JSP Worked Well Using Scripting Elements, But, Becomes a Null Using JSTL

Question

I have to bother all of you again with my JSTL problems. I am working on a JSP, which is very similar to the one that caused a lengthy discussion on this forum before. This JSP is a bit more complicated.
Again, this JSP worked well and displayed what I expected when the page was coded using the scripting elements.
I tried to use JSTL where possible and managed to correct all the compilation errors.
Now, I got a simple message: 'null' in the browser without any explanation - typical problem working with JSP: very difficult to debug.
I have tried to identify problems "one by one" and found the first encounter is where I want to set the background color for each row inside the <c:forEach ....> tag.
I examined the code again and again, I could not see what would lead to 'null'. I need a hand again.
<%@ taglib uri="/tags/struts-html-el" prefix="html" %>
<%@ taglib uri="/tags/struts-bean" prefix="bean" %>
<%@ taglib uri="/tags/struts-logic" prefix="logic" %>
<%@ taglib uri="/tags/tiles" prefix="tiles" %>
<%@ taglib uri="/tags/request" prefix="req" %>
<%@ taglib uri="/WEB-INF/c.tld" prefix="c" %>
<%@ page import="org.apache.artimus.message.utility.ParamUtil" %>
<%@ page import="org.apache.artimus.message.exceptions.AssertionException" %>
<%@ page import="org.apache.artimus.authorization.OnlineUser" %>
<%@ page import="org.apache.artimus.authorization.OnlineUserImpl" %>
<%@ page import="org.apache.artimus.filter.EnableEmotionFilter" %>
<%@ page import="org.apache.artimus.message.ThreadBean" %>
<%@ page import="org.apache.artimus.message.PostBean" %>
<%@ page import="org.apache.artimus.message.AttachmentBean" %>
<%@ page import="org.apache.artimus.MessageInboxConfig" %>
<%@ page import="org.apache.artimus.WebSiteGlobal" %>
<%@ page import="org.apache.artimus.SiteUtil" %>
<c:set var="threadBean" value="${requestScope.ThreadBean}" />
<c:set var="postRows" value="${requestScope.PostBeans}" />
<c:set var="numberOfPosts" value="${requestScope.NumberOfPosts}" />
<c:set var="previousTopic" value="${requestScope.PreviousTopic}" />
<c:set var="nextTopic" value="${requestScope.NextTopic}" />
<c:set var="offset" value="${requestScope.offset}" />
<c:set var="threadID_int" value="${requestScope.thread}" />
<c:if test="${threadBean.threadID != threadID_int}">
    The two threadID are not the same.
</c:if>
<c:set var="threadID" value="${threadID_int}" />
<%-- the code before this line works fine and I have a number of things displayed as expected --%>
<c:forEach var="postBean" items="{postRows}" varStatus="i">
<% PostBean postBean =
   ( PostBean )pageContext.getAttribute( "postBean" );
%>
   <c:choose>
      <c:when test="${(postBean.parentPostID) eq 0}">
         <c:set var="background" value="#FFCE9C" />
      </c:when>
      <c:otherwise>
         <c:choose>
            <c:when test="${(i+1)%2 ne 0}">
               <c:set var="background" value="#EEEEEE" />
            </c:when>
            <c:otherwise>
               <c:set var="background" value="#FFFFFF" />   
            </c:otherwise>
         </c:choose>
      </c:otherwise>
   </c:choose>
<table width="95%" border="0" cellspacing="1" cellpadding="5" align="center" bgcolor="#999999">
  <tr bgColor="${background}">
  </tr>
</table>
</c:forEach>

Answer 1

Where is the null occuring? Have you looked at the returned HTML code to see where the 'null' is?
You might try just a few <c:out value=""/>, like:
The postRows = <c:out value="${postRows"/><br/>
<c:forEach var="postBean" items="${postRows"/>
  The Current postBean = <c:out value="${postBean}"/><br/>
  The Current parentPostID = <c:out value="${postBean.parentPostID eq 0}"/><br/>
  <!-- Only do this set if background ends up null, to make sure the scope of background is wide enough -->
  <c:set var="background" value="#FFFFFF"/>
  <c:choose>
    <c:when test="${postBean.parentPostID eq 0}">
      <c:set var="background" value="#FFCE9C"/>
    </c:when>
    <c:when test="${(i+1)%2 ne 0}">
      <c:set var="background" value="#EEEEEE"/>
    </c:when>
    <c:otherwise>
      <c:set var="background" value="#FFFFFF"/>
    </c:otherwise>
  </c:choose>
  The Background is now = <c:out value="${background}"/><br/>
  <table width="95%" border="0" cellspacing="1" cellpadding="5" align="center" bgcolor="#999999">
    <tr bgColor="${background}">
    </tr>
  </table>
</c:forEach>By adding output like that when we are debugging we can help track down where problems that occur are coming from. That and looking at the source code have been invaluable to me...