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From "Tarek Nabil" <Tarek.Na...@tco.ae>
Subject Mixing EL and Java expressions
Date Wed, 22 Mar 2006 10:01:33 GMT
Hi everyone,

 

I'm moving from the Struts tag libraries to JSTL and I'm having a bit of
a hard time. Things don't work as I thought they would. For example, I'm
not able to use constants defined in classes for comparison, which is
something I used to do easily with Struts tags. Here's an example; I
want to do something if the display parameter equals the constant
DISPLAY_BY_SERVICE defined in some class and do something else if it has
the value DISPLAY_BY_BU. First, I tried this

 

    <c:choose>

      <c:when test="${param.display == <%=
MainPageAction.DISPLAY_BY_SERVICE %>}">

            By Service

      </c:when>

      <c:when test="${param.display == <%= MainPageAction.DISPLAY_BY_BU
%>}">

            By BU

      </c:when>

    </c:choose>

 

That didn't even compile, so I learned my first lesson: I can not use
Java expression inside EL expressions. So, I tried this

 

    <c:set var="displayParam" value="${param.display}"/>

    <c:choose>

      <c:when test="<%=
MainPageAction.DISPLAY_BY_SERVICE.equals(displayParam) %>">

      <%= MainPageAction.DISPLAY_BY_SERVICE %>

      </c:when>

      <c:when test="<%=
MainPageAction.DISPLAY_BY_BU.equals(displayParam) %>">

      <%= MainPageAction.DISPLAY_BY_BU %>

      </c:when>

    </c:choose>

 

That didn't compile either, so now I know that unlike <bean:define>,
<c:set> doesn't define a scripting variable, but only a page scope
variable. So, I tried this

 

    <c:choose>

      <c:when test="${param.display} == <%=
MainPageAction.DISPLAY_BY_SERVICE %>">

            By Service

      </c:when>

      <c:when test="${param.display} == <%= MainPageAction.DISPLAY_BY_BU
%>">

            By BU

      </c:when>

      <c:otherwise>Otherwise</c:otherwise>

    </c:choose>

 

Now, that compiled. But it didn't work!!! So, now I know that the value
of test should either be the String true or an expression that evaluates
to true, so mixing expressions won't work this way.

 

Now, this is what worked, and it's not elegant at all.

 

    <c:set var="DISPLAY_BY_SERVICE" value="<%=
MainPageAction.DISPLAY_BY_SERVICE %>"/> 

    <c:set var="DISPLAY_BY_BU" value="<%= MainPageAction.DISPLAY_BY_BU
%>"/>

    <c:choose>

      <c:when test="${param.display == DISPLAY_BY_SERVICE}">

            By Service

      </c:when>

      <c:when test="${param.display == DISPLAY_BY_BU}">

            By BU

      </c:when>

      <c:otherwise>Otherwise</c:otherwise>

    </c:choose>

 

What if I had 5 constants, do I have to re-define the 5 constants as
vars using <c:set>? Either I'm missing something or JSTL is really not
what I thought it was. I mean, maybe I should go back to the bean and
logic libraries from Struts, they used to really do the job for me.

 

Could someone please guide me on this issue?

 

Thank you.

 

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