Question

I have a class that defines the names of various session attributes, e.g.

class Constants {     public static final String ATTR_CURRENT_USER = 'current.user'; } 

I would like to use these constants within a JSP to test for the presence of these attributes, something like:

<%@ taglib prefix='c' uri='http://java.sun.com/jsp/jstl/core' %> <%@ page import='com.example.Constants' %>  <c:if test='${sessionScope[Constants.ATTR_CURRENT_USER] eq null}'>     <%-- Do somthing --%> </c:if> 

But I can’t seem to get the sytax correct. Also, to avoid repeating the rather lengthy tests above in multiple places, I’d like to assign the result to a local (page-scoped) variable, and refer to that instead. I believe I can do this with <c:set>, but again I’m struggling to find the correct syntax.

UPDATE: Further to the suggestion below, I tried:

<c:set var='nullUser' scope='session' value='${sessionScope[Constants.ATTR_CURRENT_USER] eq null}' /> 

which didn’t work. So instead, I tried substituting the literal value of the constant. I also added the constant to the content of the page, so I could verify the constant’s value when the page is being rendered

<c:set var='nullUser' scope='session' value='${sessionScope['current.user'] eq null}' /> <%= 'Constant value: ' + WebHelper.ATTR_CURRENT_PARTNER %> 

This worked fine and it printed the expected value ‘current.user’ on the page. I’m at a loss to explain why using the String literal works, but a reference to the constant doesn’t, when the two appear to have the same value. Help…..

Answer 1

It’s not working in your example because the ATTR_CURRENT_USER constant is not visible to the JSTL tags, which expect properties to be exposed by getter functions. I haven’t tried it, but the cleanest way to expose your constants appears to be the unstandard tag library.

ETA: Old link I gave didn’t work. New links can be found in this answer: Java constants in JSP

Code snippets to clarify the behavior you’re seeing: Sample class:

package com.example;  public class Constants {     // attribute, visible to the scriptlet     public static final String ATTR_CURRENT_USER = 'current.user';      // getter function;     // name modified to make it clear, later on,      // that I am calling this function     // and not accessing the constant     public String getATTR_CURRENT_USER_FUNC()     {         return ATTR_CURRENT_USER;     }   }     

Snippet of the JSP page, showing sample usage:

<%-- Set up the current user --%> <%     session.setAttribute('current.user', 'Me'); %>  <%-- scriptlets --%> <%@ page import='com.example.Constants' %> <h1>Using scriptlets</h1> <h3>Constants.ATTR_CURRENT_USER</h3> <%=Constants.ATTR_CURRENT_USER%> <br /> <h3>Session[Constants.ATTR_CURRENT_USER]</h3> <%=session.getAttribute(Constants.ATTR_CURRENT_USER)%>  <%-- JSTL --%> <%@ taglib prefix='c' uri='http://java.sun.com/jsp/jstl/core' %> <jsp:useBean id='cons' class='com.example.Constants' scope='session'/>  <h1>Using JSTL</h1> <h3>Constants.getATTR_CURRENT_USER_FUNC()</h3> <c:out value='${cons.ATTR_CURRENT_USER_FUNC}'/> <h3>Session[Constants.getATTR_CURRENT_USER_FUNC()]</h3> <c:out value='${sessionScope[cons.ATTR_CURRENT_USER_FUNC]}'/> <h3>Constants.ATTR_CURRENT_USER</h3> <c:out value='${sessionScope[Constants.ATTR_CURRENT_USER]}'/> <%-- Commented out, because otherwise will error: The class 'com.example.Constants' does not have the property 'ATTR_CURRENT_USER'.  <h3>cons.ATTR_CURRENT_USER</h3> <c:out value='${sessionScope[cons.ATTR_CURRENT_USER]}'/> --%> <hr /> 

This outputs:

Using scriptlets

Constants.ATTR_CURRENT_USER

current.user

Session[Constants.ATTR_CURRENT_USER]

Me

---

Using JSTL

Constants.getATTR_CURRENT_USER_FUNC()

current.user

Session[Constants.getATTR_CURRENT_USER_FUNC()]

Me

Constants.ATTR_CURRENT_USER

---