accessing constants in JSP (without scriptlet) [duplicate]

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

---

Answer 2

You can define Constants.ATTR_CURRENT_USER as a variable with c:set,just as below:

<c:set var="ATTR_CURRENT_USER" value="<%=Constants.ATTR_CURRENT_USER%>" />
<c:if test="${sessionScope[ATTR_CURRENT_USER] eq null}">     
    <%-- Do somthing --%> 
</c:if> 

Answer 3

the topic is quite old, but anyway..:)

I found nice solution to have Constants available through JSTL. You should prepare a map using reflection and put it wherever you want.

The map will always contain all the constants you define in Constants class. You can put it into ServletContext using listener and enjoy constants in JSTL like:

${CONSTANTS["CONSTANT_NAME_IN_JAVA_CLASS_AS_A_STRING"]}

CONSTANTS here is a key you used putting map into Context   :-)

---

The following is a piece of my code building a map of the constant fields:

Map<String, Object> map = new HashMap<String, Object>();
Class c = Constants.class;
Field[] fields = c.getDeclaredFields();
for (Field field : fields) {
   int modifier = field.getModifiers();
   if (Modifier.isPublic(modifier) && Modifier.isStatic(modifier) && Modifier.isFinal(modifier)) {
      try {
         map.put(field.getName(), field.get(null));//Obj param of get method is ignored for static fields
      } catch (IllegalAccessException e) { /* ignorable due to modifiers check */ }
   }
}

Answer 4

I am late to the discussion, but my approach is a little different. I use a custom tag handler to give JSP pages the constant values (numeric or string) it needs. Here is how I did it:

Supposed I have a class that keeps all the constants:

public class AppJspConstants implements Serializable {
    public static final int MAXLENGTH_SIGNON_ID = 100;
    public static final int MAXLENGTH_PASSWORD = 100;
    public static final int MAXLENGTH_FULLNAME = 30;
    public static final int MAXLENGTH_PHONENUMBER = 30;
    public static final int MAXLENGTH_EXTENSION = 10;
    public static final int MAXLENGTH_EMAIL = 235;
}

I also have this extremely simple custom tag:

public class JspFieldAttributes extends SimpleTagSupport {
    public void doTag() throws JspException, IOException {
        getJspContext().setAttribute("maxlength_signon_id", AppJspConstants.MAXLENGTH_SIGNON_ID);
        getJspContext().setAttribute("maxlength_password", AppJspConstants.MAXLENGTH_PASSWORD);
        getJspContext().setAttribute("maxlength_fullname", AppJspConstants.MAXLENGTH_FULLNAME);
        getJspContext().setAttribute("maxlength_phonenumber", AppJspConstants.MAXLENGTH_PHONENUMBER);
        getJspContext().setAttribute("maxlength_extension", AppJspConstants.MAXLENGTH_EXTENSION);
        getJspContext().setAttribute("maxlength_email", AppJspConstants.MAXLENGTH_EMAIL);

        getJspBody().invoke(null);
    }
}

Then I have a StringHelper.tld. Inside, I have this :

<tag>
    <name>fieldAttributes</name>
    <tag-class>package.path.JspFieldAttributes</tag-class>
    <body-content>scriptless</body-content>
    <info>This tag provide HTML field attributes that CCS is unable to do.</info>
</tag>

On the JSP, I include the StringHelper.tld the normal way:

<%@ taglib uri="/WEB-INF/tags/StringHelper.tld" prefix="stringHelper" %>

Finally, I use the tag and apply the needed values using EL.

        <stringHelper:fieldAttributes>
[snip]
            <form:input path="emailAddress" cssClass="formeffect" cssErrorClass="formEffect error" maxlength="**${maxlength_email}**"/>&nbsp;
            <form:errors path="emailAddress" cssClass="error" element="span"/>
[snip]
        </stringHelper:fieldAttributes>