Chapter 2. The contextual component model - JBoss Seam 1.0.0 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 distroyed 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", 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.

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.

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.

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.

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, automatic 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.

By default, JavaBeans are bound to the conversation context.

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 any Seam contexts apart from the EVENT and APPLICATION contexts. 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.

Almost all seam components need a name. We 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.

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 { 
    ... 
}

Unfortunately, JSF's expression language interprets a period as a property dereference. So, inside a JSF expression, we use $ to indicate a qualified component name:

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

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 the built-in component org.jboss.seam.core.hibernate. 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!

Seam components may be provided with configuration properties either via servlet context parameters, or via a properties file named seam.properties in the root of the classpath.

The configurable Seam component must expose a JavaBeans-style property setter methods for the configurable attributes. If a seam component named com.jboss.myapp.settings has a setter method named setLocale(), we can provide a property named com.jboss.myapp.settings.locale in the seam.properties file or as a servlet context parameter, and Seam will set the value of the locale attribute whenever it instantiates the component.

Note that it is not possible to configure stateless session beans or entity beans.

The same mechanism is used to configure Seam itself. For example, to set the conversation timeout, we provide a value for org.jboss.seam.core.manager.conversationTimeout in web.xml or seam.properties. (There is a built-in Seam component named org.jboss.seam.core.manager with a setter method named setConversationTimeout().)

The components.xml is a bit more powerful than property settings. It lets you:

Usually, Seam components are installed when the deployment scanner discovers a class with a @Name annotation sitting in an archive with a seam.properties file. The components.xml file lets us handle special cases where that is not the case.

For example, the following components.xml file installs the JBoss Embeddable EJB3 container:

<components>
    <component class="org.jboss.seam.core.Ejb"/>
</components>

This one installs and configures two different Seam-managed persistence contexts:

<components>

    <component name="customerDatabase" 
              class="org.jboss.seam.core.ManagedPersistenceContext">
        <property name="persistenceUnitJndiName">java:/customerEntityManagerFactory</property>
    </component>
    
    <component name="accountingDatabase"
              class="org.jboss.seam.core.ManagedPersistenceContext">
        <property name="persistenceUnitJndiName">java:/accountingEntityManagerFactory</property>
    </component>

</components>

Sometimes we want to reuse the same components.xml file with minor changes during both deployment and testing. Seam let's you place wildcards of the form @wildcard@ in the components.xml file which can be replaced either by your Ant build script (at deployment time) or by providing a file named components.properties in the classpath (at development time). You'll see this approach used in the Seam examples.

A Seam page action is an event that occurs just before we render a page. We declare page actions in WEB-INF/pages.xml. We can define a page action for either a particular JSF view id:

<pages>
    <page view-id="/hello.jsp" action="#{helloWorld.sayHello}"/>
<pages>

Or we can use a wildcard to specify an action that applies to all view ids that match the pattern:

<pages>
    <page view-id="/hello/*" action="#{helloWorld.sayHello}"/>
<pages>

If multiple wildcarded page actions match the current view-id, Seam will call all the actions, in order of least-specific to most-specific.

The page action method can return a JSF outcome. If the outcome is non-null, Seam will delegate to the defined JSF navigation rules and a different view may end up being rendered.

Furthermore, the view id mentioned in the <page> element need not correspond to a real JSP or Facelets page! So, we can reproduce the functionality of a traditional action-oriented framework like Struts or WebWork using page actions. For example:

TODO: translate struts action into page action

This is quite useful if you want to do complex things in response to non-faces requests (for example, HTTP GET requests).

Seam components can interact by simply calling each others methods. Stateful components may even implement the observer/observable pattern. But to enable components to interact in a more loosely-coupled fashion than is possible when the components call each others methods directly, Seam provides component-driven events.

We specify event listeners (observers) in WEB-INF/events.xml.

<events>
    <event type="hello">
        <action expression="#{helloListener.sayHelloBack}"/>
        <action expression="#{logger.logHello}"/>
    </event>
<events>

Where the event type is just an arbitrary string.

When an event occurs, the actions registered for that event will be called in the order they appear in events.xml. How does a component raise an event? Seam provides a built-in component for this.

@Name("helloWorld")
public class HelloWorld {
    public void sayHello() {
        FacesMessages.instance().add("Hello World!");
        Events.instance().raiseEvent("hello");
    }
}

Notice that this event producer has no dependency upon event consumers. The event listener may now be implemented with absolutely no dependency upon the producer:

@Name("helloListener")
public class HelloListener {
    public void sayHelloBack() {
        FacesMessages.instance().add("Hello to you too!");
    }
}

If you don't like the events.xml file, we can use an annotation instead:

@Name("helloListener")
public class HelloListener {
    @Observer("hello")
    public void sayHelloBack() {
        FacesMessages.instance().add("Hello to you too!");
    }
}

You might wonder why I've not mentioned anything about event objects in this discussion. In Seam, there is no need for an event object to propagate state between event producer and listener. All state is held in the Seam contexts, and is shared between components.