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Chapter 6. Events, interceptors and exception handling

6.1. Seam events
6.2. Page actions
6.3. Page parameters
6.3.1. Mapping request parameters to the model
6.4. Propagating request parameters
6.5. URL rewriting with page parameters
6.6. Conversion and Validation
6.7. Navigation
6.8. Fine-grained files for definition of navigation, page actions and parameters
6.9. Component-driven events
6.10. Contextual events
6.11. Seam interceptors
6.12. Managing exceptions
6.12.1. Exceptions and transactions
6.12.2. Enabling Seam exception handling
6.12.3. Using annotations for exception handling
6.12.4. Using XML for exception handling
6.12.5. Some common exceptions

Complementing the contextual component model, there are two further basic concepts that facilitate the extreme loose-coupling that is the distinctive feature of Seam applications. The first is a strong event model where events may be mapped to event listeners via JSF-like method binding expressions. The second is the pervasive use of annotations and interceptors to apply cross-cutting concerns to components which implement business logic.

6.1. Seam events

The Seam component model was developed for use with event-driven applications, specifically to enable the development of fine-grained, loosely-coupled components in a fine-grained eventing model. Events in Seam come in several types, most of which we have already seen:

All of these various kinds of events are mapped to Seam components via JSF EL method binding expressions. For a JSF event, this is defined in the JSF template: 


<h:commandButton value="Click me!" action="#{helloWorld.sayHello}"/>

For a jBPM transition event, it is specified in the jBPM process definition or pageflow definition: 


<start-page name="hello" view-id="/hello.jsp">

    <transition to="hello">

        <action expression="#{helloWorld.sayHello}"/>

    </transition>

</start-page>

You can find out more information about JSF events and jBPM events elsewhere. Let's concentrate for now upon the two additional kinds of events defined by Seam.

6.2. Page actions

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 as a suffix to the view-id to specify an action that applies to all view ids that match the pattern: 


<pages>

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

</pages>

Keep in mind that if the <page> element is defined in a fine-grained page descriptor, the view-id attribute can be left off since it is implied.

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 use the defined navigation rules to navigate to a view.

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

Multiple or conditional page actions my be specified using the <action> tag: 


<pages>

    <page view-id="/hello.jsp">

        <action execute="#{helloWorld.sayHello}" if="#{not validation.failed}"/>

        <action execute="#{hitCount.increment}"/>

    </page>

</pages>

Page actions are executed on both an initial (non-faces) request and a postback (faces) request. If you are using the page action to load data, this operation may conflict with the standard JSF action(s) being executed on a postback. One way to disable the page action is to setup a condition that resolves to true only on an initial request. 


<pages>

    <page view-id="/dashboard.xhtml">

        <action execute="#{dashboard.loadData}"

            if="#{not facesContext.renderKit.responseStateManager.isPostback(facesContext)}"/>

    </page>

</pages>

This condition consults the ResponseStateManager#isPostback(FacesContext) to determine if the request is a postback. The ResponseStateManager is accessed using FacesContext.getCurrentInstance().getRenderKit().getResponseStateManager().

To save you from the verbosity of JSF's API, Seam offers a built-in condition that allows you to accomplish the same result with a heck of a lot less typing. You can disable a page action on postback by simply setting the on-postback to false: 


<pages>

    <page view-id="/dashboard.xhtml">

        <action execute="#{dashboard.loadData}" on-postback="false"/>

    </page>

</pages>

For backwards compatibility reasons, the default value of the on-postback attribute is true, though likely you will end up using the opposite setting more often.

6.3. Page parameters

A JSF faces request (a form submission) encapsulates both an "action" (a method binding) and "parameters" (input value bindings). A page action might also needs parameters!

Since GET requests are bookmarkable, page parameters are passed as human-readable request parameters. (Unlike JSF form inputs, which are anything but!)

You can use page parameters with or without an action method.

Seam lets us provide a value binding that maps a named request parameter to an attribute of a model object. 


<pages>

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

          <param name="firstName" value="#{person.firstName}"/>

          <param name="lastName" value="#{person.lastName}"/>

      </page>

  </pages>

The <param> declaration is bidirectional, just like a value binding for a JSF input:

The essential idea behind all this is that however we get from any other page to /hello.jsp (or from /hello.jsp back to /hello.jsp), the value of the model attribute referred to in the value binding is "remembered", without the need for a conversation (or other server-side state).

6.4. Propagating request parameters

If just the name attribute is specified then the request parameter is propagated using the PAGE context (it isn't mapped to model property). 


<pages>

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

          <param name="firstName" />

          <param name="lastName" />

      </page>

  </pages>

Propagation of page parameters is especially useful if you want to build multi-layer master-detail CRUD pages. You can use it to "remember" which view you were previously on (e.g. when pressing the Save button), and which entity you were editing.

This all sounds pretty complex, and you're probably wondering if such an exotic construct is really worth the effort. Actually, the idea is very natural once you "get it". It is definitely worth taking the time to understand this stuff. Page parameters are the most elegant way to propagate state across a non-faces request. They are especially cool for problems like search screens with bookmarkable results pages, where we would like to be able to write our application code to handle both POST and GET requests with the same code. Page parameters eliminate repetitive listing of request parameters in the view definition and make redirects much easier to code.

6.5. URL rewriting with page parameters

Rewriting occurs based on rewrite patterns found for views in pages.xml. Seam URL rewriting does both incoming and outgoing URL rewriting based on the same pattern. Here's a simple pattern: 




<page view-id="/home.xhtml">

    <rewrite pattern="/home" />

</page>

In this case, any incoming request for /home will be sent to /home.xhtml. More interestingly, any link generated that would normally point to /home.seam will instead be rewritten as /home. Rewrite patterns only match the portion of the URL before the query parameters. So, /home.seam?conversationId=13 and /home.seam?color=red will both be matched by this rewrite rule.

Rewrite rules can take these query paramters into consideration, as shown with the following rules. 




<page view-id="/home.xhtml">

    <rewrite pattern="/home/{color}" />

    <rewrite pattern="/home" />

</page>

In this case, an incoming request for /home/red will be served as if it were a request for /home.seam?color=red. Similarly, if color is a page parameter an outgoing URL that would normally show as /home.seam?color=blue would instead be output as /home/blue. Rules are processed in order, so it is important to list more specific rules before more general rules.

Default Seam query parameters can also be mapped using URL rewriting, allowing for another option for hiding Seam's fingerprints. In the following example, /search.seam?conversationId=13 would be written as /search-13. 




<page view-id="/search.xhtml">

    <rewrite pattern="/search-{conversationId}" />

    <rewrite pattern="/search" />

</page>

Seam URL rewriting provides simple, bidirectional rewriting on a per-view basis. For more complex rewriting rules that cover non-seam components, Seam applications can continue to use the org.tuckey URLRewriteFilter or apply rewriting rules at the web server.

URL rewriting requires the Seam rewrite filter to be enable. Rewrite filter configuration is discussed in Section 30.1.4.3, “URL rewriting”.

6.6. Conversion and Validation

You can specify a JSF converter for complex model propreties: 


<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>

Alternatively: 


<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>

JSF validators, and required="true" may also be used: 


<pages>

    <page view-id="/blog.xhtml">

        <param name="date" 

               value="#{blog.date}" 

               validatorId="com.my.blog.PastDate" 

               required="true"/>

    </page>

</pages>

Alternatively: 


<pages>

    <page view-id="/blog.xhtml">

        <param name="date" 

               value="#{blog.date}" 

               validator="#{pastDateValidator}" 

               required="true"/>

    </page>

</pages>

Even better, model-based Hibernate validator annotations are automatically recognized and validated. Seam also provides a default date converter to convert a string parameter value to a date and back.

When type conversion or validation fails, a global FacesMessage is added to the FacesContext.

6.7. Navigation

You can use standard JSF navigation rules defined in faces-config.xml in a Seam application. However, JSF navigation rules have a number of annoying limitations:

A further problem is that "orchestration" logic gets scattered between pages.xml and faces-config.xml. It's better to unify this logic into pages.xml.

This JSF navigation rule: 


<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>

Can be rewritten as follows: 


<page view-id="/editDocument.xhtml">

    

    <navigation from-action="#{documentEditor.update}">

        <rule if-outcome="success">

            <redirect view-id="/viewDocument.xhtml"/>

        </rule>

    </navigation>

    

</page>

But it would be even nicer if we didn't have to pollute our DocumentEditor component with string-valued return values (the JSF outcomes). So Seam lets us write: 


<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>

    

</page>

Or even: 


<page view-id="/editDocument.xhtml">

    

    <navigation from-action="#{documentEditor.update}">

        <rule if="#{documentEditor.errors.empty}">

            <redirect view-id="/viewDocument.xhtml"/>

        </rule>

    </navigation>

    

</page>

The first form evaluates a value binding to determine the outcome value to be used by the subsequent rules. The second approach ignores the outcome and evaluates a value binding for each possible rule.

Of course, when an update succeeds, we probably want to end the current conversation. We can do that like this: 


<page view-id="/editDocument.xhtml">

    

    <navigation from-action="#{documentEditor.update}">

        <rule if="#{documentEditor.errors.empty}">

            <end-conversation/>

            <redirect view-id="/viewDocument.xhtml"/>

        </rule>

    </navigation>

    

</page>

As we've ended conversation any subsequent requests won't know which document we are interested in. We can pass the document id as a request parameter which also makes the view bookmarkable: 


<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>

    

</page>

Null outcomes are a special case in JSF. The null outcome is interpreted to mean "redisplay the page". The following navigation rule matches any non-null outcome, but not the null outcome: 


<page view-id="/editDocument.xhtml">

    

    <navigation from-action="#{documentEditor.update}">

        <rule>

            <render view-id="/viewDocument.xhtml"/>

        </rule>

    </navigation>

    

</page>

If you want to perform navigation when a null outcome occurs, use the following form instead: 


<page view-id="/editDocument.xhtml">

    

    <navigation from-action="#{documentEditor.update}">

        <render view-id="/viewDocument.xhtml"/>

    </navigation>

    

</page>

The view-id may be given as a JSF EL expression: 


<page view-id="/editDocument.xhtml">



    <navigation>

        <rule if-outcome="success">

            <redirect view-id="/#{userAgent}/displayDocument.xhtml"/>

        </rule>

    </navigation>

    

</page>

6.8. Fine-grained files for definition of navigation, page actions and parameters

If you have a lot of different page actions and page parameters, or even just a lot of navigation rules, you will almost certainly want to split the declarations up over multiple files. You can define actions and parameters for a page with the view id /calc/calculator.jsp in a resource named calc/calculator.page.xml. The root element in this case is the <page> element, and the view id is implied: 


<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>

6.9. Component-driven events

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 execute="#{helloListener.sayHelloBack}"/>

        <action execute="#{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);

    }

}

6.10. Contextual events

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.

6.11. Seam interceptors

EJB 3.0 introduced a standard interceptor model for session bean components. To add an interceptor to a bean, you need to write a class with a method annotated @AroundInvoke and annotate the bean with an @Interceptors annotation that specifies the name of the interceptor class. For example, the following interceptor checks that the user is logged in before allowing invoking an action listener method: 

public class LoggedInInterceptor {


   @AroundInvoke

   public Object checkLoggedIn(InvocationContext invocation) throws Exception {

   

      boolean isLoggedIn = Contexts.getSessionContext().get("loggedIn")!=null;

      if (isLoggedIn) {

         //the user is already logged in

         return invocation.proceed();

      }

      else {

         //the user is not logged in, fwd to login page

         return "login";

      }

   }


}

To apply this interceptor to a session bean which acts as an action listener, we must annotate the session bean @Interceptors(LoggedInInterceptor.class). This is a somewhat ugly annotation. Seam builds upon the interceptor framework in EJB3 by allowing you to use @Interceptors as a meta-annotation for class level interceptors (those annotated @Target(TYPE)). In our example, we would create an @LoggedIn annotation, as follows: 

@Target(TYPE)

@Retention(RUNTIME)

@Interceptors(LoggedInInterceptor.class)

public @interface LoggedIn {}

We can now simply annotate our action listener bean with @LoggedIn to apply the interceptor. 

@Stateless

@Name("changePasswordAction")

@LoggedIn

@Interceptors(SeamInterceptor.class)

public class ChangePasswordAction implements ChangePassword { 

    

    ...

    

    public String changePassword() { ... }

    

}

If interceptor ordering is important (it usually is), you can add @Interceptor annotations to your interceptor classes to specify a partial order of interceptors. 

@Interceptor(around={BijectionInterceptor.class,

                     ValidationInterceptor.class,

                     ConversationInterceptor.class},

             within=RemoveInterceptor.class)

public class LoggedInInterceptor

{

    ...

}

You can even have a "client-side" interceptor, that runs around any of the built-in functionality of EJB3: 

@Interceptor(type=CLIENT)

public class LoggedInInterceptor

{

    ...

}

EJB interceptors are stateful, with a lifecycle that is the same as the component they intercept. For interceptors which do not need to maintain state, Seam lets you get a performance optimization by specifying @Interceptor(stateless=true).

Much of the functionality of Seam is implemented as a set of built-in Seam interceptors, including the interceptors named in the previous example. You don't have to explicitly specify these interceptors by annotating your components; they exist for all interceptable Seam components.

You can even use Seam interceptors with JavaBean components, not just EJB3 beans!

EJB defines interception not only for business methods (using @AroundInvoke), but also for the lifecycle methods @PostConstruct, @PreDestroy, @PrePassivate and @PostActive. Seam supports all these lifecycle methods on both component and interceptor not only for EJB3 beans, but also for JavaBean components (except @PreDestroy which is not meaningful for JavaBean components).

6.12. Managing exceptions

JSF is surprisingly limited when it comes to exception handling. As a partial workaround for this problem, Seam lets you define how a particular class of exception is to be treated by annotating the exception class, or declaring the exception class in an XML file. This facility is meant to be combined with the EJB 3.0-standard @ApplicationException annotation which specifies whether the exception should cause a transaction rollback.

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 make sure we have the master servlet filter declared in web.xml: 


<filter>

    <filter-name>Seam Filter</filter-name>

    <filter-class>org.jboss.seam.servlet.SeamFilter</filter-class>

</filter>



<filter-mapping>

    <filter-name>Seam Filter</filter-name>

    <url-pattern>*.seam</url-pattern>

</filter-mapping>

You need to disable Facelets development mode in web.xml and Seam debug mode in components.xml if you want your exception handlers to fire.

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

It is important to note that Seam cannot handle exceptions that occur during JSF's RENDER_RESPONSE phase, as it is not possible to perform a redirect once the response has started being written to.

You can also use EL to specify the viewId to redirect to.

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.

You can also use EL to specify the view-id to redirect to.

You can also access the handled exception instance through EL, Seam places it in the conversation context, e.g. to access the message of the exception: 


...

throw new AuthorizationException("You are not allowed to do this!");



<pages>



    <exception class="org.jboss.seam.security.AuthorizationException">

        <end-conversation/>

        <redirect view-id="/error.xhtml">

            <message severity="WARN">#{org.jboss.seam.handledException.message}</message>

        </redirect>

    </exception>



</pages>

org.jboss.seam.handledException holds the nested exception that was actually handled by an exception handler. The outermost (wrapper) exception is also available, as org.jboss.seam.caughtException.

For the exception handlers defined in pages.xml, it is possible to declare the logging level at which the exception will be logged, or to even suppress the exception being logged altogether. The attributes log and log-level can be used to control exception logging. By setting log="false" as per the following example, then no log message will be generated when the specified exception occurs: 


    <exception class="org.jboss.seam.security.NotLoggedInException" log="false">

        <redirect view-id="/register.xhtml">

            <message severity="warn">You must be a member to use this feature</message>

        </redirect>

    </exception>

If the log attribute is not specified, then it defaults to true (i.e. the exception will be logged). Alternatively, you can specify the log-level to control at which log level the exception will be logged: 


    <exception class="org.jboss.seam.security.NotLoggedInException" log-level="info">

        <redirect view-id="/register.xhtml">

            <message severity="warn">You must be a member to use this feature</message>

        </redirect>

    </exception>

The acceptable values for log-level are: fatal, error, warn, info, debug or trace. If the log-level is not specified, or if an invalid value is configured, then it will default to error.

If you are using JPA: 


<exception class="javax.persistence.EntityNotFoundException">

   <redirect view-id="/error.xhtml">

      <message>Not found</message>

   </redirect>

</exception>



<exception class="javax.persistence.OptimisticLockException">

   <end-conversation/>

   <redirect view-id="/error.xhtml">

      <message>Another user changed the same data, please try again</message>

   </redirect>

</exception>

If you are using the Seam Application Framework: 


<exception class="org.jboss.seam.framework.EntityNotFoundException">

   <redirect view-id="/error.xhtml">

      <message>Not found</message>

   </redirect>

</exception>

If you are using Seam Security: 


<exception class="org.jboss.seam.security.AuthorizationException">

   <redirect>

      <message>You don't have permission to do this</message>

   </redirect>

</exception>

    

<exception class="org.jboss.seam.security.NotLoggedInException">

   <redirect view-id="/login.xhtml">

      <message>Please log in first</message>

   </redirect>

</exception>

And, for JSF: 


<exception class="javax.faces.application.ViewExpiredException">

   <redirect view-id="/error.xhtml">

      <message>Your session has timed out, please try again</message>

   </redirect>

</exception>

A ViewExpiredException occurs if the user posts back to a page once their session has expired. The conversation-required and no-conversation-view-id settings in the Seam page descriptor, discussed in Section 7.4, “Requiring a long-running conversation”, give you finer-grained control over session expiration if you are accessing a page used within a conversation.