Chapter 5. Events, interceptors and exception handling - JBoss Seam 1.1.6 正式版英文参考手册

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 JSFadn Seam 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).

<pages>
    <page view-id="/hello.jsp" action="#{helloWorld.sayHello}">
        <param name="firstName" value="#{person.firstName}"/>
        <param name="lastName" value="#{person.lastName}"/>
    </page>
</pages>

<pages>
    <page view-id="/hello.jsp">
        <param name="firstName" value="#{person.firstName}"/>
        <param name="lastName" value="#{person.lastName}"/>
    </page>
</pages>

<pages>
    <page view-id="/calculator.jsp" action="#{calculator.calculate}">
        <param name="x" value="#{calculator.lhs}"/>
        <param name="y" value="#{calculator.rhs}"/>
        <param name="op" converterId="com.my.calculator.OperatorConverter" value="#{calculator.op}"/>
    </page>
</pages>

<pages>
    <page view-id="/calculator.jsp" action="#{calculator.calculate}">
        <param name="x" value="#{calculator.lhs}"/>
        <param name="y" value="#{calculator.rhs}"/>
        <param name="op" converter="#{operatorConverter}" value="#{calculator.op}"/>
    </page>
</pages>

<navigation-rule>
    <from-view-id>/editDocument.xhtml</from-view-id>
    
    <navigation-case>
        <from-action>#{documentEditor.update}</from-action>
        <from-outcome>success</from-outcome>
        <to-view-id>/viewDocument.xhtml</to-view-id>
        <redirect/>
    </navigation-case>
    
</navigation-rule>

<page view-id="/editDocument.xhtml">
    
    <navigation from-action="#{documentEditor.update}">
        <rule if-outcome="success">
            <redirect view-id="/viewDocument.xhtml"/>
        <rule/>
    </navigation-case>
    
</navigation-rule>

<page view-id="/editDocument.xhtml">
    
    <navigation from-action="#{documentEditor.update}" 
                   evaluate="#{documentEditor.errors.size}">
        <rule if-outcome="0">
            <redirect view-id="/viewDocument.xhtml"/>
        <rule/>
    </navigation-case>
    
</navigation-rule>

<page view-id="/editDocument.xhtml">
    
    <navigation from-action="#{documentEditor.update}">
        <rule if="#{documentEditor.errors.empty}">
            <redirect view-id="/viewDocument.xhtml"/>
        <rule/>
    </navigation-case>
    
</navigation-rule>

<page view-id="/editDocument.xhtml">
    
    <navigation from-action="#{documentEditor.update}">
        <rule if="#{documentEditor.errors.empty}">
            <end-conversation/>
            <redirect view-id="/viewDocument.xhtml"/>
        <rule/>
    </navigation-case>
    
</navigation-rule>

<page view-id="/editDocument.xhtml">
    
    <navigation from-action="#{documentEditor.update}">
        <rule if="#{documentEditor.errors.empty}">
            <end-conversation/>
            <render view-id="/viewDocument.xhtml"/>
        <rule/>
    </navigation-case>
    
</navigation-rule>

<page view-id="/editDocument.xhtml">
    
    <navigation from-action="#{documentEditor.update}">
        <rule if="#{documentEditor.errors.empty}">
            <end-conversation/>
            <redirect view-id="/viewDocument.xhtml">
                <param name="documentId" value="#{documentEditor.documentId}"/>
            </redirect>
        <rule/>
    </navigation-case>
    
</navigation-rule>

<page view-id="/editDocument.xhtml">
    
    <navigation from-action="#{documentEditor.update}">
        <rule>
            <render view-id="/viewDocument.xhtml"/>
        <rule/>
    </navigation-case>
    
</navigation-rule>

<page view-id="/editDocument.xhtml">
    
    <navigation from-action="#{documentEditor.update}">
        <render view-id="/viewDocument.xhtml"/>
    </navigation-case>
    
</navigation-rule>

<page action="#{calculator.calculate}">
    <param name="x" value="#{calculator.lhs}"/>
    <param name="y" value="#{calculator.rhs}"/>
    <param name="op" converter="#{operatorConverter}" value="#{calculator.op}"/>
</page>

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 components.xml.

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

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 components.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");
    }
}

Or you can use an annotation.

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

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!");
    }
}

The method binding defined in components.xml above takes care of mapping the event to the consumer. If you don't like futzing about in the components.xml file, you 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. State is held in the Seam contexts, and is shared between components. However, if you really want to pass an event object, you can:

@Name("helloWorld")
public class HelloWorld {
    private String name;
    public void sayHello() {
        FacesMessages.instance().add("Hello World, my name is #0.", name);
        Events.instance().raiseEvent("hello", name);
    }
}

@Name("helloListener")
public class HelloListener {
    @Observer("hello")
    public void sayHelloBack(String name) {
        FacesMessages.instance().add("Hello #0!", name);
    }
}

Seam defines a number of built-in events that the application can use to perform special kinds of framework integration. The events are:

Seam components may observe any of these events in just the same way they observe any other component-driven events.

EJB specifies well-defined rules that let us control whether an exception immediately marks the current transaction for rollback when it is thrown by a business method of the bean: system exceptions always cause a transaction rollback, application exceptions do not cause a rollback by default, but they do if @ApplicationException(rollback=true) is specified. (An application exception is any checked exception, or any unchecked exception annotated @ApplicationException. A system exception is any unchecked exception without an @ApplicationException annotation.)

Note that there is a difference between marking a transaction for rollback, and actually rolling it back. The exception rules say that the transaction should be marked rollback only, but it may still be active after the exception is thrown.

Seam applies the EJB 3.0 exception rollback rules also to Seam JavaBean components.

But these rules only apply in the Seam component layer. What about an exception that is uncaught and propagates out of the Seam component layer, and out of the JSF layer? Well, it is always wrong to leave a dangling transaction open, so Seam rolls back any active transaction when an exception occurs and is uncaught in the Seam component layer.

To enable Seam's exception handling, we need to add a servlet filter to web.xml:

<filter>
    <filter-name>Seam Exception Filter</filter-name>
    <filter-class>org.jboss.seam.servlet.SeamExceptionFilter</filter-class>
</filter>

<filter-mapping>
    <filter-name>Seam Exception Filter</filter-name>
    <url-pattern>*.seam</url-pattern>
</filter-mapping>

The following exception results in a HTTP 404 error whenever it propagates out of the Seam component layer. It does not roll back the current transaction immediately when thrown, but the transaction will be rolled back if it the exception is not caught by another Seam component.

@HttpError(errorCode=404)
public class ApplicationException extends Exception { ... }

This exception results in a browser redirect whenever it propagates out of the Seam component layer. It also ends the current conversation. It causes an immediate rollback of the current transaction.

@Redirect(viewId="/failure.xhtml", end=true)
@ApplicationException(rollback=true)
public class UnrecoverableApplicationException extends RuntimeException { ... }

Note that @Redirect does not work for exceptions which occur during the render phase of the JSF lifecycle.

This exception results in a redirect, along with a message to the user, when it propagates out of the Seam component layer. It also immediately rolls back the current transaction.

@Redirect(viewId="/error.xhtml", message="Unexpected error")
public class SystemException extends RuntimeException { ... }

Since we can't add annotations to all the exception classes we are interested in, Seam also lets us specify this functionality in pages.xml.

<pages>
   
   <exception class="javax.persistence.EntityNotFoundException">
      <http-error error-code="404"/>
   </exception>
   
   <exception class="javax.persistence.PersistenceException">
      <end-conversation/>
      <redirect view-id="/error.xhtml">
          <message>Database access failed</message>
      </redirect>
   </exception>
   
   <exception>
      <end-conversation/>
      <redirect view-id="/error.xhtml">
          <message>Unexpected failure</message>
      </redirect>
   </exception>
   
</pages>

The last <exception> declaration does not specify a class, and is a catch-all for any exception for which handling is not otherwise specified via annotations or in pages.xml.