Facelets - Accessing local variables via EL in custom tag handler

[rajeshja]

Hi, this is a cross post from the facelets users and dev lists. I didn't get much of a response there, and I'm beginning to feel this might be a bug in com/sun/facelets/impl/DefaultFaceletContext.java, though I'm not sure yet.

Here's the description from the original post:

I'm using Facelets 1.1.13 with Seam 1.2.1 under WebLogic 10, and have written a custom tag handler, to implement a tag library in my application.

Here's what I do:

     public final class MyTagHandler extends TagHandler {
     
         private final TagAttribute value;
         // Some more attributes
      
         public MyTagHandler(TagConfig config) {
             super(config);
             this.value = this.getAttribute("value");
         }
     
         public void apply(FaceletContext ctx, UIComponent parent)
                 throws IOException, FacesException, FaceletException, ELException {
            // Some processing goes here.
            Object value = getValue(ctx);
            // More processing
           return;
         }
     
         public Object getValue(FaceletContext ctx) {
             if (this.value != null) {
                 return this.value.getObject(ctx);
             } else {
                 return null;
             }
         }
     .
     .
     .
     }

This is how I set it up in the taglib.xml:

     <facelet-taglib>
      <namespace>http://mydomain.net/tags</namespace>
      <tag>
       <tag-name>mytag</tag-name>
       <handler-class>net.mydomain.MyTagHandler</handler-class>
      </tag>
     </facelet-taglib>

This is how the tag is used in the XHTML files:

     <myprefix:mytag value="somevalue">

• If somevalue is a literal, this works fine.


• If somevalue is an EL expression of the form #{customer.name} (where the bean customer has a property name)


• a. If customer is a component defined in faces-config.xml, then it works fine


• b. If customer is outjected (using the Out Seam annotation) to - say - the session scope, then this works fine


• c. If customers (plural), is a component which has multiple customers, and I use a tag like c:forEach or h:dataTable to loop over each customer, like so:

          <c:forEach items="#{customers}" var="customer">
            <myprefix:mytag value="#{customer.name}"/>
          </c:forEach>

     then getValue(ctx) as written above in the apply method, evaluates to null.

If I use this.value.getValue(ctx); instead of this.value.getObject(ctx);, it's the same thing.
If I use code like this.value.getValueExpression(ctx,
String.class).getValue(ctx) instead, I get an empty string.
If I use code like this.value.getValueExpression(ctx,
String.class).getType(ctx) instead, I get the following exception:

Feb 20, 2008 8:06:37 PM com.sun.facelets.FaceletViewHandler
handleRenderException
SEVERE: Error Rendering View\[/mypage.xhtml\]
javax.el.PropertyNotFoundException: /mypage.xhtml @97,66
value="${customer.name}": Target Unreachable, identifier 'customer'
resolved to null
        at
com.sun.facelets.el.TagValueExpression.getType(TagValueExpression.java:6
2)
        at
net.mydomain.MyTagHandler.getValue(DisplayStringParamTagHandler.java:122
)
        at
net.mydomain.MyTagHandler.apply(DynamicDisplayStringHandler.java:81)
        at
com.sun.facelets.tag.CompositeFaceletHandler.apply(CompositeFaceletHandl
er.java:47)
        at
com.sun.facelets.tag.jsf.ComponentHandler.applyNextHandler(ComponentHand
ler.java:314)
        at
com.sun.facelets.tag.jsf.ComponentHandler.apply(ComponentHandler.java:16
9)
        at
com.sun.facelets.tag.CompositeFaceletHandler.apply(CompositeFaceletHandl
er.java:47)
        at
com.sun.facelets.tag.jsf.ComponentHandler.applyNextHandler(ComponentHand
ler.java:314)
        at
com.sun.facelets.tag.jsf.ComponentHandler.apply(ComponentHandler.java:16
9)
        at
com.sun.facelets.tag.CompositeFaceletHandler.apply(CompositeFaceletHandl
er.java:47)
        at
com.sun.facelets.tag.jsf.core.ViewHandler.apply(ViewHandler.java:109)
        at
com.sun.facelets.tag.CompositeFaceletHandler.apply(CompositeFaceletHandl
er.java:47)
        at
com.sun.facelets.compiler.NamespaceHandler.apply(NamespaceHandler.java:4
9)
        at
com.sun.facelets.tag.CompositeFaceletHandler.apply(CompositeFaceletHandl
er.java:47)
        at
com.sun.facelets.compiler.EncodingHandler.apply(EncodingHandler.java:25)
        at
com.sun.facelets.impl.DefaultFacelet.apply(DefaultFacelet.java:95)
        at
com.sun.facelets.FaceletViewHandler.buildView(FaceletViewHandler.java:50
3)
        at
com.sun.facelets.FaceletViewHandler.renderView(FaceletViewHandler.java:5
46)
        at
org.ajax4jsf.framework.ViewHandlerWrapper.renderView(ViewHandlerWrapper.
java:108)
        at
org.ajax4jsf.framework.ajax.AjaxViewHandler.renderView(AjaxViewHandler.j
ava:229)
        at
org.apache.myfaces.lifecycle.RenderResponseExecutor.execute(RenderRespon
seExecutor.java:41)
        at
org.apache.myfaces.lifecycle.LifecycleImpl.render(LifecycleImpl.java:132
)
        at
javax.faces.webapp.FacesServlet.service(FacesServlet.java:140)
        at
weblogic.servlet.internal.StubSecurityHelper$ServletServiceAction.run(St
ubSecurityHelper.java:226)
        at
weblogic.servlet.internal.StubSecurityHelper.invokeServlet(StubSecurityH
elper.java:124)
        at
weblogic.servlet.internal.ServletStubImpl.execute(ServletStubImpl.java:2
83)
        at
weblogic.servlet.internal.TailFilter.doFilter(TailFilter.java:26)
        at
weblogic.servlet.internal.FilterChainImpl.doFilter(FilterChainImpl.java:
42)
        at
org.jboss.seam.web.SeamFilter$FilterChainImpl.doFilter(SeamFilter.java:6
3)
        at
org.jboss.seam.web.RedirectFilter.doFilter(RedirectFilter.java:45)
        at
org.jboss.seam.web.SeamFilter$FilterChainImpl.doFilter(SeamFilter.java:4
9)
        at
org.jboss.seam.web.ExceptionFilter.doFilter(ExceptionFilter.java:57)

Note that code like -

          <c:forEach items="#{customers}" var="customer">
            #{customer.name}
          </c:forEach>

- works just fine.

I'm guessing that maybe the ELContext that the customer variable is stored to, is different than the one my tag tries to use. What I can't understand is Why?, and How do I fix it?

Any insights into this would be highly appreciated.

Regards,

Rajesh

[keithnaas]

Remember that TagHandlers are evaluated when the component tree is built, not when the tree is rendered.

For an example of this, you might want to look at c:forEach vs ui:repeat in Facelets.

[rajeshja]

Hey thanks! I created a Component and wired that to my component and it worked - mostly.

If the Component I write extends from UIComponentBase, things work fine.

However the purpose of my tag is to produce some output, and so I extended from UIOutput instead, overriding getValue(). However the attributes that contain the EL expressions I need, don't seem to get retrieved from the getAttributes() method.
For now I'm using syntax like -

    <myprefix:mytag literalvalue="someliteral">
         <myprefix:subtag value="#{customer.name}"/>
     </myprefix:mytag>

- where mytag is backed by a component that extends UIOutput and subtag is backed by a component that extends UIComponentBase.

That solution is obviously kludgy, but I haven't been able to figure out why it one tag but not another. Any clue?