring Framework's philosophy,
PlatformTransactionManager
is
an interface, and can thus be easily mocked or stubbed
as necessary. Nor is it tied to a lookup strategy such as JNDI:
PlatformTransactionManager
implementations
are defined like any other object (or bean) in the Spring Framework's IoC container.
This benefit alone makes it a worthwhile abstraction even when working
with JTA: transactional code can be tested much more easily than if it
used JTA directly.
Again in keeping with Spring's philosophy, the TransactionException
that can be thrown by any of the PlatformTransactionManager
interface's methods is unchecked (i.e. it extends the
java.lang.RuntimeException
class). Transaction infrastructure
failures are almost invariably fatal. In rare cases where application code can actually
recover from a transaction failure, the application developer can still choose to catch
and handle TransactionException
. The salient point is
that developers are not forced to do so.
The getTransaction(..)
method returns a
TransactionStatus
object, depending on a
TransactionDefinition
parameter. The returned
TransactionStatus
might represent a new or
existing transaction (if there were a matching transaction in the current
call stack - with the implication being that (as with J2EE transaction contexts)
a TransactionStatus
is associated with a
thread of execution).
The TransactionDefinition
interface specifies:
Isolation: the degree of isolation this transaction has from the work of other transactions. For example, can this transaction see uncommitted writes from other transactions?
Propagation: normally all code executed within a transaction scope will run in that transaction. However, there are several options specifying behavior if a transactional method is executed when a transaction context already exists: for example, simply continue running in the existing transaction (the common case); or suspending the existing transaction and creating a new transaction. Spring offers all of the transaction propagation options familiar from EJB CMT.
Timeout: how long this transaction may run before timing out (and automatically being rolled back by the underlying transaction infrastructure).
Read-only status: a read-only transaction does not modify any data. Read-only transactions can be a useful optimization in some cases (such as when using Hibernate).
These settings reflect standard transactional concepts. If necessary, please refer to a resource discussing transaction isolation levels and other core transaction concepts because understanding such core concepts is essential to using the Spring Framework or indeed any other transaction management solution.
The TransactionStatus
interface provides a simple
way for transactional code to control transaction execution and query
transaction status. The concepts should be familiar, as they are common to
all transaction APIs:
public interface TransactionStatus { boolean isNewTransaction(); void setRollbackOnly(); boolean isRollbackOnly(); }
Regardless of whether you opt for declarative or programmatic transaction
management in Spring, defining the correct
PlatformTransactionManager
implementation is
absolutely essential. In good Spring fashion, this important definition typically
is made using via Dependency Injection.
PlatformTransactionManager
implementations
normally require knowledge of the environment in which they work: JDBC, JTA,
Hibernate, etc The following examples from the
dataAccessContext-local.xml
file from Spring's
jPetStore sample application show how a local
PlatformTransactionManager
implementation can be
defined. (This will work with plain JDBC.)
We must define a JDBC DataSource
, and
then use the Spring DataSourceTransactionManager
, giving
it a reference to the DataSource
.
<bean id="dataSource" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close"> <property name="driverClassName" value="${jdbc.driverClassName}" /> <property name="url" value="${jdbc.url}" /> <property name="username" value="${jdbc.username}" /> <property name="password" value="${jdbc.password}" /> </bean>
The related PlatformTransactionManager
bean
definition will look like this:
<bean id="txManager" class="org.springframework.jdbc.datasource.DataSourceTransactionManager"> <property name="dataSource" ref="dataSource"/> </bean>
If we use JTA in a J2EE container, as in the 'dataAccessContext-jta.xml'
file from the same sample application, we use a container DataSource
,
obtained via JNDI, in conjunction with Spring's JtaTransactionManager
.
The JtaTransactionManager
doesn't need to know about the
DataSource
, or any other specific resources, as
it will use the container's global transaction management infrastructure.
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:jee="http://www.springframework.org/schema/jee"
xsi:schemaLocation="
http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.0.xsd
http://www.springframework.org/schema/jee http://www.springframework.org/schema/jee/spring-jee-2.0.xsd">
<jee:jndi-lookup id="dataSource" jndi-name="jdbc/jpetstore"/>
<bean id="txManager" class="org.springframework.transaction.jta.JtaTransactionManager" />
<!-- other <bean/>
definitions here -->
</beans>
![]() | Note |
---|---|
The above definition of the |
We can also use Hibernate local transactions easily, as shown in the
following examples from the Spring Framework's PetClinic
sample application. In this case, we need to define a Hibernate
LocalSessionFactoryBean
, which application code will
use to obtain Hibernate Session
instances.
The DataSource
bean definition will be
similar to the one shown previously (and thus is not shown). If the
DataSource
is managed by the JEE container it should
be non-transactional as the Spring Framework, rather than the JEE container, will
manage transactions.
The 'txManager'
bean in this case is of the
HibernateTransactionManager
type. In the same way as the
DataSourceTransactionManager
needs a reference to the
DataSource
, the
HibernateTransactionManager
needs a reference to the
SessionFactory
.
<bean id="sessionFactory" class="org.springframework.orm.hibernate.LocalSessionFactoryBean"> <property name="dataSource" ref="dataSource" /> <property name="mappingResources"> <list> <value>org/springframework/samples/petclinic/hibernate/petclinic.hbm.xml</value> </list> </property> <property name="hibernateProperties"> <value> hibernate.dialect=${hibernate.dialect} </value> </property> </bean> <bean id="txManager" class="org.springframework.orm.hibernate.HibernateTransactionManager"> <property name="sessionFactory" ref="sessionFactory" /> </bean>
With Hibernate and JTA transactions, we can simply use the
JtaTransactionManager
as with JDBC or any other resource strategy.
<bean id="txManager" class="org.springframework.transaction.jta.JtaTransactionManager"/>
Note that this is identical to JTA configuration for any resource, as these are global transactions, which can enlist any transactional resource.
In all these cases, application code will not need to change at
all. We can change how transactions are managed merely by changing
configuration, even if that change means moving from local to global
transactions or vice versa.
It should now be clear how different transaction managers are
created, and how they are linked to related resources which need to be
synchronized to transactions (i.e. DataSourceTransactionManager
to a JDBC DataSource
,
HibernateTransactionManager
to a Hibernate
SessionFactory
, etc). There remains the question
however of how the application code, directly or indirectly using a
persistence API (JDBC, Hibernate, JDO, etc), ensures that these resources
are obtained and handled properly in terms of proper
creation/reuse/cleanup and trigger (optionally) transaction
synchronization via the relevant PlatformTransactionManager
.
The preferred approach is to use Spring's highest level
persistence integration APIs. These do not replace the native APIs, but
internally handle resource creation/reuse, cleanup, optional
transaction synchronization of the resources and exception mapping so
that user data access code doesn't have to worry about these concerns at
all, but can concentrate purely on non-boilerplate persistence logic.
Generally, the same template approach is used
for all persistence APIs, with examples including the
JdbcTemplate
, HibernateTemplate
,
and JdoTemplate
classes (detailed in subsequent chapters
of this reference documentation.
At a lower level exist classes such as
DataSourceUtils
(for JDBC),
SessionFactoryUtils
(for Hibernate),
PersistenceManagerFactoryUtils
(for JDO), and so on.
When it is preferable for application code to deal directly with the
resource types of the native persistence APIs, these classes ensure that
proper Spring Framework-managed instances are obtained, transactions are
(optionally) synchronized, and exceptions which happen in the process
are properly mapped to a consistent API.
For example, in the case of JDBC, instead of the traditional JDBC approach of
calling the getConnection()
method on the
DataSource
, you would instead use Spring's
org.springframework.jdbc.datasource.DataSourceUtils
class as follows:
Connection conn = DataSourceUtils.getConnection(dataSource);
If an existing transaction exists, and already has a connection
synchronized (linked) to it, that instance will be returned. Otherwise,
the method call will trigger the creation of a new connection, which
will be (optionally) synchronized to any existing transaction, and
made available for subsequent reuse in that same transaction. As mentioned,
this has the added advantage that any SQLException
will be wrapped in a Spring Framework
CannotGetJdbcConnectionException
- one of the Spring
Framework's hierarchy of unchecked DataAccessExceptions. This gives you more
information than can easily be obtained from the
SQLException
, and ensures portability across
databases: even across different persistence technologies.
It should be noted that this will also work fine without Spring transaction management (transaction synchronization is optional), so you can use it whether or not you are using Spring for transaction management.
Of course, once you've used Spring's JDBC support or Hibernate
support, you will generally prefer not to use
DataSourceUtils
or the other helper classes, because
you'll be much happier working via the Spring abstraction than directly
with the relevant APIs. For example, if you use the Spring
JdbcTemplate
or jdbc.object
package to simplify your use of JDBC, correct connection retrieval happens
behind the scenes and you won't need to write any special code.
At the very lowest level exists the
TransactionAwareDataSourceProxy
class. This is a
proxy for a target DataSource
, which wraps the
target DataSource
to add awareness of Spring-managed
transactions. In this respect, it is similar to a transactional JNDI
DataSource
as provided by a J2EE server.
It should almost never be necessary or desirable to use this
class, except when existing code exists which must be called and passed
a standard JDBC DataSource
interface implementation.
In that case, it's possible to still have this code be usable, but
participating in Spring managed transactions. It is preferable to write
your new code using the higher level abstractions mentioned
above.
Most users of the Spring Framework choose declarative transaction management. It is the option with the least impact on application code, and hence is most consistent with the ideals of a non-invasive lightweight container.
The Spring Framework's declarative transaction management is made possible with Spring AOP, although, as the transactional aspects code comes with the Spring Framework distribution and may be used in a boilerplate fashion, AOP concepts do not generally have to be understood to make effective use of this code.
It may be helpful to begin by considering EJB CMT and explaining the
similarities and differences with the Spring Framework's declarative transaction
management. The basic approach is similar: it is possible to specify
transaction behavior (or lack of it) down to individual method level. It is
possible to make a setRollbackOnly()
call within a
transaction context if necessary. The differences are:
Unlike EJB CMT, which is tied to JTA, the Spring Framework's declarative transaction management works in any environment. It can work with JDBC, JDO, Hibernate or other transactions under the covers, with configuration changes only.
The Spring Framework enables declarative transaction management to be applied to any class, not merely special classes such as EJBs.
The Spring Framework offers declarative rollback rules: a feature with no EJB equivalent, which we'll discuss below. Rollback can be controlled declaratively, not merely programmatically.
The Spring Framework gives you an opportunity to customize transactional
behavior, using AOP. For example, if you want to insert custom
behavior in the case of transaction rollback, you can. You can also
add arbitrary advice, along with the transactional advice. With EJB
CMT, you have no way to influence the container's transaction
management other than setRollbackOnly()
.
The Spring Framework does not support propagation of transaction contexts across remote calls, as do high-end application servers. If you need this feature, we recommend that you use EJB. However, consider carefully before using such a feature. Normally, we do not want transactions to span remote calls.
The concept of rollback rules is important: they enable us to
specify which exceptions (and throwables) should cause automatic roll
back. We specify this declaratively, in configuration, not in Java code.
So, while we can still call setRollbackOnly()
on the
TransactionStatus
object to roll the current
transaction back programmatically, most often we can specify a rule that
MyApplicationException
must always result in
rollback. This has the significant advantage that business objects don't need
to depend on the transaction infrastructure. For example, they typically
don't need to import any Spring APIs, transaction or other.
While the EJB default behavior is for the EJB container to
automatically roll back the transaction on a system
exception (usually a runtime exception), EJB CMT does not roll
back the transaction automatically on an application exception
(i.e. a checked exception other than java.rmi.RemoteException
).
While the Spring default behavior for declarative transaction management follows
EJB convention (roll back is automatic only on unchecked exceptions), it is often
useful to customize this.
The aim of this section is to dispel the mystique that is sometimes associated
with the use of declarative transactions. It is all very well for this reference
documentation simply to tell you to annotate your classes with the
@Transactional
annotation, add the line
('<tx:annotation-driven/>'
) to your configuration,
and then expect you to understand how it all works. This section will explain the
inner workings of the Spring Framework's declarative transaction infrastructure to
help you navigate your way back upstream to calmer waters in the event of
transaction-related issues.
![]() | Tip |
---|---|
Looking at the Spring Framework source code is a good way to get a real
understanding of the transaction support. We also suggest turning the logging
level to |
The most important concepts to grasp with regard to the Spring Framework's
declarative transaction support are that this support is enabled
via AOP proxies,
and that the transactional advice is driven by metadata (currently
XML- or annotation-based). The combination of AOP with transactional metadata yields
an AOP proxy that uses a TransactionInterceptor
in conjunction
with an appropriate PlatformTransactionManager
implementation
to drive transactions around method invocations.
![]() | Note |
---|---|
Although knowledge of Spring AOP is not required to use Spring's declarative transaction support, it can help. Spring AOP is thoroughly covered in the chapter entitled Chapter 6, Aspect Oriented Programming with Spring. |
Conceptually, calling a method on a transactional proxy looks like this...
Consider the following interface, and its attendant implementation.
(The intent is to convey the concepts, and using the rote Foo
and
Bar
tropes means that you can concentrate on the transaction
usage and not have to worry about the domain model.)
// the service interface that we want to make transactional
package x.y.service;
public interface FooService {
Foo getFoo(String fooName);
Foo getFoo(String fooName, String barName);
void insertFoo(Foo foo);
void updateFoo(Foo foo);
}
// an implementation of the above interface
package x.y.service;
public class DefaultFooService implements FooService {
public Foo getFoo(String fooName) {
throw new UnsupportedOperationException();
}
public Foo getFoo(String fooName, String barName) {
throw new UnsupportedOperationException();
}
public void insertFoo(Foo foo) {
throw new UnsupportedOperationException();
}
public void updateFoo(Foo foo) {
throw new UnsupportedOperationException();
}
}
(For the purposes of this example, the fact that the
DefaultFooService
class throws
UnsupportedOperationException
instances in the body
of each implemented method is good; it will allow us to see transactions being created
and then rolled back in response to the UnsupportedOperationException
instance being thrown.)
Let's assume that the first two methods of the FooService
interface (getFoo(String)
and getFoo(String, String)
)
have to execute in the context of a transaction with read-only semantics, and that
the other methods (insertFoo(Foo)
and
updateFoo(Foo)
) have to execute in the context of a transaction
with read-write semantics. Don't worry about taking the following configuration in
all at once; everything will be explained in detail in the next few paragraphs.
<!-- from the file'context.xml'
--> <?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:aop="http://www.springframework.org/schema/aop" xmlns:tx="http://www.springframework.org/schema/tx" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.0.xsd http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-2.0.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-2.0.xsd"> <!-- this is the service object that we want to make transactional --> <bean id="fooService" class="x.y.service.DefaultFooService"/> <!-- the transactional advice (i.e. what 'happens'; see the<aop:advisor/>
bean below) --> <tx:advice id="txAdvice" transaction-manager="txManager"> <!-- the transactional semantics... --> <tx:attributes> <!-- all methods starting with'get'
are read-only --> <tx:method name="get*" read-only="true"/> <!-- other methods use the default transaction settings (see below) --> <tx:method name="*"/> </tx:attributes> </tx:advice> <!-- ensure that the above transactional advice runs for any execution of an operation defined by theFooService
interface --> <aop:config> <aop:pointcut id="fooServiceOperation" expression="execution(* x.y.service.FooService.*(..))"/> <aop:advisor advice-ref="txAdvice" pointcut-ref="fooServiceOperation"/> </aop:config> <!-- don't forget theDataSource
--> <bean id="dataSource" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close"> <property name="driverClassName" value="oracle.jdbc.driver.OracleDriver"/> <property name="url" value="jdbc:oracle:thin:@rj-t42:1521:elvis"/> <property name="username" value="scott"/> <property name="password" value="tiger"/> </bean> <!-- similarly, don't forget thePlatformTransactionManager
--> <bean id="txManager" class="org.springframework.jdbc.datasource.DataSourceTransactionManager"> <property name="dataSource" ref="dataSource"/> </bean> <!-- other<bean/>
definitions here --> </beans>
Let's pick apart the above configuration. We have a service object
(the 'fooService'
bean) that we want to make transactional.
The transaction semantics that we want to apply are encapsulated in the
<tx:advice/>
definition. The
<tx:advice/>
definition reads as
“... all methods on starting with 'get'
are to execute
in the context of a read-only transaction, and all other methods are to execute
with the default transaction semantics”. The
'transaction-manager'
attribute of the
<tx:advice/>
tag is set to the
name of the PlatformTransactionManager
bean
that is going to actually drive the transactions (in this
case the 'txManager'
bean).
![]() | Tip |
---|---|
You can actually omit the |
The <aop:config/>
definition ensures that the transactional
advice defined by the 'txAdvice'
bean actually executes at the appropriate
points in the program. First we define a pointcut that matches the execution of any
operation defined in the FooService
interface
('fooServiceOperation'
). Then we associate the pointcut with the
'txAdvice'
using an advisor. The result indicates that at the execution
of a 'fooServiceOperation'
, the advice defined by 'txAdvice'
will be run.
The expression defined within the <aop:pointcut/>
element is an AspectJ pointcut expression; see the chapter entitled Chapter 6, Aspect Oriented Programming with Spring
for more details on pointcut expressions in Spring 2.0.
A common requirement is to make an entire service layer transactional. The best way to do this is simply to change the pointcut expression to match any operation in your service layer. For example:
<aop:config> <aop:pointcut id="fooServiceMethods" expression="execution(* x.y.service.*.*(..))"/> <aop:advisor advice-ref="txAdvice" pointcut-ref="fooServiceMethods"/> </aop:config>
(This example assumes that all your service interfaces are defined
in the 'x.y.service'
package; see the chapter entitled
Chapter 6, Aspect Oriented Programming with Spring for more details.)
Now that we've analyzed the configuration, you may be asking yourself, “Okay... but what does all this configuration actually do?”.
The above configuration is going to effect the creation of a
transactional proxy around the object that is created from the
'fooService'
bean definition. The proxy will be configured
with the transactional advice, so that when an appropriate method is invoked
on the proxy, a transaction may
be started, suspended, be marked as read-only, etc., depending on the
transaction configuration associated with that method. Consider the following
program that test drives the above configuration.
public final class Boot { public static void main(final String[] args) throws Exception { ApplicationContext ctx = new ClassPathXmlApplicationContext("context.xml", Boot.class); FooService fooService = (FooService) ctx.getBean("fooService"); fooService.insertFoo (new Foo()); } }
The output from running the above program will look something
like this. (Please note that the Log4J output and the stacktrace
from the UnsupportedOperationException
thrown by the
insertFoo(..)
method of the
DefaultFooService
class have been truncated in
the interest of clarity.)
<!-- the Spring container is starting up... --> [AspectJInvocationContextExposingAdvisorAutoProxyCreator] - Creating implicit proxy for bean 'fooService' with 0 common interceptors and 1 specific interceptors <!-- theDefaultFooService
is actually proxied --> [JdkDynamicAopProxy] - Creating JDK dynamic proxy for [x.y.service.DefaultFooService] <!-- ... theinsertFoo(..)
method is now being invoked on the proxy --> [TransactionInterceptor] - Getting transaction for x.y.service.FooService.insertFoo <!-- the transactional advice kicks in here... --> [DataSourceTransactionManager] - Creating new transaction with name [x.y.service.FooService.insertFoo] [DataSourceTransactionManager] - Acquired Connection [org.apache.commons.dbcp.PoolableConnection@a53de4] for JDBC transaction <!-- theinsertFoo(..)
method fromDefaultFooService
throws an exception... --> [RuleBasedTransactionAttribute] - Applying rules to determine whether transaction should rollback on java.lang.UnsupportedOperationException [TransactionInterceptor] - Invoking rollback for transaction on x.y.service.FooService.insertFoo due to throwable [java.lang.UnsupportedOperationException] <!-- and the transaction is rolled back (by default,RuntimeException
instances cause rollback) --> [DataSourceTransactionManager] - Rolling back JDBC transaction on Connection [org.apache.commons.dbcp.PoolableConnection@a53de4] [DataSourceTransactionManager] - Releasing JDBC Connection after transaction [DataSourceUtils] - Returning JDBC Connection to DataSource Exception in thread "main" java.lang.UnsupportedOperationException at x.y.service.DefaultFooService.insertFoo(DefaultFooService.java:14) <!-- AOP infrastructure stack trace elements removed for clarity --> at $Proxy0.insertFoo(Unknown Source) at Boot.main(Boot.java:11)
The previous section outlined the basics of how to specify the transactional settings for the classes, typically service layer classes, in your application in a declarative fashion. This section describes how you can control the rollback of transactions in a simple declarative fashion.
The recommended way to indicate to the Spring Framework's
transaction infrastructure that a transaction's work is to be rolled back is to
throw an Exception
from code that is currently
executing in the context of a transaction. The Spring Framework's
transaction infrastructure code will catch any unhandled
Exception
as it bubbles up the call stack, and will
mark the transaction for rollback.
However, please note that the Spring Framework's transaction infrastructure
code will, by default, only mark a transaction for rollback in
the case of runtime, unchecked exceptions; that is, when the thrown exception is an
instance or subclass of RuntimeException
.
(Errors
will also - by default - result in a rollback.) Checked
exceptions that are thrown from a transactional method will
not result in the transaction being rolled back.
Exactly which Exception
types mark a transaction
for rollback can be configured. Find below a snippet of XML configuration that
demonstrates how one would configure rollback for a checked, application-specific
Exception
type.
<tx:advice id="txAdvice" transaction-manager="txManager">
<tx:attributes>
<tx:method name="get*" read-only="false" rollback-for="NoProductInStockException"/>
<tx:method name="*"/>
</tx:attributes>
</tx:advice>
The second way to indicate to the transaction infrastructure that a rollback is required is to do so programmatically. Although very simple, this way is quite invasive, and tightly couples your code to the Spring Framework's transaction infrastructure. Find below a snippet of code that does programmatic rollback of a Spring Framework-managed transaction:
public void resolvePosition() { try { // some business logic... } catch (NoProductInStockException ex) { // trigger rollback programmatically TransactionAspectSupport.currentTransactionStatus().setRollbackOnly(); } }
You are strongly encouraged to use the declarative approach to rollback if at all possible. Programmatic rollback is available should you need it, but its usage flies in the face of achieving a clean POJO-based application model.
Consider the scenario where you have a number of service layer objects,
and you want to apply totally different transactional configuration
to each of them. This is achieved by defining distinct <aop:advisor/>
elements with differing 'pointcut'
and 'advice-ref'
attribute values.
Let's assume that all of your service layer classes are defined in a root
'x.y.service'
package. To make all beans that are instances of classes
defined in that package (or in subpackages) and that have names ending in
'Service'
have the default transactional configuration, you would write
the following:
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:aop="http://www.springframework.org/schema/aop"
xmlns:tx="http://www.springframework.org/schema/tx"
xsi:schemaLocation="
http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.0.xsd
http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-2.0.xsd
http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-2.0.xsd">
<aop:config>
<aop:pointcut id="serviceOperation"
expression="execution(* x.y.service..*Service.*(..))"/>
<aop:advisor pointcut-ref="serviceOperation" advice-ref="txAdvice"/>
</aop:config>
<!-- these two beans will be transactional... -->
<bean id="fooService" class="x.y.service.DefaultFooService"/>
<bean id="barService" class="x.y.service.extras.SimpleBarService"/>
<!-- ... and these two beans won't -->
<bean id="anotherService" class="org.xyz.SomeService"/> <!-- (not in the right package) -->
<bean id="barManager" class="x.y.service.SimpleBarManager"/> <!-- (doesn't end in 'Service') -->
<tx:advice id="txAdvice">
<tx:attributes>
<tx:method name="get*" read-only="true"/>
<tx:method name="*"/>
</tx:attributes>
</tx:advice>
<!-- other transaction infrastructure beans such as a PlatformTransactionManager
omitted... -->
</beans>
Find below an example of configuring two distinct beans with totally different transactional settings.
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:aop="http://www.springframework.org/schema/aop" xmlns:tx="http://www.springframework.org/schema/tx" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.0.xsd http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-2.0.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-2.0.xsd"> <aop:config> <aop:pointcut id="defaultServiceOperation" expression="execution(* x.y.service.*Service.*(..))"/> <aop:pointcut id="noTxServiceOperation" expression="execution(* x.y.service.ddl.DefaultDdlManager.*(..))"/> <aop:advisor pointcut-ref="defaultServiceOperation" advice-ref="defaultTxAdvice"/> <aop:advisor pointcut-ref="noTxServiceOperation" advice-ref="noTxAdvice"/> </aop:config> <!-- this bean will be transactional (see the'defaultServiceOperation'
pointcut) --> <bean id="fooService" class="x.y.service.DefaultFooService"/> <!-- this bean will also be transactional, but with totally different transactional settings --> <bean id="anotherFooService" class="x.y.service.ddl.DefaultDdlManager"/> <tx:advice id="defaultTxAdvice"> <tx:attributes> <tx:method name="get*" read-only="true"/> <tx:method name="*"/> </tx:attributes> </tx:advice> <tx:advice id="noTxAdvice"> <tx:attributes> <tx:method name="*" propagation="NEVER"/> </tx:attributes> </tx:advice> <!-- other transaction infrastructure beans such as aPlatformTransactionManager
omitted... --> </beans>
This section summarises the various transactional settings that can be specified
using the <tx:advice/>
tag. The default
<tx:advice/>
settings are:
The propagation setting is REQUIRED
The isolation level is DEFAULT
The transaction is read/write
The transaction timeout defaults to the default timeout of the underlying transaction system, or or none if timeouts are not supported
Any RuntimeException
will trigger
rollback, and any checked Exception
will not
These default settings can, of course, be changed; the various
attributes of the <tx:method/>
tags that are nested within
<tx:advice/>
and <tx:attributes/>
tags are summarized below:
Table 9.1. <tx:method/>
settings
Attribute | Required? | Default | Description |
---|---|---|---|
name | Yes |
The method name(s) with which the transaction attributes
are to be associated. The wildcard (*) character can be used to
associate the same transaction attribute settings with a number
of methods; for example, | |
propagation | No | REQUIRED | The transaction propagation behavior |
isolation | No | DEFAULT | The transaction isolation level |
timeout | No | -1 | The transaction timeout value (in seconds) |
read-only | No | false | Is this transaction read-only? |
rollback-for | No |
The | |
no-rollback-for | No |
The |
At the time of writing it is not possible to have explicit control over the
name of a transaction, where 'name' means the transaction name that will be shown
in a transaction monitor, if applicable (for example, WebLogic's transaction
monitor), and in logging output. For declarative transactions, the transaction
name is always the fully-qualified class name + "." + method name of the
transactionally-advised class. For example
'com.foo.BusinessService.handlePayment'
.
![]() | Note |
---|---|
The functionality offered by the |
In addition to the XML-based declarative approach to transaction configuration, you can also use an annotation-based approach to transaction configuration. Declaring transaction semantics directly in the Java source code puts the declarations much closer to the affected code, and there is generally not much danger of undue coupling, since code that is meant to be used transactionally is almost always deployed that way anyway.
The ease-of-use afforded by the use of the @Transactional
annotation is best illustrated with an example, after which all of the details
will be explained. Consider the following class definition:
// the service class that we want to make transactional @Transactional public class DefaultFooService implements FooService { Foo getFoo(String fooName); Foo getFoo(String fooName, String barName); void insertFoo(Foo foo); void updateFoo(Foo foo); }
When the above POJO is defined as a bean in a Spring IoC container, the bean instance can be made transactional by adding merely one line of XML configuration, like so:
<!-- from the file'context.xml'
--> <?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:aop="http://www.springframework.org/schema/aop" xmlns:tx="http://www.springframework.org/schema/tx" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.0.xsd http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-2.0.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-2.0.xsd"> <!-- this is the service object that we want to make transactional --> <bean id="fooService" class="x.y.service.DefaultFooService"/> <!-- enable the configuration of transactional behavior based on annotations --> <tx:annotation-driven transaction-manager="txManager"/> <!-- aPlatformTransactionManager
is still required --> <bean id="txManager" class="org.springframework.jdbc.datasource.DataSourceTransactionManager"> <!-- (this dependency is defined somewhere else) --> <property name="dataSource" ref="dataSource"/> </bean> <!-- other<bean/>
definitions here --> </beans>
![]() | Tip |
---|---|
You can actually omit the |
The @Transactional
annotation may be placed
before an interface definition, a method on an interface, a class definition, or a
public method on a class. However, please note that the mere
presence of the @Transactional
annotation is not
enough to actually turn on the transactional behavior - the
@Transactional
annotation is simply metadata
that can be consumed by something that is @Transactional
-aware
and that can use the metadata to configure the appropriate beans with transactional
behavior. In the case of the above example, it is the presence of the
<tx:annotation-driven/>
element that
switches on the transactional behavior.
The Spring team's recommendation is that you only annotate concrete
classes with the @Transactional
annotation,
as opposed to annotating interfaces. You certainly can place the
@Transactional
annotation on an interface (or
an interface method), but this will only work as you would expect it to if
you are using interface-based proxies. The fact that annotations are
not inherited means that if you are using class-based
proxies then the transaction settings will not be recognised by the
class-based proxying infrastructure and the object will not be wrapped in a
transactional proxy (which would be decidedly bad).
So please do take the Spring team's advice and only annotate concrete classes
(and the methods of concrete classes) with the
@Transactional
annotation.
Table 9.2. <tx:annotation-driven/>
settings
Attribute | Required? | Default | Description |
---|---|---|---|
transaction-manager | No | transactionManager |
The name of transaction manager to use. Only required
if the name of the transaction manager is not |
proxy-target-class | No |
Controls
what type of transactional proxies are created for classes annotated
with the | |
order | No | Defines the order of the transaction advice that will be applied to
beans annotated with |
![]() | Note |
---|---|
The " |
The most derived location takes precedence when evaluating the transactional
settings for a method. In the case of the following example, the
DefaultFooService
class is annotated at the class level
with the settings for a read-only transaction, but the
@Transactional
annotation on the
updateFoo(Foo)
method in the same class takes precedence
over the transactional settings defined at the class level.
@Transactional(readOnly = true) public class DefaultFooService implements FooService { public Foo getFoo(String fooName) { // do something } // these settings have precedence for this method @Transactional(readOnly = false, propagation = Propagation.REQUIRES_NEW) public void updateFoo(Foo foo) { // do something } }
The @Transactional
annotation is
metadata that specifies that an interface, class, or method must have
transactional semantics; for example, “start a brand new read-only
transaction when this method is invoked, suspending any existing
transaction”. The default
@Transactional
settings are:
The propagation setting is PROPAGATION_REQUIRED
The isolation level is ISOLATION_DEFAULT
The transaction is read/write
The transaction timeout defaults to the default timeout of the underlying transaction system, or or none if timeouts are not supported
Any RuntimeException
will trigger
rollback, and any checked Exception
will not
These default settings can, of course, be changed; the various
properties of the @Transactional
annotation
are summarized in the following table:
Table 9.3. @Transactional
properties
Property | Type | Description |
---|---|---|
propagation
| enum: Propagation | optional propagation setting |
isolation
| enum: Isolation | optional isolation level |
readOnly
| boolean | read/write vs. read-only transaction |
timeout
| int (in seconds granularity) | the transaction timeout |
rollbackFor
| an array of Class objects, which
must be derived from Throwable | an optional array of exception classes which must cause rollback |
rollbackForClassname
| an array of class names. Classes
must be derived from Throwable | an optional array of names of exception classes that must cause rollback |
noRollbackFor
| an array of Class objects, which
must be derived from Throwable | an optional array of exception classes that must not cause rollback. |
noRollbackForClassname
| an array of String class names, which
must be derived from Throwable | an optional array of names of exception classes that must not cause rollback |
At the time of writing it is not possible to have explicit
control over the name of a transaction, where 'name' means the
transaction name that will be shown in a transaction monitor, if
applicable (for example, WebLogic's transaction monitor), and in
logging output. For declarative transactions, the transaction
name is always the fully-qualified class name + "." + method name
of the transactionally-advised class. For example
'com.foo.BusinessService.handlePayment'
.
Consider the situation where you have an instance of a class, and
you would like to execute both transactional
and (to keep things simple) some basic profiling
advice. So how do you effect this in the context of using
<tx:annotation-driven/>
?
What we want to see when we invoke the updateFoo(Foo)
method is:
the configured profiling aspect starting up,
then the transactional advice executing,
then the method on the advised object executing
then the transaction committing (we'll assume a sunny day scenario here),
and then finally the profiling aspect reporting (somehow) exactly how long the whole transactional method invocation took
![]() | Note |
---|---|
This chapter is not concerned with explaining AOP in any great detail (except as it applies to transactions). Please see the chapter entitled Chapter 6, Aspect Oriented Programming with Spring for detailed coverage of the various bits and pieces of the following AOP configuration (and AOP in general). |
Here is the code for a simple profiling aspect. (The
ordering of advice is controlled via the Ordered
interface. For full details on advice ordering, see the section entitled
Section 6.2.4.7, “Advice ordering”.)
package x.y; import org.aspectj.lang.ProceedingJoinPoint; import org.springframework.util.StopWatch; import org.springframework.core.Ordered; public class SimpleProfiler implements Ordered { private int order; // allows us to control the ordering of advice public int getOrder() { return this.order; } public void setOrder(int order) { this.order = order; } // this method is the around advice public Object profile(ProceedingJoinPoint call) throws Throwable { Object returnValue; StopWatch clock = new StopWatch(getClass().getName()); try { clock.start(call.toShortString()); returnValue = call.proceed(); } finally { clock.stop(); System.out.println(clock.prettyPrint()); } return returnValue; } }
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:aop="http://www.springframework.org/schema/aop" xmlns:tx="http://www.springframework.org/schema/tx" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.0.xsd http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-2.0.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-2.0.xsd"> <bean id="fooService" class="x.y.service.DefaultFooService"/> <!-- this is the aspect --> <bean id="profiler" class="x.y.SimpleProfiler"> <!-- execute before the transactional advice (hence the lower order number) --> <property name="order" value="1"/> </bean> <tx:annotation-driven transaction-manager="txManager"/> <aop:config> <!-- this advice will execute around the transactional advice --> <aop:aspect id="profilingAspect" ref="profiler"> <aop:pointcut id="serviceMethodWithReturnValue" expression="execution(!void x.y..*Service.*(..))"/> <aop:around method="profile" pointcut-ref="serviceMethodWithReturnValue"/> </aop:aspect> </aop:config> <bean id="dataSource" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close"> <property name="driverClassName" value="oracle.jdbc.driver.OracleDriver"/> <property name="url" value="jdbc:oracle:thin:@rj-t42:1521:elvis"/> <property name="username" value="scott"/> <property name="password" value="tiger"/> </bean> <bean id="txManager" class="org.springframework.jdbc.datasource.DataSourceTransactionManager"> <property name="dataSource" ref="dataSource"/> </bean> </beans>
The result of the above configuration will be a 'fooService'
bean that has profiling and transactional aspects applied to it
in that order. The configuration of any number of additional
aspects is effected in a similar fashion.
Finally, find below some example configuration for effecting the same setup as above, but using the purely XML declarative approach.
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:aop="http://www.springframework.org/schema/aop" xmlns:tx="http://www.springframework.org/schema/tx" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.0.xsd http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-2.0.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-2.0.xsd"> <bean id="fooService" class="x.y.service.DefaultFooService"/> <!-- the profiling advice --> <bean id="profiler" class="x.y.SimpleProfiler"> <!-- execute before the transactional advice (hence the lower order number) --> <property name="order" value="1"/> </bean> <aop:config> <aop:pointcut id="entryPointMethod" expression="execution(* x.y..*Service.*(..))"/> <!-- will execute after the profiling advice (c.f. the order attribute) --> <aop:advisor advice-ref="txAdvice" pointcut-ref="entryPointMethod" order="2"/> <!-- order value is higher than the profiling aspect --> <aop:aspect id="profilingAspect" ref="profiler"> <aop:pointcut id="serviceMethodWithReturnValue" expression="execution(!void x.y..*Service.*(..))"/> <aop:around method="profile" pointcut-ref="serviceMethodWithReturnValue"/> </aop:aspect> </aop:config> <tx:advice id="txAdvice" transaction-manager="txManager"> <tx:attributes> <tx:method name="get*" read-only="true"/> <tx:method name="*"/> </tx:attributes> </tx:advice> <!-- other <bean/> definitions such as aDataSource
and aPlatformTransactionManager
here --> </beans>
The result of the above configuration will be a 'fooService'
bean that has profiling and transactional aspects applied to it
in that order. If we wanted the profiling advice to execute
after the transactional advice on the way in, and
before the transactional advice on the way out, then we would
simply swap the value of the profiling aspect bean's 'order'
property such that it was higher than the transactional advice's order value.
The configuration of any number of additional aspects is effected in a similar fashion.
It is also possible to use the Spring Framework's
@Transactional
support outside of a Spring container
by means of an AspectJ aspect. To use this support you must first
annotate your classes (and optionally your classes' methods with the
@Transactional
annotation, and then you must link
(weave) your application with the
org.springframework.transaction.aspectj.AnnotationTransactionAspect
defined in the spring-aspects.jar
file.
The aspect must also be configured with a transaction manager. You could of course
use the Spring Framework's IoC container to take care of dependency injecting the
aspect, but since we're focusing here on applications running outside of a Spring
container, we'll show you how to do it programmatically.
![]() | Note |
---|---|
Prior to continuing, you may well want to read the previous sections entitled
Section 9.5.6, “Using |
// construct an appropriate transaction manager
DataSourceTransactionManager txManager = new DataSourceTransactionManager(getDataSource());
// configure the AnnotationTransactionAspect
to use it; this must be done before executing any transactional methods
AnnotationTransactionAspect.aspectOf().setTransactionManager(txManager);
![]() | Note |
---|---|
When using this aspect, you must annotate the implementation class (and/or methods within that class), not the interface (if any) that the class implements. AspectJ follows Java's rule that annotations on interfaces are not inherited. |
The @Transactional
annotation on a class specifies
the default transaction semantics for the execution of any public
method in the class.
The @Transactional
annotation on a method within
the class overrides the default transaction semantics given by the class annotation
(if present). Any method may be annotated, regardless of visibility: annotating
non-public methods directly is the only way to get transaction demarcation for the
execution of such methods.
To weave your applications with the AnnotationTransactionAspect
you must either build your application with AspectJ (see the
AspectJ Development Guide)
or use load-time weaving. See the section entitled Section 6.8.4, “Using AspectJ Load-time weaving (LTW) with Spring applications” for a
discussion of load-time weaving with AspectJ.
The Spring Framework provides two means of programmatic transaction management:
Using the TransactionTemplate
.
Using a PlatformTransactionManager
implementation directly.
If you are going to use programmatic transaction management, the Spring
team generally recommend, namely that of using the TransactionTemplate
).
The second approach is similar to using the JTA UserTransaction
API (although exception handling is less cumbersome).
The TransactionTemplate
adopts the same
approach as other Spring templates such as the
JdbcTemplate
. It uses a callback approach, to
free application code from having to do the boilerplate acquisition
and release of transactional resources, and results in code that is
intention driven, in that the code that is written focuses solely
on what the developer wants to do.
![]() | Note |
---|---|
As you will immediately see in the examples that follow, using
the |
Application code that must execute in a transactional context,
and that will use the TransactionTemplate
explicitly,
looks like this. You, as an application developer, will write a
TransactionCallback
implementation (typically
expressed as an anonymous inner class) that will contain all of the code
that you need to have execute in the context of a transaction. You will then
pass an instance of your custom TransactionCallback
to the execute(..)
method exposed on the
TransactionTemplate
.
public class SimpleService implements Service { // singleTransactionTemplate
shared amongst all methods in this instance private final TransactionTemplate transactionTemplate; // use constructor-injection to supply thePlatformTransactionManager
public SimpleService(PlatformTransactionManager transactionManager) { Assert.notNull(transactionManager, "The 'transactionManager' argument must not be null."); this.transactionTemplate = new TransactionTemplate(transactionManager); } public Object someServiceMethod() { return transactionTemplate.execute(new TransactionCallback() { // the code in this method executes in a transactional context public Object doInTransaction(TransactionStatus status) { updateOperation1(); return resultOfUpdateOperation2(); } }); } }
If there is no return value, use the convenient
TransactionCallbackWithoutResult
class via an
anonymous class like so:
transactionTemplate.execute(new TransactionCallbackWithoutResult() {
protected void doInTransactionWithoutResult(TransactionStatus status) {
updateOperation1();
updateOperation2();
}
});
Code within the callback can roll the transaction back by calling
the setRollbackOnly()
method on the supplied
TransactionStatus
object.
transactionTemplate.execute(new TransactionCallbackWithoutResult() {
protected void doInTransactionWithoutResult(TransactionStatus status) {
try {
updateOperation1();
updateOperation2();
} catch (SomeBusinessExeption ex) {
status.setRollbackOnly();
}
}
});
Transaction settings such as the propagation mode, the
isolation level, the timeout, and so forth can be set
on the TransactionTemplate
either
programmatically or in configuration. TransactionTemplate
instances by default have the default transactional settings.
Find below an example of programmatically customizing the
transactional settings for a specific TransactionTemplate
.
public class SimpleService implements Service { private final TransactionTemplate transactionTemplate; public SimpleService(PlatformTransactionManager transactionManager) { Assert.notNull(transactionManager, "The 'transactionManager' argument must not be null."); this.transactionTemplate = new TransactionTemplate(transactionManager); // the transaction settings can be set here explicitly if so desired this.transactionTemplate.setIsolationLevel(TransactionDefinition.ISOLATION_READ_UNCOMMITTED); this.transactionTemplate.setTimeout(30); // 30 seconds // and so forth... } }
Find below an example of defining a TransactionTemplate
with some custom
transactional settings, using Spring XML configuration. The 'sharedTransactionTemplate
'
can then be injected into as many services as are required.
<bean id="sharedTransactionTemplate" class="org.springframework.transaction.support.TransactionTemplate"> <property name="isolationLevelName" value="ISOLATION_READ_UNCOMMITTED"/> <property name="timeout" value="30"/> </bean>"
Finally, instances of the TransactionTemplate
class are
threadsafe, in that instances do not maintain any conversational state.
TransactionTemplate
instances do
however maintain configuration state, so while a number of classes
may choose to share a single instance of a TransactionTemplate
, if a class needed
to use a TransactionTemplate
with different settings
(for example, a different isolation level), then two distinct
TransactionTemplate
instances would need to be
created and used.
You can also use the
org.springframework.transaction.PlatformTransactionManager
directly to manage your transaction. Simply pass the implementation of
the PlatformTransactionManager
you're
using to your bean via a bean reference. Then, using the
TransactionDefinition
and
TransactionStatus
objects you can
initiate transactions, rollback and commit.
DefaultTransactionDefinition def = new DefaultTransactionDefinition(); // explicitly setting the transaction name is something that can only be done programmatically def.setName("SomeTxName"); def.setPropagationBehavior(TransactionDefinition.PROPAGATION_REQUIRED); TransactionStatus status = txManager.getTransaction(def); try { // execute your business logic here } catch (MyException ex) { txManager.rollback(status); throw ex; } txManager.commit(status);
Programmatic transaction management is usually a good idea only if
you have a small number of transactional operations. For example, if you
have a web application that require transactions only for certain update
operations, you may not want to set up transactional proxies using Spring
or any other technology. In this case, using the TransactionTemplate
may be a good approach. Being able to set the transaction name
explicitly is also something that can only be done using the programmatic
approach to transaction management.
On the other hand, if your application has numerous transactional operations, declarative transaction management is usually worthwhile. It keeps transaction management out of business logic, and is not difficult to configure. When using the Spring Framework, rather than EJB CMT, the configuration cost of declarative transaction management is greatly reduced.
Spring's transaction abstraction is generally application server
agnostic. Additionally, Spring's JtaTransactionManager
class,
which can optionally perform a JNDI lookup for the JTA
UserTransaction
and
TransactionManager
objects, can be set to autodetect
the location for the latter object, which varies by application server. Having
access to the TransactionManager
instance does allow
enhanced transaction semantics. Please see the
JtaTransactionManager
Javadocs for more details.
In a WebLogic 7.0, 8.1 or higher environment, you will generally
prefer to use WebLogicJtaTransactionManager
instead
of the stock JtaTransactionManager
class. This
special WebLogic-specific subclass of the normal
JtaTransactionManager
. It supports the full power of
Spring's transaction definitions in a WebLogic managed transaction
environment, beyond standard JTA semantics: features include transaction
names, per-transaction isolation levels, and proper resuming of
transactions in all cases.
In a WebSphere 5.1, 5.0 and 4 environment, you may wish to use
Spring's WebSphereTransactionManagerFactoryBean
class. This is a factory bean which retrieves the JTA
TransactionManager
in a WebSphere environment, which
is done via WebSphere's static
access methods.
(These methods are different for each version of WebSphere.)
Once the JTA TransactionManager
instance has
been obtained via this factory bean, Spring's
JtaTransactionManager
may be configured with a
reference to it, for enhanced transaction semantics over the use of only
the JTA UserTransaction
object.
Please see the Javadocs for full details.
You should take care to use the correct
PlatformTransactionManager
implementation for their requirements. It is important to understand
how the the Spring Framework's transaction abstraction works with JTA global
transactions. Used properly, there is no conflict here: the Spring Framework
merely provides a straightforward and portable abstraction. If you are using
global transactions, you must use the
org.springframework.transaction.jta.JtaTransactionManager
class
(or an
application server-specific subclass
of it) for all your transactional operations. Otherwise the transaction infrastructure
will attempt to perform local transactions on resources such as container
DataSource
instances. Such local transactions
don't make sense, and a good application server will treat them as errors.
Find below links to further resources about the Spring Framework's transaction support.
This book, Java Transaction Design Strategies from InfoQ is a well-paced introduction to transactions in the Java space. It also includes side-by-side examples of how to configure and use transactions in both the Spring Framework and EJB3.
The Data Access Object (DAO) support in Spring is primarily aimed at making it easy to work with data access technologies like JDBC, Hibernate or JDO in a standardized way. This allows one to switch between the aforementioned persistence technologies fairly easily and it also allows one to code without worrying about catching exceptions that are specific to each technology.
Spring provides a convenient translation from technology specific
exceptions like SQLException
to its own exception
hierarchy with the DataAccessException
as the root
exception. These exceptions wrap the original exception so there is never
any risk that one might lose any information as to what might have gone
wrong.
In addition to JDBC exceptions, Spring can also wrap Hibernate-specific exceptions, converting them from proprietary, checked exceptions (in the case of versions of Hibernate prior to Hibernate 3.0), to a set of abstracted runtime exceptions (the same is true for JDO exceptions). This allows one to handle most persistence exceptions, which are non-recoverable, only in the appropriate layers, without annoying boilerplate catch and throw blocks, and exception declarations. (One can still trap and handle exceptions anywhere one needs to though.) As mentioned above, JDBC exceptions (including database-specific dialects) are also converted to the same hierarchy, meaning that one can perform some operations with JDBC within a consistent programming model.
The above holds true for the various template-based versions of the ORM access
framework. If one uses the interceptor-based classes then the application
must care about handling HibernateException
s and
JDOException
s itself, preferably via delegating to
SessionFactoryUtils
'
convertHibernateAccessException
or
convertJdoAccessException
methods respectively. These
methods convert the exceptions to ones that are compatible with the
org.springframework.dao
exception hierarchy. As
JDOException
s are unchecked, they can simply get thrown
too, sacrificing generic DAO abstraction in terms of exceptions though.
The exception hierarchy that Spring uses is outlined in the following image:
(Please note that the class hierarchy detailed in the above image shows only
a subset of the whole, rich, DataAccessException
hierarchy.)
To make it easier to work with a variety of data access technologies such as JDBC, JDO and Hibernate in a consistent way, Spring provides a set of abstract DAO classes that one can extend. These abstract classes have methods for providing the data source and any other configuration settings that are specific to the technology one currently is using.
Dao support classes:
JdbcDaoSupport
- super class for JDBC data
access objects. Requires a DataSource
to be provided; in turn, this class provides a
JdbcTemplate
instance initialized from the
supplied DataSource
to subclasses.
HibernateDaoSupport
- super class for
Hibernate data access objects. Requires a
SessionFactory
to be provided;
in turn, this class provides a
HibernateTemplate
instance initialized from the
supplied SessionFactory
to subclasses. Can
alternatively be initialized directly via a
HibernateTemplate
, to reuse the latter's settings
like SessionFactory
, flush mode, exception
translator, etc.
JdoDaoSupport
- super class for JDBC data
access objects. Requires a PersistenceManagerFactory
to be provided; in turn, this class provides a
JdoTemplate
instance initialized from the
supplied PersistenceManagerFactory
to
subclasses.
JpaDaoSupport
- super class for JPA data
access objects. Requires a EntityManagerFactory
to be provided; in turn, this class provides a
JpaTemplate
instance initialized from the
supplied EntityManagerFactory
to
subclasses.
The value-add provided by the Spring Framework's JDBC abstraction framework is perhaps best shown by the following list (note that only the italicized lines need to be coded by an application developer):
Define connection parameters
Open the connection
Specify the statement
Prepare and execute the statement
Set up the loop to iterate through the results (if any)
Do the work for each iteration
Process any exception
Handle transactions
Close the connection
The Spring Framework takes care of all the grungy, low-level details that can make JDBC such a tedious API to develop with.
The Spring Framework's JDBC abstraction framework consists of four
different packages, namely core
,
dataSource
, object
, and
support
.
The org.springframework.jdbc.core
package
contains the JdbcTemplate
class and its various
callback interfaces, plus a variety of related classes.
The org.springframework.jdbc.datasource
package
contains a utility class for easy DataSource
access, and various simple DataSource
implementations that can be used for testing and running
unmodified JDBC code outside of a J2EE container. The utility class
provides static methods to obtain connections from JNDI and to close
connections if necessary. It has support for thread-bound connections,
e.g. for use with DataSourceTransactionManager
.
Next, the org.springframework.jdbc.object
package
contains classes that represent RDBMS queries, updates, and stored
procedures as thread safe, reusable objects. This approach is modeled by
JDO, although of course objects returned by queries are
“disconnected” from the database. This higher level of JDBC
abstraction depends on the lower-level abstraction in the
org.springframework.jdbc.core
package.
Finally the org.springframework.jdbc.support
package is where you find the SQLException
translation
functionality and some utility classes.
Exceptions thrown during JDBC processing are translated to
exceptions defined in the org.springframework.dao
package. This means that code using the Spring JDBC abstraction layer does
not need to implement JDBC or RDBMS-specific error handling. All
translated exceptions are unchecked giving you the option of catching the
exceptions that you can recover from while allowing other exceptions to be
propagated to the caller.
The JdbcTemplate
class is the central class in
the JDBC core package. It simplifies the use of JDBC since it handles the
creation and release of resources. This helps to avoid common errors such
as forgetting to always close the connection. It executes the core JDBC
workflow like statement creation and execution, leaving application code
to provide SQL and extract results. This class executes SQL queries, update
statements or stored procedure calls, imitating iteration over
ResultSet
s and extraction of returned
parameter values. It also catches JDBC exceptions and
translates them to the generic, more informative, exception hierarchy
defined in the org.springframework.dao
package.
Code using the JdbcTemplate
only need to implement
callback interfaces, giving them a clearly defined contract. The
PreparedStatementCreator
callback
interface creates a prepared statement given a
Connection
provided by this class,
providing SQL and any necessary parameters. The same is true for the
CallableStatementCreator
interface
which creates callable statement. The
RowCallbackHandler
interface extracts
values from each row of a ResultSet
.
The JdbcTemplate
can be used within a DAO
implementation via direct instantiation with a DataSource
reference, or be configured in a Spring IOC container and given to DAOs as a
bean reference. Note: the DataSource
should
always be configured as a bean in the Spring IoC container, in the first
case given to the service directly, in the second case to the prepared
template.
Finally, all of the SQL issued by this class is logged at the
'DEBUG'
level under the category corresponding
to the fully qualified class name of the template instance (typically
JdbcTemplate
, but it may be different if a custom
subclass of the JdbcTemplate
class is being used).
Find below some examples of using the JdbcTemplate
class. (These examples are not an exhaustive list of all of the functionality
exposed by the JdbcTemplate
; see the attendant Javadocs for that).
A simple query for getting the number of rows in a relation.
int rowCount = this.jdbcTemplate.queryForInt("select count(0) from t_accrual");
A simple query using a bind variable.
int countOfActorsNamedJoe = this.jdbcTemplate.queryForInt("select count(0) from t_actors where first_name = ?", new Object[]{"Joe"});
Querying for a String
.
String surname = (String) this.jdbcTemplate .queryForObject("select surname from t_actor where id = ?", new Object[]{new Long(1212)}, String.class);
Querying and populating a single domain object.
Actor actor = (Actor) this.jdbcTemplate.queryForObject( "select first_name, surname from t_actor where id = ?", new Object[]{new Long(1212)}, new RowMapper() { public Object mapRow(ResultSet rs, int rowNum) throws SQLException { Actor actor = new Actor(); actor.setFirstName(rs.getString("first_name")); actor.setSurname(rs.getString("surname")); return actor; } });
Querying and populating a number of domain objects.
Collection actors = this.jdbcTemplate.query( "select first_name, surname from t_actor", new RowMapper() { public Object mapRow(ResultSet rs, int rowNum) throws SQLException { Actor actor = new Actor(); actor.setFirstName(rs.getString("first_name")); actor.setSurname(rs.getString("surname")); return actor; } });
If the last two snippets of code actually existed in the same application,
it would make sense to remove the duplication present in the two
RowMapper
anonymous inner classes,
and extract them out into a single class (typically a static
inner class) that can then be referenced by DAO methods as needed. For example,
the last code snippet might be better off written like so:
public Collection findAllActors() { return this.jdbcTemplate.query( "select first_name, surname from t_actor", new ActorMapper()); } private static final class ActorMapper implements RowMapper { public Object mapRow(ResultSet rs, int rowNum) throws SQLException { Actor actor = new Actor(); actor.setFirstName(rs.getString("first_name")); actor.setSurname(rs.getString("surname")); return actor; } }
this.jdbcTemplate.update("insert into t_actor (first_name, surname) values (?, ?)", new Object[] {"Leonor", "Watling"});
this.jdbcTemplate.update("update t_actor set weapon = ? where id = ?", new Object[] {"Banjo", new Long(5276)});
this.jdbcTemplate.update("delete from orders"); // :)
The execute(..)
method can be used to execute any
arbitrary SQL, and as such is often used for DDL statements. It is heavily overloaded
with variants taking callback interfaces, bind variable arrays, and suchlike.
this.jdbcTemplate.execute("create table mytable (id integer, name varchar(100))");
Invoking a simple stored procedure (more sophisticated stored procedure support is covered later).
this.jdbcTemplate.update("call SUPPORT.REFRESH_ACTORS_SUMMARY(?)", new Object[]{new Long(unionId)});
Instances of the JdbcTemplate
class are
threadsafe once configured. This is important because it
means that you can configure a single instance of a JdbcTemplate
and then safely inject this shared reference into multiple
DAOs (or repositories). To be clear, the JdbcTemplate
is stateful, in that it maintains a reference to a DataSource
,
but this state is not conversational state.
A common idiom when using the JdbcTemplate
class
(and the associated SimpleJdbcTemplate
and NamedParameterJdbcTemplate
classes)
is to configure a DataSource
in your Spring configuration file,
and then dependency inject that shared DataSource
bean
into your DAO classes; the JdbcTemplate
is created in the setter
for the DataSource
. This leads to DAOs that look
in part like this:
public class JdbcCorporateEventDao implements CorporateEventDao {
private JdbcTemplate jdbcTemplate;
public void setDataSource(DataSource dataSource) {
this.jdbcTemplate = new JdbcTemplate(dataSource);
}
// JDBC-backed implementations of the methods on the CorporateEventDao
follow...
}
The attendant configuration might look like this.
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans/spring-beans-2.0.xsd">
<bean id="corporateEventDao" class="com.example.JdbcCorporateEventDao">
<property name="dataSource" ref="dataSource"/>
</bean>
<!-- the DataSource
(parameterized for configuration via a PropertyPlaceHolderConfigurer
) -->
<bean id="dataSource" destroy-method="close" class="org.apache.commons.dbcp.BasicDataSource">
<property name="driverClassName" value="${jdbc.driverClassName}"/>
<property name="url" value="${jdbc.url}"/>
<property name="username" value="${jdbc.username}"/>
<property name="password" value="${jdbc.password}"/>
</bean>
</beans>
If you are using Spring's JdbcDaoSupport
class, and
your various JDBC-backed DAO classes extend from it, then you inherit a
setDataSource(..)
method for free from said superclass. It is
totally up to you as to whether or not you inherit from said class, you certainly are
not forced to. If you look at the source for the JdbcDaoSupport
class you will see that there is not a whole lot to it... it is provided as a convenience
only.
Regardless of which of the above template initialization styles you choose
to use (or not), there is (almost) certainly no need to create a brand new
instance of a JdbcTemplate
class each and every time you wish
to execute some SQL... remember, once configured, a JdbcTemplate
instance is threadsafe. A reason for wanting multiple JdbcTemplate
instances would be when you have an application that accesses multiple databases, which
requires multiple DataSources
, and subsequently
multiple differently configured JdbcTemplates
.
The NamedParameterJdbcTemplate
class adds support
for programming JDBC statements using named parameters (as opposed to
programming JDBC statements using only classic placeholder
('?'
) arguments. The NamedParameterJdbcTemplate
class wraps
a JdbcTemplate
, and delegates to the
wrapped JdbcTemplate
to do much of its work.
This section will describe only those areas of the
NamedParameterJdbcTemplate
class that differ from
the JdbcTemplate
itself; namely, programming
JDBC statements using named parameters.
// some JDBC-backed DAO class...
private NamedParameterJdbcTemplate namedParameterJdbcTemplate;
public void setDataSource(DataSource dataSource) {
this.namedParameterJdbcTemplate = new NamedParameterJdbcTemplate(dataSource);
}
public int countOfActorsByFirstName(String firstName) {
String sql = "select count(0) from T_ACTOR where first_name = :first_name";
SqlParameterSource namedParameters = new MapSqlParameterSource("first_name", firstName);
return namedParameterJdbcTemplate.queryForInt(sql, namedParameters);
}
Notice the use of the named parameter notation in the value assigned
to the 'sql'
variable, and the corresponding value
that is plugged into the 'namedParameters'
variable
(of type MapSqlParameterSource
).
If you like, you can also pass along named parameters (and their
corresponding values) to a NamedParameterJdbcTemplate
instance using the (perhaps more familiar)
Map
-based style.
(The rest of the methods exposed by the
NamedParameterJdbcOperations
- and implemented by the NamedParameterJdbcTemplate
class)
follow a similar pattern and will not be covered here.)
// some JDBC-backed DAO class...
private NamedParameterJdbcTemplate namedParameterJdbcTemplate;
public void setDataSource(DataSource dataSource) {
this.namedParameterJdbcTemplate = new NamedParameterJdbcTemplate(dataSource);
}
public int countOfActorsByFirstName(String firstName) {
String sql = "select count(0) from T_ACTOR where first_name = :first_name";
Map namedParameters = Collections.singletonMap("first_name", firstName);
return this.namedParameterJdbcTemplate.queryForInt(sql, namedParameters);
}
Another nice feature related to the NamedParameterJdbcTemplate
(and existing in the same Java package) is the SqlParameterSource
interface. You have already seen an example of an implementation of this
interface in one of the preceding code snippets (the MapSqlParameterSource
class). The entire point of the SqlParameterSource
is to serve as
a source of named parameter values to a NamedParameterJdbcTemplate
.
The MapSqlParameterSource
class is a very simple implementation, that
is simply an adapter around a java.util.Map
, where the keys
are the paramter names and the values are the parameter values.
Another SqlParameterSource
implementation is the
BeanPropertySqlParameterSource
class. This class wraps an arbitrary
JavaBean (that is, an instance of a class that adheres to
the JavaBean conventions),
and uses the properties of the wrapped JavaBean as the source of named parameter values.
public class Actor {
private Long id;
private String firstName;
private String lastName;
public String getFirstName() {
return this.firstName;
}
public String getLastName() {
return this.lastName;
}
public Long getId() {
return this.id;
}
// setters omitted...
}
// some JDBC-backed DAO class...
private NamedParameterJdbcTemplate namedParameterJdbcTemplate;
public void setDataSource(DataSource dataSource) {
this.namedParameterJdbcTemplate = new NamedParameterJdbcTemplate(dataSource);
}
public int countOfActors(Actor exampleActor) {
// notice how the named parameters match the properties of the above 'Actor
' class
String sql = "select count(0) from T_ACTOR where first_name = :firstName and last_name = :lastName";
SqlParameterSource namedParameters = new BeanPropertySqlParameterSource(exampleActor);
return this.namedParameterJdbcTemplate.queryForInt(sql, namedParameters);
}
Remember that the NamedParameterJdbcTemplate
class
wraps a classic JdbcTemplate
template; if you need access to the wrapped JdbcTemplate
instance
(to access some of the functionality only present in the JdbcTemplate
class), then you can use the getJdbcOperations()
method to access the wrapped JdbcTemplate
via the JdbcOperations
interface.
See also the section entitled Section 11.2.1.2, “JdbcTemplate
idioms (best practices)” for some
advice on how to best use the NamedParameterJdbcTemplate
class
in the context of an application.
![]() | Note |
---|---|
The functionality offered by the |
The SimpleJdbcTemplate
class is a wrapper around
the classic JdbcTemplate
that takes advantage
of Java 5 language features such as varargs and autoboxing. The
SimpleJdbcTemplate
class is somewhat of a sop
to the syntactic-sugar-like features of Java 5, but as anyone who has
developed on Java 5 and then had to move back to developing on a
previous version of the JDK will know, those syntactic-sugar-like features sure are nice.
The value-add of the SimpleJdbcTemplate
class
in the area of syntactic-sugar is best illustrated with a
'before and after' example. The following code
snippet shows first some data access code using the classic
JdbcTemplate
, followed immediately thereafter
by a code snippet that does the same job, only this time using the
SimpleJdbcTemplate
.
// classic JdbcTemplate
-style...
private JdbcTemplate jdbcTemplate;
public void setDataSource(DataSource dataSource) {
this.jdbcTemplate = new JdbcTemplate(dataSource);
}
public Actor findActor(long id) {
String sql = "select id, first_name, last_name from T_ACTOR where id = ?";
RowMapper mapper = new RowMapper() {
public Object mapRow(ResultSet rs, int rowNum) throws SQLException {
Actor actor = new Actor();
actor.setId(rs.getLong("id"));
actor.setFirstName(rs.getString("first_name"));
actor.setLastName(rs.getString("last_name"));
return actor;
}
};
// notice the cast, the wrapping up of the 'id' argument
// in an array, and the boxing of the 'id' argument as a reference type
return (Actor) jdbcTemplate.queryForObject(sql, mapper, new Object[] {Long.valueOf(id)});
}
Here is the same method, only this time using the SimpleJdbcTemplate
;
notice how much 'cleaner'the code is.
// SimpleJdbcTemplate
-style...
private SimpleJdbcTemplate simpleJdbcTemplate;
public void setDataSource(DataSource dataSource) {
this.simpleJdbcTemplate = new SimpleJdbcTemplate(dataSource);
}
public Actor findActor(long id) {
String sql = "select id, first_name, last_name from T_ACTOR where id = ?";
ParameterizedRowMapper<Actor> mapper = new ParameterizedRowMapper<Actor>() {
// notice the return type with respect to Java 5 covariant return types
public Actor mapRow(ResultSet rs, int rowNum) throws SQLException {
Actor actor = new Actor();
actor.setId(rs.getLong("id"));
actor.setFirstName(rs.getString("first_name"));
actor.setLastName(rs.getString("last_name"));
return actor;
}
};
return this.simpleJdbcTemplate.queryForObject(sql, mapper, id);
}
See also the section entitled Section 11.2.1.2, “JdbcTemplate
idioms (best practices)” for some
advice on how to best use the SimpleJdbcTemplate
class
in the context of an application.
![]() | Note |
---|---|
The |
In order to work with data from a database, one needs to obtain a
connection to the database. The way Spring does this is through a
DataSource
. A
DataSource
is part
of the JDBC specification and can be seen as a generalized connection
factory. It allows a container or a framework to hide connection pooling
and transaction management issues from the application code. As a
developer, you don not need to know any details about how to connect to
the database, that is the responsibility for the administrator that sets
up the datasource. You will most likely have to fulfill both roles while
you are developing and testing you code though, but you will not
necessarily have to know how the production data source is configured.
When using Spring's JDBC layer, you can either obtain a data
source from JNDI or you can configure your own, using an implementation
that is provided in the Spring distribution. The latter comes in handy
for unit testing outside of a web container. We will use the
DriverManagerDataSource
implementation for this
section but there are several additional implementations that will be
covered later on. The DriverManagerDataSource
works
the same way that you probably are used to work when you obtain a JDBC
connection. You have to specify the fully qualified class name of the
JDBC driver that you are using so that the
DriverManager
can load the driver class. Then you
have to provide a url that varies between JDBC drivers. You have to
consult the documentation for your driver for the correct value to use
here. Finally you must provide a username and a password that will be
used to connect to the database. Here is an example of how to configure
a DriverManagerDataSource
:
DriverManagerDataSource dataSource = new DriverManagerDataSource(); dataSource.setDriverClassName("org.hsqldb.jdbcDriver"); dataSource.setUrl("jdbc:hsqldb:hsql://localhost:"); dataSource.setUsername("sa"); dataSource.setPassword("");
SQLExceptionTranslator
is an interface to be
implemented by classes that can translate between SQLExceptions
and Spring's own data-access-strategy-agnostic
org.springframework.dao.DataAccessException
. Implementations can
be generic (for example, using SQLState codes for JDBC) or proprietary (for example,
using Oracle error codes) for greater precision.
SQLErrorCodeSQLExceptionTranslator
is the
implementation of SQLExceptionTranslator
that is used by default. This
implementation uses specific vendor codes. More precise than
SQLState
implementation, but vendor specific. The
error code translations are based on codes held in a JavaBean type class
named SQLErrorCodes
. This class is created and
populated by an SQLErrorCodesFactory
which as the
name suggests is a factory for creating SQLErrorCodes
based on the contents of a configuration file named
'sql-error-codes.xml'
. This file is populated
with vendor codes and based on the DatabaseProductName taken from the
DatabaseMetaData
, the codes for the current database are used.
The SQLErrorCodeSQLExceptionTranslator
applies
the following matching rules:
Try custom translation implemented by any subclass. Note that this class is concrete and is typically used itself, in which case this rule does not apply.
Apply error code matching. Error codes are obtained from the
SQLErrorCodesFactory
by default. This looks
up error codes from the classpath and keys into them from the
database name from the database metadata.
Use the fallback translator. SQLStateSQLExceptionTranslator
is the default fallback translator.
SQLErrorCodeSQLExceptionTranslator
can be extended the following way:
public class MySQLErrorCodesTranslator extends SQLErrorCodeSQLExceptionTranslator { protected DataAccessException customTranslate(String task, String sql, SQLException sqlex) { if (sqlex.getErrorCode() == -12345) { return new DeadlockLoserDataAccessException(task, sqlex); } return null; } }
In this example the specific error code '-12345'
is
translated and any other errors are simply left to be translated by the
default translator implementation. To use this custom translator, it is
necessary to pass it to the JdbcTemplate
using the
method setExceptionTranslator
and to use this
JdbcTemplate
for all of the data access processing
where this translator is needed. Here is an example of how this custom
translator can be used:
// create aJdbcTemplate
and set data source JdbcTemplate jt = new JdbcTemplate(); jt.setDataSource(dataSource); // create a custom translator and set theDataSource
for the default translation lookup MySQLErrorCodesTransalator tr = new MySQLErrorCodesTransalator(); tr.setDataSource(dataSource); jt.setExceptionTranslator(tr); // use theJdbcTemplate
for thisSqlUpdate
SqlUpdate su = new SqlUpdate(); su.setJdbcTemplate(jt); su.setSql("update orders set shipping_charge = shipping_charge * 1.05"); su.compile(); su.update();
The custom translator is passed a data source because we still want
the default translation to look up the error codes in
sql-error-codes.xml
.
To execute an SQL statement, there is very little code needed. All
you need is a DataSource
and a
JdbcTemplate
. Once you have that, you can use a
number of convenience methods that are provided with the
JdbcTemplate
. Here is a short example showing what
you need to include for a minimal but fully functional class that
creates a new table.
import javax.sql.DataSource; import org.springframework.jdbc.core.JdbcTemplate; public class ExecuteAStatement { private JdbcTemplate jdbcTemplate; public void setDataSource(DataSource dataSource) { this.jdbcTemplate = new JdbcTemplate(dataSource); } public void doExecute() { this.jdbcTemplate.execute("create table mytable (id integer, name varchar(100))"); } }
In addition to the execute methods, there is a large number of
query methods. Some of these methods are intended to be used for queries
that return a single value. Maybe you want to retrieve a count or a
specific value from one row. If that is the case then you can use
queryForInt(..)
, queryForLong(..)
or
queryForObject(..)
. The latter will convert the returned
JDBC Type
to the Java class that is passed in as an argument. If the
type conversion is invalid, then an
InvalidDataAccessApiUsageException
will be thrown.
Here is an example that contains two query methods, one for an
int
and one that queries for a String
.
import javax.sql.DataSource; import org.springframework.jdbc.core.JdbcTemplate; public class RunAQuery { private JdbcTemplate jdbcTemplate; public void setDataSource(DataSource dataSource) { this.jdbcTemplate = new JdbcTemplate(dataSource); } public int getCount() { return this.jdbcTemplate.queryForInt("select count(*) from mytable"); } public String getName() { return (String) this.jdbcTemplate.queryForObject("select name from mytable", String.class); } public void setDataSource(DataSource dataSource) { this.dataSource = dataSource; } }
In addition to the single results query methods there are several
methods that return a List with an entry for each row that the query
returned. The most generic method is queryForList(..)
which
returns a List
where each entry is a
Map
with each entry in the map representing the
column value for that row. If we add a method to the above example to
retrieve a list of all the rows, it would look like this:
private JdbcTemplate jdbcTemplate; public void setDataSource(DataSource dataSource) { this.jdbcTemplate = new JdbcTemplate(dataSource); } public List getList() { return this.jdbcTemplate.queryForList("select * from mytable"); }
The list returned would look something like this:
[{name=Bob, id=1}, {name=Mary, id=2}]
There are also a number of update methods that you can use. Find below an example where a column is updated for a certain primary key. In this example an SQL statement is used that has placeholders for row parameters. Note that the parameter values are passed in as an array of objects (and thus primitives have to be wrapped in the primitive wrapper classes).
import javax.sql.DataSource; import org.springframework.jdbc.core.JdbcTemplate; public class ExecuteAnUpdate { private JdbcTemplate jdbcTemplate; public void setDataSource(DataSource dataSource) { this.jdbcTemplate = new JdbcTemplate(dataSource); } public void setName(int id, String name) { this.jdbcTemplate.update("update mytable set name = ? where id = ?", new Object[] {name, new Integer(id)}); } }
One of the update
convenience methods provides support
for acquiring the primary keys generated by the database (part of the JDBC 3.0 standard
- see chapter 13.6 of the specification for details).
The method takes a PreparedStatementCreator
as its first argument,
and this is the way the required insert statement is specified. The other argument is a
KeyHolder
, which will contain the generated key on successful
return from the update. There is not a standard single way to create an
appropriate PreparedStatement
(which explains why the method
signature is the way it is). An example that works on Oracle and may work on other
platforms is:
final String INSERT_SQL = "insert into my_test (name) values(?)";
final String name = "Rob";
KeyHolder keyHolder = new GeneratedKeyHolder();
jdbcTemplate.update(
new PreparedStatementCreator() {
public PreparedStatement createPreparedStatement(Connection connection) throws SQLException {
PreparedStatement ps =
connection.prepareStatement(INSERT_SQL, new String[] {"id"});
ps.setString(1, name);
return ps;
}
},
keyHolder);
// keyHolder.getKey() now contains the generated key
The DataSourceUtils
class is a convenient and
powerful helper class that provides static
methods to
obtain connections from JNDI and close connections if necessary. It has
support for thread-bound connections, for example for use with
DataSourceTransactionManager
.
![]() | Note |
---|---|
The |
The SmartDataSource
interface is to
be implemented by classes that can provide a connection to a relational
database. Extends the DataSource
interface to
allow classes using it to query whether or not the connection should be
closed after a given operation. This can sometimes be useful for efficiency,
in the cases where one knows that one wants to reuse a connection.
This is an abstract
base class for Spring's
DataSource
implementations, that takes
care of the "uninteresting" glue. This is the class one would extend if
one was writing one's own DataSource
implementation.
The SingleConnectionDataSource
class is an
implementation of the SmartDataSource
interface that wraps a single
Connection
that is not
closed after use. Obviously, this is not multi-threading capable.
If client code will call close in the assumption of a pooled
connection, like when using persistence tools, set
suppressClose
to true
. This will
return a close-suppressing proxy instead of the physical connection.
Be aware that you will not be able to cast this to a native Oracle
Connection
or the like anymore.
This is primarily a test class. For example, it enables easy
testing of code outside an application server, in conjunction with a
simple JNDI environment. In contrast to
DriverManagerDataSource
, it reuses the same
connection all the time, avoiding excessive creation of physical
connections.
The DriverManagerDataSource
class is an
implementation of the SmartDataSource
interface that configures a plain old JDBC Driver via bean properties,
and returns a new connection every time.
This is potentially useful for test or standalone environments
outside of a J2EE container, either as a DataSource
bean in a Spring IoC container, or in conjunction with a simple
JNDI environment. Pool-assuming Connection.close()
calls will simply close the connection, so any DataSource
-aware
persistence code should work. However, using JavaBean style connection
pools such as commons-dbcp is so easy, even in a test environment, that
it is almost always preferable to use such a connection pool over
DriverManagerDataSource
.
TransactionAwareDataSourceProxy
is a proxy for a target
DataSource
, which wraps that target
DataSource
to add awareness of
Spring-managed transactions. In this respect it is similar to a
transactional JNDI DataSource
as provided by a J2EE
server.
![]() | Note |
---|---|
It should almost never be necessary or desireable to use this
class, except when existing code exists which must be called and passed
a standard JDBC |
(See the TransactionAwareDataSourceProxy
Javadocs for more details.)
The DataSourceTransactionManager
class is a
PlatformTransactionManager
implementation
for single JDBC datasources. It binds a JDBC connection from the specified
data source to the currently executing thread, potentially allowing for
one thread connection per data source.
Application code is required to retrieve the JDBC connection via
DataSourceUtils.getConnection(DataSource)
instead of
J2EE's standard DataSource.getConnection
. This is
recommended anyway, as it throws unchecked
org.springframework.dao
exceptions instead of checked
SQLExceptions
. All framework classes like
JdbcTemplate
use this strategy implicitly. If not
used with this transaction manager, the lookup strategy behaves exactly
like the common one - it can thus be used in any case.
The DataSourceTransactionManager
class supports
custom isolation levels, and timeouts that get applied as
appropriate JDBC statement query timeouts. To support the latter,
application code must either use JdbcTemplate
or call
DataSourceUtils.applyTransactionTimeout(..)
method for
each created statement.
This implementation can be used instead of
JtaTransactionManager
in the single resource case, as
it does not require the container to support JTA. Switching between both
is just a matter of configuration, if you stick to the required
connection lookup pattern. Note that JTA does not support custom
isolation levels!
The org.springframework.jdbc.object
package
contains classes that allow one to access the database in a more
object-oriented manner. By way of an example, one can execute queries
and get the results back as a list containing business objects with
the relational column data mapped to the properties of the business
object. One can also execute stored procedures and run update, delete
and insert statements.
![]() | Note |
---|---|
There is a view borne from experience acquired in the field
amongst some of the Spring developers that the various RDBMS
operation classes described below (with the exception of the
It must be stressed however that this is just a view... if you feel that you are getting measurable value from using the RDBMS operation classes, feel free to continue using these classes. |
SqlQuery
is a reusable, threadsafe class that
encapsulates an SQL query. Subclasses must implement the
newRowMapper(..)
method to provide a
RowMapper
instance that
can create one object per row obtained from iterating over the
ResultSet
that is created during the
execution of the query. The SqlQuery
class is
rarely used directly since the MappingSqlQuery
subclass provides a much more convenient implementation for mapping
rows to Java classes. Other implementations that extend
SqlQuery
are
MappingSqlQueryWithParameters
and
UpdatableSqlQuery
.
MappingSqlQuery
is a reusable query in which
concrete subclasses must implement the abstract
mapRow(..)
method to convert each row of
the supplied ResultSet
into an object.
Find below a brief example of a custom query that maps the data from the customer
relation to an instance of the Customer
class.
private class CustomerMappingQuery extends MappingSqlQuery { public CustomerMappingQuery(DataSource ds) { super(ds, "SELECT id, name FROM customer WHERE id = ?"); super.declareParameter(new SqlParameter("id", Types.INTEGER)); compile(); } public Object mapRow(ResultSet rs, int rowNumber) throws SQLException { Customer cust = new Customer(); cust.setId((Integer) rs.getObject("id")); cust.setName(rs.getString("name")); return cust; } }
We provide a constructor for this customer query that takes the
DataSource
as the only parameter. In this constructor
we call the constructor on the superclass with the
DataSource
and the SQL that should be executed to
retrieve the rows for this query. This SQL will be used to create a
PreparedStatement
so it may contain place holders for
any parameters to be passed in during execution. Each parameter must be
declared using the declareParameter
method passing in
an SqlParameter
. The SqlParameter
takes a name and the JDBC type as defined in
java.sql.Types
. After all parameters have been
defined we call the compile()
method so the statement
can be prepared and later be executed.
public Customer getCustomer(Integer id) { CustomerMappingQuery custQry = new CustomerMappingQuery(dataSource); Object[] parms = new Object[1]; parms[0] = id; List customers = custQry.execute(parms); if (customers.size() > 0) { return (Customer) customers.get(0); } else { return null; } }
The method in this example retrieves the customer with the id that
is passed in as the only parameter. After creating an instance of the
CustomerMappingQuery
class we create an array of
objects that will contain all parameters that are passed in. In this
case there is only one parameter and it is passed in as an
Integer
. Now we are ready to execute the query using
this array of parameters and we get a List
that
contains a Customer
object for each row that was
returned for our query. In this case it will only be one entry if there
was a match.
The SqlUpdate
class encapsulates an SQL update.
Like a query, an update object is reusable, and like all
RdbmsOperation
classes, an update
can have parameters and is defined in SQL. This class provides a number of
update(..)
methods analogous to the
execute(..)
methods of query objects.
This class is concrete. Although it can be subclassed (for example
to add a custom update method) it can easily be parameterized by setting
SQL and declaring parameters.
import java.sql.Types;
import javax.sql.DataSource;
import org.springframework.jdbc.core.SqlParameter;
import org.springframework.jdbc.object.SqlUpdate;
public class UpdateCreditRating extends SqlUpdate {
public UpdateCreditRating(DataSource ds) {
setDataSource(ds);
setSql("update customer set credit_rating = ? where id = ?");
declareParameter(new SqlParameter(Types.NUMERIC));
declareParameter(new SqlParameter(Types.NUMERIC));
compile();
}
/**
* @param id for the Customer to be updated
* @param rating the new value for credit rating
* @return number of rows updated
*/
public int run(int id, int rating) {
Object[] params =
new Object[] {
new Integer(rating),
new Integer(id)};
return update(params);
}
}
The StoredProcedure
class is a superclass for object
abstractions of RDBMS stored procedures. This class is abstract
,
and its various execute(..)
methods have
protected
access, preventing use other than through a
subclass that offers tighter typing.
The inherited sql
property will be the name of the
stored procedure in the RDBMS. Note that JDBC 3.0 introduces named
parameters, although the other features provided by this class are
still necessary in JDBC 3.0.
Here is an example of a program that calls a function, sysdate()
,
that comes with any Oracle database. To use the stored procedure
functionality one has to create a class that extends
StoredProcedure
. There are no input parameters, but
there is an output parameter that is declared as a date type using the class
SqlOutParameter
. The execute()
method returns a map with an entry for each declared output parameter
using the parameter name as the key.
import java.sql.Types;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import javax.sql.DataSource;
import org.springframework.jdbc.core.SqlOutParameter;
import org.springframework.jdbc.datasource.*;
import org.springframework.jdbc.object.StoredProcedure;
public class TestStoredProcedure {
public static void main(String[] args) {
TestStoredProcedure t = new TestStoredProcedure();
t.test();
System.out.println("Done!");
}
void test() {
DriverManagerDataSource ds = new DriverManagerDataSource();
ds.setDriverClassName("oracle.jdbc.OracleDriver");
ds.setUrl("jdbc:oracle:thin:@localhost:1521:mydb");
ds.setUsername("scott");
ds.setPassword("tiger");
MyStoredProcedure sproc = new MyStoredProcedure(ds);
Map results = sproc.execute();
printMap(results);
}
private class MyStoredProcedure extends StoredProcedure {
private static final String SQL = "sysdate";
public MyStoredProcedure(DataSource ds) {
setDataSource(ds);
setFunction(true);
setSql(SQL);
declareParameter(new SqlOutParameter("date", Types.DATE));
compile();
}
public Map execute() {
// the 'sysdate' sproc has no input parameters, so an empty Map is supplied...
return execute(new HashMap());
}
}
private static void printMap(Map results) {
for (Iterator it = results.entrySet().iterator(); it.hasNext(); ) {
System.out.println(it.next());
}
}
}
Find below an example of a StoredProcedure
that
has two output parameters (in this case Oracle cursors).
import oracle.jdbc.driver.OracleTypes;
import org.springframework.jdbc.core.SqlOutParameter;
import org.springframework.jdbc.object.StoredProcedure;
import javax.sql.DataSource;
import java.util.HashMap;
import java.util.Map;
public class TitlesAndGenresStoredProcedure extends StoredProcedure {
private static final String SPROC_NAME = "AllTitlesAndGenres";
public TitlesAndGenresStoredProcedure(DataSource dataSource) {
super(dataSource, SPROC_NAME);
declareParameter(new SqlOutParameter("titles", OracleTypes.CURSOR, new TitleMapper()));
declareParameter(new SqlOutParameter("genres", OracleTypes.CURSOR, new GenreMapper()));
compile();
}
public Map execute() {
// again, this sproc has no input parameters, so an empty Map is supplied...
return super.execute(new HashMap());
}
}
Notice how the overloaded variants of the declareParameter(..)
method that have been used in the TitlesAndGenresStoredProcedure
constructor are passed RowMapper
implementation
instances; this is a very convenient and powerful way to reuse existing
functionality. (The code for the two RowMapper
implementations is provided below in the interest of completeness.)
Firstly the TitleMapper
class, which simply
maps a ResultSet
to a Title
domain object for each row in the supplied ResultSet
.
import com.foo.sprocs.domain.Title; import org.springframework.jdbc.core.RowMapper; import java.sql.ResultSet; import java.sql.SQLException; public final class TitleMapper implements RowMapper { public Object mapRow(ResultSet rs, int rowNum) throws SQLException { Title title = new Title(); title.setId(rs.getLong("id")); title.setName(rs.getString("name")); return title; } }
Secondly, the GenreMapper
class, which again simply
maps a ResultSet
to a Genre
domain object for each row in the supplied ResultSet
.
import org.springframework.jdbc.core.RowMapper; import java.sql.ResultSet; import java.sql.SQLException; import com.foo.domain.Genre; public final class GenreMapper implements RowMapper { public Object mapRow(ResultSet rs, int rowNum) throws SQLException { return new Genre(rs.getString("name")); } }
If one needs to pass parameters to a stored procedure (that is
the stored procedure has been declared as having one or more input
parameters in its definition in the RDBMS), one would code a
strongly typed execute(..)
method which would delegate
to the superclass' (untyped) execute(Map parameters)
(which has protected
access); for example:
import oracle.jdbc.driver.OracleTypes; import org.springframework.jdbc.core.SqlOutParameter; import org.springframework.jdbc.object.StoredProcedure; import javax.sql.DataSource; import java.util.HashMap; import java.util.Map; public class TitlesAfterDateStoredProcedure extends StoredProcedure { private static final String SPROC_NAME = "TitlesAfterDate"; private static final String CUTOFF_DATE_PARAM = "cutoffDate"; public TitlesAfterDateStoredProcedure(DataSource dataSource) { super(dataSource, SPROC_NAME); declareParameter(new SqlParameter(CUTOFF_DATE_PARAM, Types.DATE); declareParameter(new SqlOutParameter("titles", OracleTypes.CURSOR, new TitleMapper())); compile(); } public Map execute(Date cutoffDate) { Map inputs = new HashMap(); inputs.put(CUTOFF_DATE_PARAM, cutoffDate); return super.execute(inputs); } }
The SqlFunction
RDBMS operation class encapsulates an
SQL "function" wrapper for a query that returns a single row of
results. The default behavior is to return an int
, but that
can be overridden by using the methods with an extra return type parameter.
This is similar to using the queryForXxx
methods of
the JdbcTemplate
. The advantage with
SqlFunction
is that you don't have to create the
JdbcTemplate
, it is done behind the scenes.
This class is intended to use to call SQL functions that return a
single result using a query like "select user()" or "select sysdate from
dual". It is not intended for calling more complex stored functions or
for using a CallableStatement
to invoke a stored
procedure or stored function. (Use the StoredProcedure
or
SqlCall
classes for this type of processing).
SqlFunction
is a concrete class, and there is typically
no need to subclass it. Code using this package can create an object of this type,
declaring SQL and parameters, and then invoke the appropriate run method
repeatedly to execute the function. Here is an example of retrieving the
count of rows from a table:
public int countRows() { SqlFunction sf = new SqlFunction(dataSource, "select count(*) from mytable"); sf.compile(); return sf.run(); }
The Spring Framework provides integration with Hibernate, JDO, Oracle TopLink, iBATIS SQL Maps and JPA: in terms of resource management, DAO implementation support, and transaction strategies. For example for Hibernate, there is first-class support with lots of IoC convenience features, addressing many typical Hibernate integration issues. All of these support packages for O/R (Object Relational) mappers comply with Spring's generic transaction and DAO exception hierarchies. There are usually two integration styles: either using Spring's DAO 'templates' or coding DAOs against plain Hibernate/JDO/TopLink/etc APIs. In both cases, DAOs can be configured through Dependency Injection and participate in Spring's resource and transaction management.
Spring adds significant support when using the O/R mapping layer of your choice to create data access applications. First of all, you should know that once you started using Spring's support for O/R mapping, you don't have to go all the way. No matter to what extent, you're invited to review and leverage the Spring approach, before deciding to take the effort and risk of building a similar infrastructure in-house. Much of the O/R mapping support, no matter what technology you're using may be used in a library style, as everything is designed as a set of reusable JavaBeans. Usage inside a Spring IoC container does provide additional benefits in terms of ease of configuration and deployment; as such, most examples in this section show configuration inside a Spring container.
Some of the benefits of using the Spring Framework to create your ORM DAOs include:
Ease of testing. Spring's IoC approach makes it
easy to swap the implementations and config locations of Hibernate
SessionFactory
instances, JDBC DataSource
instances,
transaction managers, and mapper object implementations (if needed).
This makes it much easier to isolate and test each piece of
persistence-related code in isolation.
Common data access exceptions. Spring can wrap exceptions from your O/R mapping tool of choice, converting them from proprietary (potentially checked) exceptions to a common runtime DataAccessException hierarchy. This allows you to handle most persistence exceptions, which are non-recoverable, only in the appropriate layers, without annoying boilerplate catches/throws, and exception declarations. You can still trap and handle exceptions anywhere you need to. Remember that JDBC exceptions (including DB specific dialects) are also converted to the same hierarchy, meaning that you can perform some operations with JDBC within a consistent programming model.
General resource management. Spring
application contexts can handle the location and configuration of
Hibernate SessionFactory
instances,
JDBC DataSource
instances, iBATIS SQL
Maps configuration objects, and other related resources. This makes
these values easy to manage and change. Spring offers efficient, easy
and safe handling of persistence resources. For example: related code
using Hibernate generally needs to use the same Hibernate
Session
for efficiency and proper
transaction handling. Spring makes it easy to transparently create and
bind a Session
to the current thread,
either by using an explicit 'template' wrapper class at the Java code
level or by exposing a current Session
through the Hibernate SessionFactory
(for DAOs based on plain Hibernate3 API). Thus Spring solves many of
the issues that repeatedly arise from typical Hibernate usage, for any
transaction environment (local or JTA).
Integrated transaction management. Spring allows you to wrap your O/R mapping code with either a declarative, AOP style method interceptor, or an explicit 'template' wrapper class at the Java code level. In either case, transaction semantics are handled for you, and proper transaction handling (rollback, etc) in case of exceptions is taken care of. As discussed below, you also get the benefit of being able to use and swap various transaction managers, without your Hibernate/JDO related code being affected: for example, between local transactions and JTA, with the same full services (such as declarative transactions) available in both scenarios. As an additional benefit, JDBC-related code can fully integrate transactionally with the code you use to do O/R mapping. This is useful for data access that's not suitable for O/R mapping, such as batch processing or streaming of BLOBs, which still needs to share common transactions with ORM operations.
The PetClinic sample in the Spring distribution offers alternative DAO implementations and application context configurations for JDBC, Hibernate, Oracle TopLink, and JPA. PetClinic can therefore serve as working sample app that illustrates the use of Hibernate, TopLink and JPA in a Spring web application. It also leverages declarative transaction demarcation with different transaction strategies.
The JPetStore sample illustrates the use of iBATIS SQL Maps in a Spring environment. It also features two web tier versions: one based on Spring Web MVC, one based on Struts.
Beyond the samples shipped with Spring, there is a variety of Spring-based O/R mapping samples provided by specific vendors: for example, the JDO implementations JPOX (http://www.jpox.org/) and Kodo (http://www.bea.com/kodo).
We will start with a coverage of Hibernate in a Spring environment, using it to demonstrate the approach that Spring takes towards integrating O/R mappers. This section will cover many issues in detail and show different variations of DAO implementations and transaction demarcations. Most of these patterns can be directly translated to all other supported O/R mapping tools. The following sections in this chapter will then cover the other O/R mappers, showing briefer examples there.
The following discussion focuses on Hibernate 3: this is the current
major production ready version of Hibernate. Hibernate 2.x, which has been
supported in Spring since its inception continues to be supported... it is
just that the following examples all use the Hibernate 3 classes and
configuration. All of this can (pretty much) be applied to Hibernate 2.x
as-is, using the analogous Hibernate 2.x support package:
org.springframework.orm.hibernate
, mirroring
org.springframework.orm.hibernate3
with analogous
support classes for Hibernate 2.x. Furthermore, all references to the
org.hibernate
package need to be replaced with
net.sf.hibernate
, following the root package change in
Hibernate 3. Simply adapt the package names (as used in the examples)
accordingly.
Typical business applications are often cluttered with repetitive
resource management code. Many projects try to invent their own
solutions for this issue, sometimes sacrificing proper handling of
failures for programming convenience. Spring advocates strikingly simple
solutions for proper resource handling, namely IoC via
templating; for example infrastructure classes with callback interfaces, or
applying AOP interceptors. The infrastructure cares for proper resource
handling, and for appropriate conversion of specific API exceptions to
an unchecked infrastructure exception hierarchy. Spring introduces a DAO
exception hierarchy, applicable to any data access strategy. For direct
JDBC, the JdbcTemplate
class mentioned in a
previous section cares for connection handling, and for proper
conversion of SQLException
to the
DataAccessException
hierarchy, including
translation of database-specific SQL error codes to meaningful exception
classes. It supports both JTA and JDBC transactions, via respective
Spring transaction managers.
Spring also offers Hibernate and JDO support, consisting of a
HibernateTemplate
/
JdoTemplate
analogous to
JdbcTemplate
, a
HibernateInterceptor
/
JdoInterceptor
, and a Hibernate / JDO transaction
manager. The major goal is to allow for clear application layering, with
any data access and transaction technology, and for loose coupling of
application objects. No more business service dependencies on the data
access or transaction strategy, no more hard-coded resource lookups, no
more hard-to-replace singletons, no more custom service registries. One
simple and consistent approach to wiring up application objects, keeping
them as reusable and free from container dependencies as possible. All
the individual data access features are usable on their own but
integrate nicely with Spring's application context concept, providing
XML-based configuration and cross-referencing of plain JavaBean
instances that don't need to be Spring-aware. In a typical Spring app,
many important objects are JavaBeans: data access templates, data access
objects (that use the templates), transaction managers, business
services (that use the data access objects and transaction managers),
web view resolvers, web controllers (that use the business services),and so on.
To avoid tying application objects to hard-coded resource lookups,
Spring allows you to define resources like a JDBC DataSource
or a
Hibernate SessionFactory
as beans in an
application context. Application objects that need to access resources
just receive references to such pre-defined instances via bean
references (the DAO definition in the next section illustrates this).
The following excerpt from an XML application context definition shows
how to set up a JDBC DataSource
and a Hibernate
SessionFactory
on top of it:
<beans> <bean id="myDataSource" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close"> <property name="driverClassName" value="org.hsqldb.jdbcDriver"/> <property name="url" value="jdbc:hsqldb:hsql://localhost:9001"/> <property name="username" value="sa"/> <property name="password" value=""/> </bean> <bean id="mySessionFactory" class="org.springframework.orm.hibernate3.LocalSessionFactoryBean"> <property name="dataSource" ref="myDataSource"/> <property name="mappingResources"> <list> <value>product.hbm.xml</value> </list> </property> <property name="hibernateProperties"> <value> hibernate.dialect=org.hibernate.dialect.HSQLDialect </value> </property> </bean> </beans>
Note that switching from a local Jakarta Commons DBCP
BasicDataSource
to a JNDI-located
DataSource
(usually managed by an
application server) is just a matter of configuration:
<beans> <bean id="myDataSource" class="org.springframework.jndi.JndiObjectFactoryBean"> <property name="jndiName" value="java:comp/env/jdbc/myds"/> </bean> </beans>
You can also access a JNDI-located SessionFactory
,
using Spring's JndiObjectFactoryBean
to retrieve and expose it.
However, that is typically not common outside of an EJB context.
The basic programming model for templating looks as follows, for
methods that can be part of any custom data access object or business
service. There are no restrictions on the implementation of the
surrounding object at all, it just needs to provide a Hibernate
SessionFactory
. It can get the latter
from anywhere, but preferably as bean reference from a Spring
IoC container - via a simple
setSessionFactory(..)
bean property setter. The
following snippets show a DAO definition in a Spring container, referencing the
above defined SessionFactory
, and an example
for a DAO method implementation.
<beans> <bean id="myProductDao" class="product.ProductDaoImpl"> <property name="sessionFactory" ref="mySessionFactory"/> </bean> </beans>
public class ProductDaoImpl implements ProductDao { private HibernateTemplate hibernateTemplate; public void setSessionFactory(SessionFactory sessionFactory) { this.hibernateTemplate = new HibernateTemplate(sessionFactory); } public Collection loadProductsByCategory(String category) throws DataAccessException { return this.hibernateTemplate.find("from test.Product product where product.category=?", category); } }
The HibernateTemplate
class provides many methods
that mirror the methods exposed on the Hibernate Session
interface, in addition to a number of convenience methods such as the one shown above.
If you need access to the Session
to invoke
methods that are not exposed on the HibernateTemplate
, you
can always drop down to a callback-based approach like so.
public class ProductDaoImpl implements ProductDao { private HibernateTemplate hibernateTemplate; public void setSessionFactory(SessionFactory sessionFactory) { this.hibernateTemplate = new HibernateTemplate(sessionFactory); } public Collection loadProductsByCategory(final String category) throws DataAccessException { return this.hibernateTemplate.execute(new HibernateCallback() { public Object doInHibernate(Session session) { Criteria criteria = session.createCriteria(Product.class); criteria.add(Expression.eq("category", category)); criteria.setMaxResults(6); return criteria.list(); } }; } }
A callback implementation effectively can be used for any
Hibernate data access. HibernateTemplate
will
ensure that Session
instances are
properly opened and closed, and automatically participate in
transactions. The template instances are thread-safe and reusable, they
can thus be kept as instance variables of the surrounding class. For
simple single step actions like a single find, load, saveOrUpdate, or
delete call, HibernateTemplate
offers alternative
convenience methods that can replace such one line callback
implementations. Furthermore, Spring provides a convenient
HibernateDaoSupport
base class that provides a
setSessionFactory(..)
method for receiving a
SessionFactory
, and
getSessionFactory()
and
getHibernateTemplate()
for use by subclasses. In
combination, this allows for very simple DAO implementations for typical
requirements:
public class ProductDaoImpl extends HibernateDaoSupport implements ProductDao { public Collection loadProductsByCategory(String category) throws DataAccessException { return this.getHibernateTemplate().find( "from test.Product product where product.category=?", category); } }
As alternative to using Spring's
HibernateTemplate
to implement DAOs, data access
code can also be written in a more traditional fashion, without wrapping
the Hibernate access code in a callback, while still respecting and
participating in Spring's generic DataAccessException
hierarchy.
The HibernateDaoSupport
base class offers
methods to access the current transactional
Session
and to convert exceptions in such
a scenario; similar methods are also available as static helpers on the
SessionFactoryUtils
class. Note that such code
will usually pass 'false
' as the value of the
getSession(..)
methods 'allowCreate
'
argument, to enforce running within a transaction (which
avoids the need to close the returned Session
, as
its lifecycle is managed by the transaction).
public class HibernateProductDao extends HibernateDaoSupport implements ProductDao { public Collection loadProductsByCategory(String category) throws DataAccessException, MyException { Session session = getSession(false); try { Query query = session.createQuery("from test.Product product where product.category=?"); query.setString(0, category); List result = query.list(); if (result == null) { throw new MyException("No search results."); } return result; } catch (HibernateException ex) { throw convertHibernateAccessException(ex); } } }
The advantage of such direct Hibernate access code is that
it allows any checked application exception to
be thrown within the data access code; contrast this to the
HibernateTemplate
class which is
restricted to throwing only unchecked exceptions within the callback.
Note that you can often defer the corresponding checks and the throwing
of application exceptions to after the callback, which still allows
working with HibernateTemplate
. In general, the
HibernateTemplate
class' convenience methods are
simpler and more convenient for many scenarios.
Hibernate 3.0.1 introduced a feature called "contextual Sessions",
where Hibernate itself manages one current
Session
per transaction. This is roughly
equivalent to Spring's synchronization of one Hibernate
Session
per transaction. A corresponding
DAO implementation looks like as follows, based on the plain Hibernate API:
public class ProductDaoImpl implements ProductDao { private SessionFactory sessionFactory; public void setSessionFactory(SessionFactory sessionFactory) { this.sessionFactory = sessionFactory; } public Collection loadProductsByCategory(String category) { return this.sessionFactory.getCurrentSession() .createQuery("from test.Product product where product.category=?") .setParameter(0, category) .list(); } }
This style is very similar to what you will find in the Hibernate
reference documentation and examples, except for holding the
SessionFactory
in an instance variable.
We strongly recommend such an instance-based setup over the old-school
static
HibernateUtil
class
from Hibernate's CaveatEmptor sample application. (In general, do not
keep any resources in static
variables unless
absolutely necessary.)
The above DAO follows the Dependency Injection pattern: it fits
nicely into a Spring IoC container, just like it would if
coded against Spring's HibernateTemplate
. Of course,
such a DAO can also be set up in plain Java (for example, in unit tests):
simply instantiate it and call setSessionFactory(..)
with the desired factory reference. As a Spring bean definition, it would
look as follows:
<beans> <bean id="myProductDao" class="product.ProductDaoImpl"> <property name="sessionFactory" ref="mySessionFactory"/> </bean> </beans>
The main advantage of this DAO style is that it depends on Hibernate API only; no import of any Spring class is required. This is of course appealing from a non-invasiveness perspective, and will no doubt feel more natural to Hibernate developers.
However, the DAO throws plain HibernateException
(which is unchecked, so does not have to be declared or caught), which means
that callers can only treat exceptions as generally fatal - unless they want to
depend on Hibernate's own exception hierarchy. Catching specific causes such as
an optimistic locking failure is not possible without tieing the caller to
the implementation strategy. This tradeoff might be acceptable to
applications that are strongly Hibernate-based and/or do not need any
special exception treatment.
Fortunately, Spring's
LocalSessionFactoryBean
supports Hibernate's
SessionFactory.getCurrentSession()
method for any
Spring transaction strategy, returning the current Spring-managed
transactional Session
even with
HibernateTransactionManager
. Of course, the
standard behavior of that method remains: returning the current
Session
associated with the ongoing JTA
transaction, if any (no matter whether driven by Spring's
JtaTransactionManager
, by EJB CMT, or by JTA).
In summary: DAOs can be implemented based on the plain Hibernate3 API, while still being able to participate in Spring-managed transactions.
Transactions can be demarcated in a higher level of the application,
on top of such lower-level data access services spanning any number
of operations. There are no restrictions on the implementation of the
surrounding business service here as well, it just needs a Spring
PlatformTransactionManager
. Again, the latter can
come from anywhere, but preferably as bean reference via a
setTransactionManager(..)
method - just like
the productDAO
should be set via a
setProductDao(..)
method. The following snippets
show a transaction manager and a business service definition in a Spring
application context, and an example for a business method implementation.
<beans> <bean id="myTxManager" class="org.springframework.orm.hibernate3.HibernateTransactionManager"> <property name="sessionFactory" ref="mySessionFactory"/> </bean> <bean id="myProductService" class="product.ProductServiceImpl"> <property name="transactionManager" ref="myTxManager"/> <property name="productDao" ref="myProductDao"/> </bean> </beans>
public class ProductServiceImpl implements ProductService {
private TransactionTemplate transactionTemplate;
private ProductDao productDao;
public void setTransactionManager(PlatformTransactionManager transactionManager) {
this.transactionTemplate = new TransactionTemplate(transactionManager);
}
public void setProductDao(ProductDao productDao) {
this.productDao = productDao;
}
public void increasePriceOfAllProductsInCategory(final String category) {
this.transactionTemplate.execute(new TransactionCallbackWithoutResult() {
public void doInTransactionWithoutResult(TransactionStatus status) {
List productsToChange = this.productDao.loadProductsByCategory(category);
// do the price increase...
}
}
);
}
}
Alternatively, one can use Spring's declarative transaction support, which essentially enables you to replace explicit transaction demarcation API calls in your Java code with an AOP transaction interceptor configured in a Spring container. This allows you to keep business services free of repetitive transaction demarcation code, and allows you to focus on adding business logic which is where the real value of your application lies. Furthermore, transaction semantics like propagation behavior and isolation level can be changed in a configuration file and do not affect the business service implementations.
<beans>
<bean id="myTxManager" class="org.springframework.orm.hibernate3.HibernateTransactionManager">
<property name="sessionFactory" ref="mySessionFactory"/>
</bean>
<bean id="myProductService" class="org.springframework.aop.framework.ProxyFactoryBean">
<property name="proxyInterfaces" value="product.ProductService"/>
<property name="target">
<bean class="product.DefaultProductService">
<property name="productDao" ref="myProductDao"/>
</bean>
</property>
<property name="interceptorNames">
<list>
<value>myTxInterceptor</value> <!-- the transaction interceptor (configured elsewhere) -->
</list>
</property>
</bean>
</beans>
public class ProductServiceImpl implements ProductService { private ProductDao productDao; public void setProductDao(ProductDao productDao) { this.productDao = productDao; } // notice the absence of transaction demarcation code in this method // Spring's declarative transaction infrastructure will be demarcating transactions on your behalf public void increasePriceOfAllProductsInCategory(final String category) { List productsToChange = this.productDao.loadProductsByCategory(category); // ... } }
Spring's TransactionInterceptor
allows any
checked application exception to be thrown with the callback code, while
TransactionTemplate
is restricted to unchecked
exceptions within the callback.
TransactionTemplate
will trigger a rollback in
case of an unchecked application exception, or if the transaction has
been marked rollback-only by the application (via TransactionStatus
).
TransactionInterceptor
behaves the same way by
default but allows configurable rollback policies per method.
The following higher level approach to declarative transactions
doesn't use the ProxyFactoryBean
, and as such may
be easier to use if you have a large number of service objects that you
wish to make transactional.
![]() | Note |
---|---|
You are strongly encouraged to read the section entitled Section 9.5, “Declarative transaction management” if you have not done so already prior to continuing. |
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:aop="http://www.springframework.org/schema/aop"
xmlns:tx="http://www.springframework.org/schema/tx"
xsi:schemaLocation="
http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.0.xsd
http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-2.0.xsd
http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-2.0.xsd">
<!-- SessionFactory
, DataSource
, etc. omitted -->
<bean id="myTxManager" class="org.springframework.orm.hibernate3.HibernateTransactionManager">
<property name="sessionFactory" ref="mySessionFactory"/>
</bean>
<aop:config>
<aop:pointcut id="productServiceMethods" expression="execution(* product.ProductService.*(..))"/>
<aop:advisor advice-ref="txAdvice" pointcut-ref="productServiceMethods"/>
</aop:config>
<tx:advice id="txAdvice" transaction-manager="myTxManager">
<tx:attributes>
<tx:method name="increasePrice*" propagation="REQUIRED"/>
<tx:method name="someOtherBusinessMethod" propagation="REQUIRES_NEW"/>
<tx:method name="*" propagation="SUPPORTS" read-only="true"/>
</tx:attributes>
</tx:advice>
<bean id="myProductService" class="product.SimpleProductService">
<property name="productDao" ref="myProductDao"/>
</bean>
</beans>
Both TransactionTemplate
and
TransactionInterceptor
delegate the actual
transaction handling to a
PlatformTransactionManager
instance, which can be
a HibernateTransactionManager
(for a single
Hibernate SessionFactory
, using a
ThreadLocal
Session
under the hood) or a
JtaTransactionManager
(delegating to the JTA
subsystem of the container) for Hibernate applications. You could even
use a custom PlatformTransactionManager
implementation. So switching from native Hibernate transaction
management to JTA, such as when facing distributed transaction requirements
for certain deployments of your application, is just a matter of
configuration. Simply replace the Hibernate transaction manager with
Spring's JTA transaction implementation. Both transaction demarcation
and data access code will work without changes, as they just use the
generic transaction management APIs.
For distributed transactions across multiple Hibernate session
factories, simply combine JtaTransactionManager
as a transaction strategy with multiple
LocalSessionFactoryBean
definitions. Each of your
DAOs then gets one specific SessionFactory
reference passed into it's respective bean property. If all underlying JDBC
data sources are transactional container ones, a business service can demarcate
transactions across any number of DAOs and any number of session
factories without special regard, as long as it is using
JtaTransactionManager
as the strategy.
<beans> <bean id="myDataSource1" class="org.springframework.jndi.JndiObjectFactoryBean"> <property name="jndiName value="java:comp/env/jdbc/myds1"/> </bean> <bean id="myDataSource2" class="org.springframework.jndi.JndiObjectFactoryBean"> <property name="jndiName" value="java:comp/env/jdbc/myds2"/> </bean> <bean id="mySessionFactory1" class="org.springframework.orm.hibernate3.LocalSessionFactoryBean"> <property name="dataSource" ref="myDataSource1"/> <property name="mappingResources"> <list> <value>product.hbm.xml</value> </list> </property> <property name="hibernateProperties"> <value> hibernate.dialect=org.hibernate.dialect.MySQLDialect hibernate.show_sql=true </value> </property> </bean> <bean id="mySessionFactory2" class="org.springframework.orm.hibernate3.LocalSessionFactoryBean"> <property name="dataSource" ref="myDataSource2"/> <property name="mappingResources"> <list> <value>inventory.hbm.xml</value> </list> </property> <property name="hibernateProperties"> <value> hibernate.dialect=org.hibernate.dialect.OracleDialect </value> </property> </bean> <bean id="myTxManager" class="org.springframework.transaction.jta.JtaTransactionManager"/> <bean id="myProductDao" class="product.ProductDaoImpl"> <property name="sessionFactory" ref="mySessionFactory1"/> </bean> <bean id="myInventoryDao" class="product.InventoryDaoImpl"> <property name="sessionFactory" ref="mySessionFactory2"/> </bean> <!-- this shows the Spring 1.x style of declarative transaction configuration --> <!-- it is totally supported, 100% legal in Spring 2.x, but see also above for the sleeker, Spring 2.0 style --> <bean id="myProductService" class="org.springframework.transaction.interceptor.TransactionProxyFactoryBean"> <property name="transactionManager" ref="myTxManager"/> <property name="target"> <bean class="product.ProductServiceImpl"> <property name="productDao" ref="myProductDao"/> <property name="inventoryDao" ref="myInventoryDao"/> </bean> </property> <property name="transactionAttributes"> <props> <prop key="increasePrice*">PROPAGATION_REQUIRED</prop> <prop key="someOtherBusinessMethod">PROPAGATION_REQUIRES_NEW</prop> <prop key="*">PROPAGATION_SUPPORTS,readOnly</prop> </props> </property> </bean> </beans>
Both HibernateTransactionManager
and
JtaTransactionManager
allow for proper JVM-level
cache handling with Hibernate - without container-specific transaction
manager lookup or JCA connector (as long as not using EJB to initiate
transactions).
HibernateTransactionManager
can export the
JDBC Connection
used by Hibernate to
plain JDBC access code, for a specific
DataSource
. This allows for high-level
transaction demarcation with mixed Hibernate/JDBC data access completely
without JTA, as long as you are just accessing one database!
HibernateTransactionManager
will automatically
expose the Hibernate transaction as JDBC transaction if the passed-in
SessionFactory
has been set up with a
DataSource
(through the "dataSource"
property of the LocalSessionFactoryBean
class).
Alternatively, the DataSource
that the
transactions are supposed to be exposed for can also be specified
explicitly, through the "dataSource" property of the
HibernateTransactionManager
class.
Spring's resource management allows for simple switching between a
JNDI SessionFactory
and a local one,
without having to change a single line of application code. The decision
as to whether to keep the resource definitions in the container or
locally within the application, is mainly a matter of the transaction
strategy being used. Compared to a Spring-defined local
SessionFactory
, a manually registered
JNDI SessionFactory
does not provide any
benefits. Deploying a SessionFactory
through Hibernate's JCA connector provides the added value of
participating in the J2EE server's management infrastructure, but does
not add actual value beyond that.
An important benefit of Spring's transaction support is that it
isn't bound to a container at all. Configured to any other strategy than
JTA, it will work in a standalone or test environment too. Especially
for the typical case of single-database transactions, this is a very
lightweight and powerful alternative to JTA. When using local EJB
Stateless Session Beans to drive transactions, you depend both on an EJB
container and JTA - even if you just access a single database anyway,
and just use SLSBs for declarative transactions via CMT. The alternative
of using JTA programmatically requires a J2EE environment as well. JTA
does not just involve container dependencies in terms of JTA itself and
of JNDI DataSource
instances. For
non-Spring JTA-driven Hibernate transactions, you have to use the
Hibernate JCA connector, or extra Hibernate transaction code with the
TransactionManagerLookup
being configured
for proper JVM-level caching.
Spring-driven transactions can work with a locally defined
Hibernate SessionFactory
nicely, just
like with a local JDBC DataSource
- if
accessing a single database, of course. Therefore you just have to fall
back to Spring's JTA transaction strategy when actually facing
distributed transaction requirements. Note that a JCA connector needs
container-specific deployment steps, and obviously JCA support in the
first place. This is far more hassle than deploying a simple web app
with local resource definitions and Spring-driven transactions. And you
often need the Enterprise Edition of your container, as for example WebLogic
Express does not provide JCA. A Spring application with local resources
and transactions spanning one single database will work in any J2EE web
container (without JTA, JCA, or EJB) - like Tomcat, Resin, or even plain
Jetty. Additionally, such a middle tier can be reused in desktop
applications or test suites easily.
All things considered: if you do not use EJB, stick with local
SessionFactory
setup and Spring's
HibernateTransactionManager
or
JtaTransactionManager
. You will get all of the
benefits including proper transactional JVM-level caching and
distributed transactions, without any container deployment hassle. JNDI
registration of a Hibernate
SessionFactory
via the JCA connector
really only adds value when used in conjunction with EJBs.
In some JTA environments with very strict
XADataSource
implementations -- currently
only some WebLogic and WebSphere versions -- when using Hibernate configured
without any awareness of the JTA PlatformTransactionManager
object for that environment, it is possible for spurious warning or exceptions to
show up in the application server log. These warnings or exceptions will say something
to the effect that the connection being accessed is no longer valid, or JDBC access
is no longer valid, possibly because the transaction is no longer active. As an
example, here is an actual exception from WebLogic:
java.sql.SQLException: The transaction is no longer active - status: 'Committed'. No further JDBC access is allowed within this transaction.
This warning is easy to resolve by simply making Hibernate aware of the JTA
PlatformTransactionManager
instance, to which it will
also synchronize (along with Spring). This may be done in two ways:
If in your application context you are already directly
obtaining the JTA
PlatformTransactionManager
object
(presumably from JNDI via JndiObjectFactoryBean
)
and feeding it for example to Spring's
JtaTransactionManager
, then the easiest way
is to simply specify a reference to this as the value of
LocalSessionFactoryBean
's
jtaTransactionManager property. Spring will
then make the object available to Hibernate.
More likely you do not already have the JTA
PlatformTransactionManager
instance
(since Spring's JtaTransactionManager
can
find it itself) so you need to instead configure Hibernate to also
look it up directly. This is done by configuring an AppServer
specific TransactionManagerLookup
class in the
Hibernate configuration, as described in the Hibernate
manual.
It is not necessary to read any more for proper usage, but the
full sequence of events with and without Hibernate being aware of the
JTA PlatformTransactionManager
will now
be described.
When Hibernate is not configured with any awareness of the JTA
PlatformTransactionManager
, the sequence
of events when a JTA transaction commits is as follows:
JTA transaction commits
Spring's JtaTransactionManager
is
synchronized to the JTA transaction, so it is called back via an
afterCompletion callback by the JTA transaction
manager.
Among other activities, this can trigger a callback by Spring
to Hibernate, via Hibernate's
afterTransactionCompletion
callback (used to
clear the Hibernate cache), followed by an explicit
close()
call on the Hibernate Session, which
results in Hibernate trying to close()
the JDBC
Connection.
In some environments, this Connection.close()
call
then triggers the warning or error, as the application server no longer considers the
Connection
usable at all, since the transaction has
already been committed.
When Hibernate is configured with awareness of the JTA
PlatformTransactionManager
, the sequence
of events when a JTA transaction commits is instead as follows:
JTA transaction is ready to commit
Spring's JtaTransactionManager
is
synchronized to the JTA transaction, so it is called back via a
beforeCompletion callback by the JTA
transaction manager.
Spring is aware that Hibernate itself is synchronized to the JTA transaction,
and behaves differently than in the previous scenario. Assuming the Hibernate
Session
needs to be closed at all, Spring will close it now.
JTA Transaction commits
Hibernate is synchronized to the JTA transaction, so it is called back via an afterCompletion callback by the JTA transaction manager, and can properly clear its cache.
Spring supports the standard JDO 1.0/2.0 API as data access
strategy, following the same style as the Hibernate support. The
corresponding integration classes reside in the
org.springframework.orm.jdo
package.
Spring provides a
LocalPersistenceManagerFactoryBean
class that
allows for defining a local JDO
PersistenceManagerFactory
within a Spring
application context:
<beans> <bean id="myPmf" class="org.springframework.orm.jdo.LocalPersistenceManagerFactoryBean"> <property name="configLocation" value="classpath:kodo.properties"/> </bean> </beans>
Alternatively, a
PersistenceManagerFactory
can also be set
up through direct instantiation of a
PersistenceManagerFactory
implementation
class. A JDO PersistenceManagerFactory
implementation class is supposed to follow the JavaBeans pattern, just
like a JDBC DataSource
implementation
class, which is a natural fit for a Spring bean definition. This setup
style usually supports a Spring-defined JDBC
DataSource
, passed into the
"connectionFactory" property. For example, for the open source JDO
implementation JPOX (http://www.jpox.org):
<beans> <bean id="dataSource" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close"> <property name="driverClassName" value="${jdbc.driverClassName}"/> <property name="url" value="${jdbc.url}"/> <property name="username" value="${jdbc.username}"/> <property name="password" value="${jdbc.password}"/> </bean> <bean id="myPmf" class="org.jpox.PersistenceManagerFactoryImpl" destroy-method="close"> <property name="connectionFactory" ref="dataSource"/> <property name="nontransactionalRead" value="true"/> </bean> </beans>
A JDO PersistenceManagerFactory
can
also be set up in the JNDI environment of a J2EE application server,
usually through the JCA connector provided by the particular JDO
implementation. Spring's standard
JndiObjectFactoryBean
can be used to retrieve and
expose such a PersistenceManagerFactory
.
However, outside an EJB context, there is often no compelling benefit in
holding the PersistenceManagerFactory
in
JNDI: only choose such setup for a good reason. See "container resources
versus local resources" in the Hibernate section for a discussion; the
arguments there apply to JDO as well.
Each JDO-based DAO will then receive the
PersistenceManagerFactory
through
dependency injection. Such a DAO could be coded against plain JDO API,
working with the given
PersistenceManagerFactory
, but will
usually rather be used with the Spring Framework's
JdoTemplate
:
<beans> <bean id="myProductDao" class="product.ProductDaoImpl"> <property name="persistenceManagerFactory" ref="myPmf"/> </bean> </beans>
public class ProductDaoImpl implements ProductDao {
private JdoTemplate jdoTemplate;
public void setPersistenceManagerFactory(PersistenceManagerFactory pmf) {
this.jdoTemplate = new JdoTemplate(pmf);
}
public Collection loadProductsByCategory(final String category) throws DataAccessException {
return (Collection) this.jdoTemplate.execute(new JdoCallback() {
public Object doInJdo(PersistenceManager pm) throws JDOException {
Query query = pm.newQuery(Product.class, "category = pCategory");
query.declareParameters("String pCategory");
List result = query.execute(category);
// do some further stuff with the result list
return result;
}
});
}
}
A callback implementation can effectively be used for any JDO data
access. JdoTemplate
will ensure that
PersistenceManager
s are properly opened and
closed, and automatically participate in transactions. The template
instances are thread-safe and reusable, they can thus be kept as
instance variables of the surrounding class. For simple single-step
actions such as a single find
,
load
, makePersistent
, or
delete
call, JdoTemplate
offers alternative convenience methods that can replace such one line
callback implementations. Furthermore, Spring provides a convenient
JdoDaoSupport
base class that provides a
setPersistenceManagerFactory(..)
method for receiving
a PersistenceManagerFactory
, and
getPersistenceManagerFactory()
and
getJdoTemplate()
for use by subclasses. In
combination, this allows for very simple DAO implementations for typical
requirements:
public class ProductDaoImpl extends JdoDaoSupport implements ProductDao { public Collection loadProductsByCategory(String category) throws DataAccessException { return getJdoTemplate().find( Product.class, "category = pCategory", "String category", new Object[] {category}); } }
As alternative to working with Spring's
JdoTemplate
, you can also code Spring-based DAOs
at the JDO API level, explicitly opening and closing a
PersistenceManager
. As elaborated in the
corresponding Hibernate section, the main advantage of this approach is
that your data access code is able to throw checked exceptions.
JdoDaoSupport
offers a variety of support methods
for this scenario, for fetching and releasing a transactional
PersistenceManager
as well as for
converting exceptions.
DAOs can also be written against plain JDO API, without any Spring
dependencies, directly using an injected
PersistenceManagerFactory
. A
corresponding DAO implementation looks like as follows:
public class ProductDaoImpl implements ProductDao { private PersistenceManagerFactory persistenceManagerFactory; public void setPersistenceManagerFactory(PersistenceManagerFactory pmf) { this.persistenceManagerFactory = pmf; } public Collection loadProductsByCategory(String category) { PersistenceManager pm = this.persistenceManagerFactory.getPersistenceManager(); try { Query query = pm.newQuery(Product.class, "category = pCategory"); query.declareParameters("String pCategory"); return query.execute(category); } finally { pm.close(); } } }
As the above DAO still follows the Dependency Injection pattern,
it still fits nicely into a Spring container, just like it
would if coded against Spring's JdoTemplate
:
<beans> <bean id="myProductDao" class="product.ProductDaoImpl"> <property name="persistenceManagerFactory" ref="myPmf"/> </bean> </beans>
The main issue with such DAOs is that they always get a new
PersistenceManager
from the factory. To
still access a Spring-managed transactional
PersistenceManager
, consider defining a
TransactionAwarePersistenceManagerFactoryProxy
(as included in Spring) in front of your target
PersistenceManagerFactory
, passing the
proxy into your DAOs.
<beans> <bean id="myPmfProxy" class="org.springframework.orm.jdo.TransactionAwarePersistenceManagerFactoryProxy"> <property name="targetPersistenceManagerFactory" ref="myPmf"/> </bean> <bean id="myProductDao" class="product.ProductDaoImpl"> <property name="persistenceManagerFactory" ref="myPmfProxy"/> </bean> </beans>
Your data access code will then receive a transactional
PersistenceManager
(if any) from the
PersistenceManagerFactory.getPersistenceManager()
method that it calls. The latter method call goes through the proxy,
which will first check for a current transactional
PersistenceManager
before getting a new
one from the factory. close()
calls on the
PersistenceManager
will be ignored in
case of a transactional PersistenceManager
.
If your data access code will always run within an active
transaction (or at least within active transaction synchronization), it
is safe to omit the PersistenceManager.close()
call
and thus the entire finally
block, which you might
prefer to keep your DAO implementations concise:
public class ProductDaoImpl implements ProductDao { private PersistenceManagerFactory persistenceManagerFactory; public void setPersistenceManagerFactory(PersistenceManagerFactory pmf) { this.persistenceManagerFactory = pmf; } public Collection loadProductsByCategory(String category) { PersistenceManager pm = this.persistenceManagerFactory.getPersistenceManager(); Query query = pm.newQuery(Product.class, "category = pCategory"); query.declareParameters("String pCategory"); return query.execute(category); } }
With such DAOs that rely on active transactions, it is recommended
to enforce active transactions through turning
TransactionAwarePersistenceManagerFactoryProxy
's
"allowCreate" flag off:
<beans> <bean id="myPmfProxy" class="org.springframework.orm.jdo.TransactionAwarePersistenceManagerFactoryProxy"> <property name="targetPersistenceManagerFactory" ref="myPmf"/> <property name="allowCreate" value="false"/> </bean> <bean id="myProductDao" class="product.ProductDaoImpl"> <property name="persistenceManagerFactory" ref="myPmfProxy"/> </bean> </beans>
The main advantage of this DAO style is that it depends on JDO API only; no import of any Spring class is required. This is of course appealing from a non-invasiveness perspective, and might feel more natural to JDO developers.
However, the DAO throws plain JDOException
(which is unchecked, so does not have to be declared or caught), which
means that callers can only treat exceptions as generally fatal - unless
they want to depend on JDO's own exception structure. Catching specific
causes such as an optimistic locking failure is not possible without
tying the caller to the implementation strategy. This tradeoff might be
acceptable to applications that are strongly JDO-based and/or do not
need any special exception treatment.
In summary: DAOs can be implemented based on plain JDO API, while
still being able to participate in Spring-managed transactions. This
might in particular appeal to people already familiar with JDO, feeling
more natural to them. However, such DAOs will throw plain
JDOException
; conversion to Spring's
DataAccessException
would have to happen
explicitly (if desired).
To execute service operations within transactions, you can use Spring's common declarative transaction facilities. For example:
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:aop="http://www.springframework.org/schema/aop" xmlns:tx="http://www.springframework.org/schema/tx" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.0.xsd http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-2.0.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-2.0.xsd"> <bean id="myTxManager" class="org.springframework.orm.jdo.JdoTransactionManager"> <property name="persistenceManagerFactory" ref="myPmf"/> </bean> <bean id="myProductService" class="product.ProductServiceImpl"> <property name="productDao" ref="myProductDao"/> </bean> <tx:advice id="txAdvice" transaction-manager="txManager"> <tx:attributes> <tx:method name="increasePrice*" propagation="REQUIRED"/> <tx:method name="someOtherBusinessMethod" propagation="REQUIRES_NEW"/> <tx:method name="*" propagation="SUPPORTS" read-only="true"/> </tx:attributes> </tx:advice> <aop:config> <aop:pointcut id="productServiceMethods" expression="execution(* product.ProductService.*(..))"/> <aop:advisor advice-ref="txAdvice" pointcut-ref="productServiceMethods"/> </aop:config> </beans>
Note that JDO requires an active transaction when modifying a
persistent object. There is no concept like a non-transactional flush in
JDO, in contrast to Hibernate. For this reason, the chosen JDO
implementation needs to be set up for a specific environment: in
particular, it needs to be explicitly set up for JTA synchronization, to
detect an active JTA transaction itself. This is not necessary for local
transactions as performed by Spring's
JdoTransactionManager
, but it is necessary for
participating in JTA transactions (whether driven by Spring's
JtaTransactionManager
or by EJB CMT / plain
JTA).
JdoTransactionManager
is capable of
exposing a JDO transaction to JDBC access code that accesses the same
JDBC DataSource
, provided that the
registered JdoDialect
supports retrieval of the
underlying JDBC Connection
. This is by
default the case for JDBC-based JDO 2.0 implementations; for JDO 1.0
implementations, a custom JdoDialect
needs to be
used. See next section for details on the
JdoDialect
mechanism.
As an advanced feature, both JdoTemplate
and interfacename
support a custom
JdoDialect
, to be passed into the "jdoDialect"
bean property. In such a scenario, the DAOs won't receive a
PersistenceManagerFactory
reference but
rather a full JdoTemplate
instance instead (for
example, passed into JdoDaoSupport
's
"jdoTemplate" property). A JdoDialect
implementation can enable some advanced features supported by Spring,
usually in a vendor-specific manner:
applying specific transaction semantics (such as custom isolation level or transaction timeout)
retrieving the transactional JDBC
Connection
(for exposure to
JDBC-based DAOs)
applying query timeouts (automatically calculated from Spring-managed transaction timeout)
eagerly flushing a
PersistenceManager
(to make
transactional changes visible to JDBC-based data access code)
advanced translation of JDOExceptions
to
Spring DataAccessExceptions
This is particularly valuable for JDO 1.0 implementations, where
none of those features are covered by the standard API. On JDO 2.0, most
of those features are supported in a standard manner: Hence, Spring's
DefaultJdoDialect
uses the corresponding JDO 2.0 API
methods by default (as of Spring 1.2). For special transaction semantics
and for advanced translation of exception, it is still valuable to
derive vendor-specific JdoDialect
subclasses.
See the JdoDialect
Javadoc for more details
on its operations and how they are used within Spring's JDO support.
Since Spring 1.2, Spring supports Oracle TopLink (http://www.oracle.com/technology/products/ias/toplink) as
data access strategy, following the same style as the Hibernate support.
Both TopLink 9.0.4 (the production version as of Spring 1.2) and 10.1.3
(still in beta as of Spring 1.2) are supported. The corresponding
integration classes reside in the
org.springframework.orm.toplink
package.
Spring's TopLink support has been co-developed with the Oracle TopLink team. Many thanks to the TopLink team, in particular to Jim Clark who helped to clarify details in all areas!
TopLink itself does not ship with a SessionFactory abstraction.
Instead, multi-threaded access is based on the concept of a central
ServerSession
, which in turn is able to spawn
ClientSession
instances for single-threaded usage.
For flexible setup options, Spring defines a
SessionFactory
abstraction for TopLink,
enabling to switch between different
Session
creation strategies.
As a one-stop shop, Spring provides a
LocalSessionFactoryBean
class that allows for
defining a TopLink SessionFactory
with
bean-style configuration. It needs to be configured with the location of
the TopLink session configuration file, and usually also receives a
Spring-managed JDBC DataSource
to use.
<beans> <bean id="dataSource" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close"> <property name="driverClassName" value="${jdbc.driverClassName}"/> <property name="url" value="${jdbc.url}"/> <property name="username" value="${jdbc.username}"/> <property name="password" value="${jdbc.password}"/> </bean> <bean id="mySessionFactory" class="org.springframework.orm.toplink.LocalSessionFactoryBean"> <property name="configLocation" value="toplink-sessions.xml"/> <property name="dataSource" ref="dataSource"/> </bean> </beans>
<toplink-configuration> <session> <name>Session</name> <project-xml>toplink-mappings.xml</project-xml> <session-type> <server-session/> </session-type> <enable-logging>true</enable-logging> <logging-options/> </session> </toplink-configuration>
Usually, LocalSessionFactoryBean
will hold
a multi-threaded TopLink ServerSession
underneath and
create appropriate client Session
s for
it: either a plain Session
(typical), a
managed ClientSession
, or a transaction-aware
Session
(the latter are mainly used
internally by Spring's TopLink support). It might also hold a
single-threaded TopLink DatabaseSession
; this is
rather unusual, though.
Each TopLink-based DAO will then receive the
SessionFactory
through dependency
injection, i.e. through a bean property setter or through a constructor
argument. Such a DAO could be coded against plain TopLink API, fetching
a Session
from the given
SessionFactory
, but will usually rather
be used with Spring's TopLinkTemplate
:
<beans> <bean id="myProductDao" class="product.ProductDaoImpl"> <property name="sessionFactory" ref="mySessionFactory"/> </bean> </beans>
public class TopLinkProductDao implements ProductDao {
private TopLinkTemplate tlTemplate;
public void setSessionFactory(SessionFactory sessionFactory) {
this.tlTemplate = new TopLinkTemplate(sessionFactory);
}
public Collection loadProductsByCategory(final String category) throws DataAccessException {
return (Collection) this.tlTemplate.execute(new TopLinkCallback() {
public Object doInTopLink(Session session) throws TopLinkException {
ReadAllQuery findOwnersQuery = new ReadAllQuery(Product.class);
findOwnersQuery.addArgument("Category");
ExpressionBuilder builder = this.findOwnersQuery.getExpressionBuilder();
findOwnersQuery.setSelectionCriteria(
builder.get("category").like(builder.getParameter("Category")));
Vector args = new Vector();
args.add(category);
List result = session.executeQuery(findOwnersQuery, args);
// do some further stuff with the result list
return result;
}
});
}
}
A callback implementation can effectively be used for any TopLink
data access. TopLinkTemplate
will ensure that
Session
s are properly opened and closed,
and automatically participate in transactions. The template instances
are thread-safe and reusable, they can thus be kept as instance
variables of the surrounding class. For simple single-step actions such
as a single executeQuery
, readAll
,
readById
, or merge
call,
JdoTemplate
offers alternative convenience
methods that can replace such one line callback implementations.
Furthermore, Spring provides a convenient
TopLinkDaoSupport
base class that provides a
setSessionFactory(..)
method for receiving a
SessionFactory
, and
getSessionFactory()
and
getTopLinkTemplate()
for use by subclasses. In
combination, this allows for simple DAO implementations for typical
requirements:
public class ProductDaoImpl extends TopLinkDaoSupport implements ProductDao { public Collection loadProductsByCategory(String category) throws DataAccessException { ReadAllQuery findOwnersQuery = new ReadAllQuery(Product.class); findOwnersQuery.addArgument("Category"); ExpressionBuilder builder = this.findOwnersQuery.getExpressionBuilder(); findOwnersQuery.setSelectionCriteria( builder.get("category").like(builder.getParameter("Category"))); return getTopLinkTemplate().executeQuery(findOwnersQuery, new Object[] {category}); } }
Side note: TopLink query objects are thread-safe and can be cached within the DAO, i.e. created on startup and kept in instance variables.
As alternative to working with Spring's
TopLinkTemplate
, you can also code your TopLink data
access based on the raw TopLink API, explicitly opening and closing a
Session
. As elaborated in the
corresponding Hibernate section, the main advantage of this approach is
that your data access code is able to throw checked exceptions.
TopLinkDaoSupport
offers a variety of support methods
for this scenario, for fetching and releasing a transactional
Session
as well as for converting
exceptions.
DAOs can also be written against plain TopLink API, without any
Spring dependencies, directly using an injected TopLink
Session
. The latter will usually be based
on a SessionFactory
defined by a
LocalSessionFactoryBean
, exposed for bean
references of type Session
through
Spring's TransactionAwareSessionAdapter
.
The getActiveSession()
method defined on
TopLink's Session
interface will return
the current transactional Session
in such
a scenario. If there is no active transaction, it will return the shared
TopLink ServerSession
as-is, which is only supposed
to be used directly for read-only access. There is also an analogous
getActiveUnitOfWork()
method, returning the TopLink
UnitOfWork
associated with the current transaction,
if any (returning null
else).
A corresponding DAO implementation looks like as follows:
public class ProductDaoImpl implements ProductDao { private Session session; public void setSession(Session session) { this.session = session; } public Collection loadProductsByCategory(String category) { ReadAllQuery findOwnersQuery = new ReadAllQuery(Product.class); findOwnersQuery.addArgument("Category"); ExpressionBuilder builder = this.findOwnersQuery.getExpressionBuilder(); findOwnersQuery.setSelectionCriteria( builder.get("category").like(builder.getParameter("Category"))); Vector args = new Vector(); args.add(category); return session.getActiveSession().executeQuery(findOwnersQuery, args); } }
As the above DAO still follows the Dependency Injection pattern,
it still fits nicely into a Spring application context, analogous to
like it would if coded against Spring's
TopLinkTemplate
. Spring's
TransactionAwareSessionAdapter
is used to expose a
bean reference of type Session
, to be
passed into the DAO:
<beans> <bean id="mySessionAdapter" class="org.springframework.orm.toplink.support.TransactionAwareSessionAdapter"> <property name="sessionFactory" ref="mySessionFactory"/> </bean> <bean id="myProductDao" class="product.ProductDaoImpl"> <property name="session" ref="mySessionAdapter"/> </bean> </beans>
The main advantage of this DAO style is that it depends on TopLink API only; no import of any Spring class is required. This is of course appealing from a non-invasiveness perspective, and might feel more natural to TopLink developers.
However, the DAO throws plain TopLinkException
(which is unchecked, so does not have to be declared or caught), which
means that callers can only treat exceptions as generally fatal - unless
they want to depend on TopLink's own exception structure. Catching
specific causes such as an optimistic locking failure is not possible
without tying the caller to the implementation strategy. This tradeoff
might be acceptable to applications that are strongly TopLink-based
and/or do not need any special exception treatment.
A further disadvantage of that DAO style is that TopLink's
standard getActiveSession()
feature just works within
JTA transactions. It does not work with any other transaction strategy
out-of-the-box, in particular not with local TopLink transactions.
Fortunately, Spring's
TransactionAwareSessionAdapter
exposes a
corresponding proxy for the TopLink ServerSession
which supports TopLink's Session.getActiveSession()
and Session.getActiveUnitOfWork()
methods for any
Spring transaction strategy, returning the current Spring-managed
transactional Session
even with
TopLinkTransactionManager
. Of course, the standard
behavior of that method remains: returning the current
Session
associated with the ongoing JTA
transaction, if any (no matter whether driven by Spring's
JtaTransactionManager
, by EJB CMT, or by plain
JTA).
In summary: DAOs can be implemented based on plain TopLink API,
while still being able to participate in Spring-managed transactions.
This might in particular appeal to people already familiar with TopLink,
feeling more natural to them. However, such DAOs will throw plain
TopLinkException
; conversion to Spring's
DataAccessException
would have to happen
explicitly (if desired).
To execute service operations within transactions, you can use Spring's common declarative transaction facilities. For example:
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:aop="http://www.springframework.org/schema/aop" xmlns:tx="http://www.springframework.org/schema/tx" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.0.xsd http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-2.0.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-2.0.xsd"> <bean id="myTxManager" class="org.springframework.orm.toplink.TopLinkTransactionManager"> <property name="sessionFactory" ref="mySessionFactory"/> </bean> <bean id="myProductService" class="product.ProductServiceImpl"> <property name="productDao" ref="myProductDao"/> </bean> <aop:config> <aop:pointcut id="productServiceMethods" expression="execution(* product.ProductService.*(..))"/> <aop:advisor advice-ref="txAdvice" pointcut-ref="productServiceMethods"/> </aop:config> <tx:advice id="txAdvice" transaction-manager="myTxManager"> <tx:attributes> <tx:method name="increasePrice*" propagation="REQUIRED"/> <tx:method name="someOtherBusinessMethod" propagation="REQUIRES_NEW"/> <tx:method name="*" propagation="SUPPORTS" read-only="true"/> </tx:attributes> </tx:advice> </beans>
Note that TopLink requires an active UnitOfWork
for modifying a persistent object. (You should never modify objects
returned by a plain TopLink Session
-
those are usually read-only objects, directly taken from the
second-level cache!) There is no concept like a non-transactional flush
in TopLink, in contrast to Hibernate. For this reason, TopLink needs to
be set up for a specific environment: in particular, it needs to be
explicitly set up for JTA synchronization, to detect an active JTA
transaction itself and expose a corresponding active
Session
and
UnitOfWork
. This is not necessary for local
transactions as performed by Spring's
TopLinkTransactionManager
, but it is necessary for
participating in JTA transactions (whether driven by Spring's
JtaTransactionManager
or by EJB CMT / plain
JTA).
Within your TopLink-based DAO code, use the
Session.getActiveUnitOfWork()
method to access the
current UnitOfWork
and perform write operations
through it. This will only work within an active transaction (both
within Spring-managed transactions and plain JTA transactions). For
special needs, you can also acquire separate
UnitOfWork
instances that won't participate in the
current transaction; this is hardly needed, though.
TopLinkTransactionManager
is capable of
exposing a TopLink transaction to JDBC access code that accesses the
same JDBC DataSource
, provided that
TopLink works with JDBC in the backend and is thus able to expose the
underlying JDBC Connection
. The
DataSource
to expose the transactions for
needs to be specified explicitly; it won't be autodetected.
The iBATIS support in the Spring Framework much resembles the JDBC / Hibernate support in that it supports the same template style programming and just as with JDBC or Hibernate, the iBATIS support works with Spring's exception hierarchy and let's you enjoy the all IoC features Spring has.
Transaction management can be handled through Spring's standard
facilities. There are no special transaction strategies for iBATIS, as
there is no special transactional resource involved other than a JDBC
Connection
. Hence, Spring's standard JDBC
DataSourceTransactionManager
or
JtaTransactionManager
are perfectly
sufficient.
![]() | Note |
---|---|
Spring does actually support both iBatis 1.x and 2.x. However, only support for iBatis 2.x is actually shipped with the core Spring distribution. The iBatis 1.x support classes were moved to the Spring Modules project as of Spring 2.0, and you are directed there for documentation. |
If we want to map the previous Account class with iBATIS 2.x we
need to create the following SQL map
'Account.xml'
:
<sqlMap namespace="Account"> <resultMap id="result" class="examples.Account"> <result property="name" column="NAME" columnIndex="1"/> <result property="email" column="EMAIL" columnIndex="2"/> </resultMap> <select id="getAccountByEmail" resultMap="result"> select ACCOUNT.NAME, ACCOUNT.EMAIL from ACCOUNT where ACCOUNT.EMAIL = #value# </select> <insert id="insertAccount"> insert into ACCOUNT (NAME, EMAIL) values (#name#, #email#) </insert> </sqlMap>
The configuration file for iBATIS 2 looks like this:
<sqlMapConfig> <sqlMap resource="example/Account.xml"/> </sqlMapConfig>
Remember that iBATIS loads resources from the class path, so be
sure to add the 'Account.xml'
file to the class
path.
We can use the SqlMapClientFactoryBean
in the
Spring container. Note that with iBATIS SQL Maps 2.x, the
JDBC DataSource
is usually specified on
the SqlMapClientFactoryBean
, which enables lazy
loading.
<beans> <bean id="dataSource" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close"> <property name="driverClassName" value="${jdbc.driverClassName}"/> <property name="url" value="${jdbc.url}"/> <property name="username" value="${jdbc.username}"/> <property name="password" value="${jdbc.password}"/> </bean> <bean id="sqlMapClient" class="org.springframework.orm.ibatis.SqlMapClientFactoryBean"> <property name="configLocation" value="WEB-INF/sqlmap-config.xml"/> <property name="dataSource" ref="dataSource"/> </bean> </beans>
The SqlMapClientDaoSupport
class offers a
supporting class similar to the SqlMapDaoSupport
.
We extend it to implement our DAO:
public class SqlMapAccountDao extends SqlMapClientDaoSupport implements AccountDao { public Account getAccount(String email) throws DataAccessException { return (Account) getSqlMapClientTemplate().queryForObject("getAccountByEmail", email); } public void insertAccount(Account account) throws DataAccessException { getSqlMapClientTemplate().update("insertAccount", account); } }
In the DAO, we use the pre-configured
SqlMapClientTemplate
to execute the queries,
after setting up the SqlMapAccountDao
in the
application context and wiring it with our
SqlMapClient
instance:
<beans> <bean id="accountDao" class="example.SqlMapAccountDao"> <property name="sqlMapClient" ref="sqlMapClient"/> </bean> </beans>
Note that a SqlMapTemplate
instance could
also be created manually, passing in the
SqlMapClient
as constructor argument. The
SqlMapClientDaoSupport
base class simply
pre-initializes a SqlMapClientTemplate
instance
for us.
The SqlMapClientTemplate
also offers a
generic execute
method, taking a custom
SqlMapClientCallback
implementation as argument.
This can, for example, be used for batching:
public class SqlMapAccountDao extends SqlMapClientDaoSupport implements AccountDao { public void insertAccount(Account account) throws DataAccessException { getSqlMapClientTemplate().execute(new SqlMapClientCallback() { public Object doInSqlMapClient(SqlMapExecutor executor) throws SQLException { executor.startBatch(); executor.update("insertAccount", account); executor.update("insertAddress", account.getAddress()); executor.executeBatch(); } }); } }
In general, any combination of operations offered by the native
SqlMapExecutor
API can be used in such a callback.
Any SQLException
thrown will automatically get
converted to Spring's generic
DataAccessException
hierarchy.
DAOs can also be written against plain iBATIS API, without any
Spring dependencies, directly using an injected
SqlMapClient
. A corresponding DAO implementation
looks like as follows:
public class SqlMapAccountDao implements AccountDao { private SqlMapClient sqlMapClient; public void setSqlMapClient(SqlMapClient sqlMapClient) { this.sqlMapClient = sqlMapClient; } public Account getAccount(String email) { try { return (Account) this.sqlMapClient.queryForObject("getAccountByEmail", email); } catch (SQLException ex) { throw new MyDaoException(ex); } } public void insertAccount(Account account) throws DataAccessException { try { this.sqlMapClient.update("insertAccount", account); } catch (SQLException ex) { throw new MyDaoException(ex); } } }
In such a scenario, the SQLException
thrown
by the iBATIS API needs to be handled in a custom fashion: usually,
wrapping it in your own application-specific DAO exception. Wiring in
the application context would still look like before, due to the fact
that the plain iBATIS-based DAO still follows the Dependency Injection
pattern:
<beans> <bean id="accountDao" class="example.SqlMapAccountDao"> <property name="sqlMapClient" ref="sqlMapClient"/> </bean> </beans>
Spring JPA (available under the
org.springframework.orm.jpa
package) offers
comprehensive support for the Java
Persistence API in a similar manner to the integration with
Hibernate or JDO, while being aware of the underlying implementation in
order to provide additional features.
Spring JPA offers two ways of setting up JPA
EntityManagerFactory
:
The LocalEntityManagerFactoryBean
creates
an EntityManager
suitable for
environments which solely use JPA for data access. The factory bean
will use the JPA PersistenceProvider
autodetection mechanism and in most cases, requires only the
persistence unit name:
<beans> <bean id="entityManagerFactory" class="org.springframework.orm.jpa.LocalEntityManagerFactoryBean"> <property name="persistenceUnitName" value="myPersistenceUnit"/> </bean> </beans>
Switching to a JNDI EntityManagerFactory
(for example in JTA environments), is just a matter of changing the XML
configuration:
<beans> <jndi:lookup id="entityManagerFactory" jndi-name="jpa/myPersistenceUnit"/> </beans>
The
LocalContainerEntityManagerFactoryBean
gives
full control over the JPA
EntityManagerFactory
and is appropriate
for environments where fine customizations are required. The
LocalContainerEntityManagerFactoryBean
will
create a PersistenceUnitInfo
based on
the 'persistence.xml'
file, the supplied
dataSourceLookup
strategy and the
loadTimeWeaver
. It is thus possible to work with
custom datasources outside of JNDI and control the weaving
process.
<beans> <bean id="entityManagerFactory" class="org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean"> <property name="dataSource" ref="someDataSource"/> <property name="loadTimeWeaver"> <bean class="org.springframework.instrument.classloading.InstrumentationLoadTimeWeaver"/> </property> </bean> </beans>
The LoadTimeWeaver
interface is a
Spring-provided class that allows JPA
ClassTransformer
instances to be
plugged in a specific manner depending on the environment (web
container/application server). Hooking
ClassTransformers
through a JDK 5.0 agent
is typically not efficient - the agents work against the
entire virtual machine and inspect
every class that is loaded - something that is
typically undesirable in a production server enviroment.
Spring provides a number of
LoadTimeWeaver
implementations for
various environments, allowing
ClassTransformer
instances to be
applied only per classloader and not per
VM.
Jakarta
Tomcat's default classloader does not support class
transformation but allows custom classloaders to be used. Spring
offers the TomcatInstrumentableClassLoader
(inside the
org.springframework.instrument.classloading.tomcat
package) which extends the Tomcat classloader
(WebappClassLoader
) and allows JPA
ClassTransformer
instances to 'enhance' all
classes loaded by it. In short, JPA transformers will be applied
only inside a specific web application (which uses the
TomcatInstrumentableClassLoader
).
In order to use the custom classloader on:
Copy spring-tomcat-weaver.jar
into
$CATALINA_HOME/server/lib (where
$CATALINA_HOME represents the root of the
Tomcat installation).
Instruct Tomcat to use the custom classloader (instead of the default one) by editing the web application context file:
<Context path="/myWebApp" docBase="/my/webApp/location"> <Loader loaderClass="org.springframework.instrument.classloading.tomcat.TomcatInstrumentableClassLoader" /> </Context>
Tomcat 5.0.x and 5.5.x series support several context locations: server configuration file ($CATALINA_HOME/conf/server.xml), the default context configuration ($CATALINA_HOME/conf/context.xml) that affects all deployed web applications and per-webapp configurations, deployed on the server ($CATALINA_HOME/conf/[enginename]/[hostname]/my-webapp-context.xml) side or along with the webapp (your-webapp.war/META-INF/context.xml). For efficiency, inside the web-app configuration style is recommended since only applications which use JPA will use the custom classloader. See the Tomcat 5.x documentation for more details about available context locations.
Note that versions prior to 5.5.20 contained a bug in the
XML configuration parsing preventing usage of Loader
tag inside server.xml (no matter
if a classloader is specified or not (be it the official or a
custom one). See Tomcat's bugzilla for more
details.
If you are using Tomcat 5.5.20+ you can set useSystemClassLoaderAsParent to
false
to fix the problem:
<Context path="/myWebApp" docBase="/my/webApp/location"> <Loader loaderClass="org.springframework.instrument.classloading.tomcat.TomcatInstrumentableClassLoader" useSystemClassLoaderAsParent="false"/> </Context>
In Tomcat 4.x, one can use the same contex.xml and place it under $CATALINA_HOME/webapps or modify $CATALINA_HOME/conf/server.xml to use the custom classloader by default. See the Tomcat 4.x documentation for more information.
Copy spring-tomcat-weaver.jar
into
$CATALINA_HOME/lib (where
$CATALINA_HOME represents the root of the
Tomcat installation).
Instruct Tomcat to use the custom classloader (instead of the default one) by editing the web application context file:
<Context path="/myWebApp" docBase="/my/webApp/location"> <Loader loaderClass="org.springframework.instrument.classloading.tomcat.TomcatInstrumentableClassLoader"/> </Context>
Tomcat 6.0.x (similar to 5.0.x/5.5.x) series support several context locations: server configuration file ($CATALINA_HOME/conf/server.xml), the default context configuration ($CATALINA_HOME/conf/context.xml) that affects all deployed web applications and per-webapp configurations, deployed on the server ($CATALINA_HOME/conf/[enginename]/[hostname]/my-webapp-context.xml) side or along with the webapp (your-webapp.war/META-INF/context.xml). For efficiency, inside the web-app configuration style is recommended since only applications which use JPA will use the custom classloader. See the Tomcat 5.x documentation for more details about available context locations.
Tomcat 4.x/5.x/5.5.x
Tomcat 6.0.x
The last step required on all Tomcat versions, is to use the appropriate
the LoadTimeWeaver
when configuring LocalContainerEntityManagerFactoryBean
:
<bean id="entityManagerFactory" class="org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean"> <property name="loadTimeWeaver"> <bean class="org.springframework.instrument.classloading.ReflectiveLoadTimeWeaver"/> </property> </bean>
Using this technique, JPA applications relying on instrumentation, can run in Tomcat without the need of an agent. This is important especially when hosting applications which rely on different JPA implementations since the JPA transformers are applied only at classloader level and thus, are isolated from each other.
![]() | Note |
---|---|
If TopLink is being used a JPA provider under Tomcat, please place the toplink-essentials jar under $CATALINA_HOME/shared/lib folder instead of your war. |
As Oracle's OC4J
classloader has native bytecode transformation support, switching
from an JDK agent to a LoadTimeWeaver
can be done
just through the application Spring configuration:
<bean id="entityManagerFactory" class="org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean"> <property name="loadTimeWeaver"> <bean class="org.springframework.instrument.classloading.oc4j.OC4JLoadTimeWeaver"/> </property> </bean>
GlassFish application server provides out of the box, an instrumentat