EL expressions not evaluated in custom tag attribute

Question

I'm writing a custom tag implementation where I need to have some attribute values
dynamically evaluated using EL, such that I can write:
<mytaglib:mytag id="${somevar}"/>
I cannot seem to accomplish this, however. My taglib tag definition looks like this:
     <tag>
          <name>mytag</name>
          <tag-class>myclass</tag-class>
          <body-content>empty</body-content>
          <description>
Foo
          </description>
          <attribute>
               <name>articleId</name>
               <required>true</required>
               <rtexprvalue>true</rtexprvalue>
               <type>java.lang.String</type>
          </attribute>
     </tag>
The implementation of myclass extends TagSupport (since I need to get at the PageContext). The setArticleId() method looks like this:
public void setArticleId(String idStr) throws JspException {
if (logger.isDebugEnabled()) {
logger.debug("idStr = " + idStr);
try {
/* XXX: Somehow, the evaluation should happen automagically, but
* I can't figure out why that doesn't happen!
String articleIdStr = (String)pageContext.getExpressionEvaluator().evaluate(
idStr,
java.lang.String.class, pageContext.getVariableResolver(), null);
this.articleId = Integer.parseInt(articleIdStr);
} catch (ELException e) {
throw new JspException("Failed to parse article id expression");
Why does it not work automagically?

Answer 1

Make sure you are working with a JSP 2 container (like Tomcat 5). Also, you should make sure that your web.xml is configured to use the newest Servlet Spec. Read reply #6 on: http://forum.java.sun.com/thread.jspa?threadID=629437 to make sure it is set up properly.
If you are not using a JSP 2.0 container, then you will not be able to use EL in your custom tag. See: http://forum.java.sun.com/thread.jspa?forumID=45&threadID=725503 for alternatives.