Chapter 3. The contextual component model - JBoss Seam 1.2.1 正式版英文参考手册

Components which are truly stateless (stateless session beans, primarily) always live in the stateless context (this is really a non-context). Stateless components are not very interesting, and are arguably not very object-oriented. Nevertheless, they are important and often useful.

The event context is the "narrowest" stateful context, and is a generalization of the notion of the web request context to cover other kinds of events. Nevertheless, the event context associated with the lifecycle of a JSF request is the most important example of an event context, and the one you will work with most often. Components associated with the event context are destroyed at the end of the request, but their state is available and well-defined for at least the lifecycle of the request.

When you invoke a Seam component via RMI, or Seam Remoting, the event context is created and destroyed just for the invocation.

The page context allows you to associate state with a particular instance of a rendered page. You can initialize state in your event listener, or while actually rendering the page, and then have access to it from any event that originates from that page. This is especially useful for functionality like clickable lists, where the list is backed by changing data on the server side. The state is actually serialized to the client, so this construct is extremely robust with respect to multi-window operation and the back button.

The conversation context is a truly central concept in Seam. A conversation is a unit of work from the point of view of the user. It might span several interactions with the user, several requests, and several database transactions. But to the user, a conversation solves a single problem. For example, "book hotel", "approve contract", "create order" are all conversations. You might like to think of a conversation implementing a single "use case" or "user story", but the relationship is not necessarily quite exact.

A conversation holds state associated with "what the user is doing now, in this window". A single user may have multiple conversations in progress at any point in time, usually in multiple windows. The conversation context allows us to ensure that state from the different conversations does not collide and cause bugs.

It might take you some time to get used to thinking of applications in terms of conversations. But once you get used to it, we think you'll love the notion, and never be able to not think in terms of conversations again!

Some conversations last for just a single request. Conversations that span multiple requests must be demarcated using annotations provided by Seam.

Some conversations are also tasks. A task is a conversation that is significant in terms of a long-running business process, and has the potential to trigger a business process state transition when it is successfully completed. Seam provides a special set of annotations for task demarcation.

Conversations may be nested, with one conversation taking place "inside" a wider conversation. This is an advanced feature.

Usually, conversation state is actually held by Seam in the servlet session between requests. Seam implements configurable conversation timeout, automatically destroying inactive conversations, and thus ensuring that the state held by a single user login session does not grow without bound if the user abandons conversations.

Seam serializes processing of concurrent requests that take place in the same long-running conversation context, in the same process.

Alternatively, Seam may be configured to keep conversational state in the client browser.

A session context holds state associated with the user login session. While there are some cases where it is useful to share state between several conversations, we generally frown on the use of session context for holding components other than global information about the logged in user.

In a JSR-168 portal environment, the session context represents the portlet session.

The business process context holds state associated with the long running business process. This state is managed and made persistent by the BPM engine (JBoss jBPM). The business process spans multiple interactions with multiple users, so this state is shared between multiple users, but in a well-defined manner. The current task determines the current business process instance, and the lifecycle of the business process is defined externally using a process definition language, so there are no special annotations for business process demarcation.

The application context is the familiar servlet context from the servlet spec. Application context is mainly useful for holding static information such as configuration data, reference data or metamodels. For example, Seam stores its own configuration and metamodel in the application context.

A context defines a namespace, a set of context variables. These work much the same as session or request attributes in the servlet spec. You may bind any value you like to a context variable, but usually we bind Seam component instances to context variables.

So, within a context, a component instance is identified by the context variable name (this is usually, but not always, the same as the component name). You may programatically access a named component instance in a particular scope via the Contexts class, which provides access to several thread-bound instances of the Context interface:

User user = (User) Contexts.getSessionContext().get("user");

You may also set or change the value associated with a name:

Contexts.getSessionContext().set("user", user);

Usually, however, we obtain components from a context via injection, and put component instances into a context via outjection.

Sometimes, as above, component instances are obtained from a particular known scope. Other times, all stateful scopes are searched, in priority order. The order is as follows:

You can perform a priority search by calling Contexts.lookupInStatefulContexts(). Whenever you access a component by name from a JSF page, a priority search occurs.

Neither the servlet nor EJB specifications define any facilities for managing concurrent requests originating from the same client. The servlet container simply lets all threads run concurrently and leaves enforcing threadsafeness to application code. The EJB container allows stateless components to be accessed concurrently, and throws an exception if multiple threads access a stateful session bean.

This behavior might have been okay in old-style web applications which were based around fine-grained, synchronous requests. But for modern applications which make heavy use of many fine-grained, asynchronous (AJAX) requests, concurrency is a fact of life, and must be supported by the programming model. Seam weaves a concurrency management layer into its context model.

The Seam session and application contexts are multithreaded. Seam will allow concurrent requests in a context to be processed concurrently. The event and page contexts are by nature single threaded. The business process context is strictly speaking multi-threaded, but in practice concurrency is sufficiently rare that this fact may be disregarded most of the time. Finally, Seam enforces a single thread per conversation per process model for the conversation context by serializing concurrent requests in the same long-running conversation context.

Since the session context is multithreaded, and often contains volatile state, session scope components are always protected by Seam from concurrent access. Seam serializes requests to session scope session beans and JavaBeans by default (and detects and breaks any deadlocks that occur). This is not the default behaviour for application scoped components however, since application scoped components do not usually hold volatile state and because synchronization at the global level is extremely expensive. However, you can force a serialized threading model on any session bean or JavaBean component by adding the @Synchronized annotation.

This concurrency model means that AJAX clients can safely use volatile session and conversational state, without the need for any special work on the part of the developer.

Stateless session bean components are not able to hold state across multiple invocations. Therefore, they usually work by operating upon the state of other components in the various Seam contexts. They may be used as JSF action listeners, but cannot provide properties to JSF components for display.

Stateless session beans always live in the stateless context.

Stateless session beans are the least interesting kind of Seam component.

Stateful session bean components are able to hold state not only across multiple invocations of the bean, but also across multiple requests. Application state that does not belong in the database should usually be held by stateful session beans. This is a major difference between Seam and many other web application frameworks. Instead of sticking information about the current conversation directly in the HttpSession, you should keep it in instance variables of a stateful session bean that is bound to the conversation context. This allows Seam to manage the lifecycle of this state for you, and ensure that there are no collisions between state relating to different concurrent conversations.

Stateful session beans are often used as JSF action listener, and as backing beans that provide properties to JSF components for display or form submission.

By default, stateful session beans are bound to the conversation context. They may never be bound to the page or stateless contexts.

Concurrent requests to session-scoped stateful session beans are always serialized by Seam.

Entity beans may be bound to a context variable and function as a seam component. Because entities have a persistent identity in addition to their contextual identity, entity instances are usually bound explicitly in Java code, rather than being instantiated implicitly by Seam.

Entity bean components do not support bijection or context demarcation. Nor does invocation of an entity bean trigger validation.

Entity beans are not usually used as JSF action listeners, but do often function as backing beans that provide properties to JSF components for display or form submission. In particular, it is common to use an entity as a backing bean, together with a stateless session bean action listener to implement create/update/delete type functionality.

By default, entity beans are bound to the conversation context. They may never be bound to the stateless context.

Note that it in a clustered environment is somewhat less efficient to bind an entity bean directly to a conversation or session scoped Seam context variable than it would be to hold a reference to the entity bean in a stateful session bean. For this reason, not all Seam applications define entity beans to be Seam components.

Javabeans may be used just like a stateless or stateful session bean. However, they do not provide the functionality of a session bean (declarative transaction demarcation, declarative security, efficient clustered state replication, EJB 3.0 persistence, timeout methods, etc).

In a later chapter, we show you how to use Seam and Hibernate without an EJB container. In this use case, components are JavaBeans instead of session beans. Note, however, that in many application servers it is somewhat less efficient to cluster conversation or session scoped Seam JavaBean components than it is to cluster stateful session bean components.

By default, JavaBeans are bound to the event context.

Concurrent requests to session-scoped JavaBeans are always serialized by Seam.

Message-driven beans may function as a seam component. However, message-driven beans are called quite differently to other Seam components - instead of invoking them via the context variable, they listen for messages sent to a JMS queue or topic.

Message-driven beans may not be bound to a Seam context. Nor do they have access to the session or conversation state of their "caller". However, they do support bijection and some other Seam functionality.

In order to perform its magic (bijection, context demarcation, validation, etc), Seam must intercept component invocations. For JavaBeans, Seam is in full control of instantiation of the component, and no special configuration is needed. For entity beans, interception is not required since bijection and context demarcation are not defined. For session beans, we must register an EJB interceptor for the session bean component. We could use an annotation, as follows:

@Stateless
@Interceptors(SeamInterceptor.class)
public class LoginAction implements Login { 
    ... 
}

But a much better way is to define the interceptor in ejb-jar.xml.

<interceptors>
   <interceptor>
      <interceptor-class>org.jboss.seam.ejb.SeamInterceptor</interceptor-class>
   </interceptor>
</interceptors>
   
<assembly-descriptor>
   <interceptor-binding>
      <ejb-name>*</ejb-name>
      <interceptor-class>org.jboss.seam.ejb.SeamInterceptor</interceptor-class>
   </interceptor-binding>
</assembly-descriptor>

All seam components need a name. We can assign a name to a component using the @Name annotation:

@Name("loginAction")
@Stateless
public class LoginAction implements Login { 
    ... 
}

This name is the seam component name and is not related to any other name defined by the EJB specification. However, seam component names work just like JSF managed bean names and you can think of the two concepts as identical.

@Name is not the only way to define a component name, but we always need to specify the name somewhere. If we don't, then none of the other Seam annotations will function.

Just like in JSF, a seam component instance is usually bound to a context variable with the same name as the component name. So, for example, we would access the LoginAction using Contexts.getStatelessContext().get("loginAction"). In particular, whenever Seam itself instantiates a component, it binds the new instance to a variable with the component name. However, again like JSF, it is possible for the application to bind a component to some other context variable by programmatic API call. This is only useful if a particular component serves more than one role in the system. For example, the currently logged in User might be bound to the currentUser session context variable, while a User that is the subject of some administration functionality might be bound to the user conversation context variable.

For very large applications, and for built-in seam components, qualified names are often used.

@Name("com.jboss.myapp.loginAction")
@Stateless
@Interceptors(SeamInterceptor.class)
public class LoginAction implements Login { 
    ... 
}

We may use the qualified component name both in Java code and in JSF's expression language:

<h:commandButton type="submit" value="Login"
                 action="#{com.jboss.myapp.loginAction.login}"/>

Since this is noisy, Seam also provides a means of aliasing a qualified name to a simple name. Add a line like this to the components.xml file:

<factory name="loginAction" scope="STATELESS" value="#{com.jboss.myapp.loginAction}"/>

All of the built-in Seam components have qualified names, but most of them are aliased to a simple name by the components.xml file included in the Seam jar.

We can override the default scope (context) of a component using the @Scope annotation. This lets us define what context a component instance is bound to, when it is instantiated by Seam.

@Name("user")
@Entity
@Scope(SESSION)
public class User { 
    ... 
}

org.jboss.seam.ScopeType defines an enumeration of possible scopes.

Some Seam component classes can fulfill more than one role in the system. For example, we often have a User class which is usually used as a session-scoped component representing the current user but is used in user administration screens as a conversation-scoped component. The @Role annotation lets us define an additional named role for a component, with a different scope—it lets us bind the same component class to different context variables. (Any Seam component instance may be bound to multiple context variables, but this lets us do it at the class level, and take advantage of auto-instantiation.)

@Name("user")
@Entity
@Scope(CONVERSATION)
@Role(name="currentUser", scope=SESSION)
public class User { 
    ... 
}

The @Roles annotation lets us specify as many additional roles as we like.

@Name("user")
@Entity
@Scope(CONVERSATION)
@Roles({@Role(name="currentUser", scope=SESSION)
        @Role(name="tempUser", scope=EVENT)})
public class User { 
    ... 
}

Like many good frameworks, Seam eats its own dogfood and is implemented mostly as a set of built-in Seam interceptors (see later) and Seam components. This makes it easy for applications to interact with built-in components at runtime or even customize the basic functionality of Seam by replacing the built-in components with custom implementations. The built-in components are defined in the Seam namespace org.jboss.seam.core and the Java package of the same name.

The built-in components may be injected, just like any Seam components, but they also provide convenient static instance() methods:

FacesMessages.instance().add("Welcome back, #{user.name}!");

Seam was designed to integrate tightly in a Java EE 5 environment. However, we understand that there are many projects which are not running in a full EE environment. We also realize the critical importance of easy unit and integration testing using frameworks such as TestNG and JUnit. So, we've made it easy to run Seam in Java SE environments by allowing you to boostrap certain critical infrastructure normally only found in EE environments by installing built-in Seam components.

For example, you can run your EJB3 components in Tomcat or an integration test suite just by installing the built-in component org.jboss.seam.core.ejb, which automatically bootstraps the JBoss Embeddable EJB3 container and deploys your EJB components.

Or, if you're not quite ready for the Brave New World of EJB 3.0, you can write a Seam application that uses only JavaBean components, together with Hibernate3 for persistence, by installing a built-in component that manages a Hibernate SessionFactory. When using Hibernate outside of a J2EE environment, you will also probably need a JTA transaction manager and JNDI server, which are available via the built-in component org.jboss.seam.core.microcontainer. This lets you use the bulletproof JTA/JCA pooling datasource from JBoss application server in an SE environment like Tomcat!