Recipe 23.3 Using the Core JSTL Tags

Problem

You want to use the core JSTL tags in a JSP.

Solution

Use the taglib directive with the core uri attribute value to make the tags available in the JSP.

Discussion

This recipe demonstrates several JSTL tags that you use all the time: c:set, c:out, c:forEach, and c:if. Here are the tag summaries:

The c:set tag sets object attributes to page, request, session, or application scopes.
The c:out tag displays text literals or the values of variables or bean properties in your JSPs.
The c:forEach tag iterates over Maps, Collections, and arrays.
The c:if tag tests expressions for true or false values, then conditionally executes the code nested in the c:if body.

Remember to use the prefix for the certain functional area of the JSTL, such as c, followed by a colon, and the tag name, as in "c:forEach."

Example 23-1 is a helper class that I find necessary to properly return a String array of TimeZone IDs to the JSP in Example 23-2.

Example 23-1. A helper class to help display TimeZone IDs

package com.jspservletcookbook;           

import java.util.TimeZone;

public class ZoneWrapper  {

    public ZoneWrapper( ){}
        
        public String[] getAvailableIDs( ){
        
            return TimeZone.getAvailableIDs( );
        
        }

}

Example 23-2 shows how to use a number of the core JSTL tags. The code uses the jsp:useBean standard action to create ZoneWrapper (Example 23-1) and java.util.Date objects for use by the tags.

Example 23-2. Using core JSTL 1.0 tags in a JSP

<%@ taglib uri="http://java.sun.com/jstl/core" prefix="c" %>

<jsp:useBean id="zone" class="com.jspservletcookbook.ZoneWrapper" /> 

<jsp:useBean id="date" class="java.util.Date" />

<html>
<head><title>Using the Core JSTL tags</title></head>
<body>
<h2>Here are the available Time Zone IDs on your system</h2>
 

<c:if test="${date.time != 0}" >

    <c:out value=
      "Phew, time has not stopped yet...<br /><br />" escapeXml="false"/>

</c:if>

<%-- The variable 'zones' contains a String array of TimeZone IDs; it is stored as a 
'session' object attribute. The '${zone.availableIDs}' expression is the equivalent of 
calling the ZoneWrapper.getAvailableIDs( ) method --%>

<c:set var="zones" value="${zone.availableIDs}" scope="session" />

<c:forEach var="id" items="${zones}">

        <c:out value="${id}<br />" escapeXml="false" />


</c:forEach>

</body>
</html>

The c:if tag uses an EL phrase to test whether the Date object's getTime( ) method returns a value that is not zero (of course it does! I'm just demonstrating how to use the c:if tag).

${date.time != 0}

The prior code represents a boolean expression that returns true if Date.getTime( ) is greater than zero. If true, then the code executes the nested c:out tag, which writes a message that the client's browser displays.

The escapeXml="false" code displays the characters <br /><br /> correctly in the HTML output by the c:out tag. See Table 23-3.

Example 23-2 sets an object attribute to session scope. This object is a String[] type containing time zone IDs, such as "Pacific/Tahiti." The c:forEach tag then iterates over all of these array members, displaying each ID with the c:out tag:

<c:forEach var="id" items="${zones}">
    <c:out value="${id}<br />" escapeXml="false" />
</c:forEach>

The var attribute of the c:forEach tag stores the current array member as c:forEach cycles over the collection. The c:out tag uses an EL expression to access the value of the current array member:

<c:out value="${id}<br />" escapeXml="false" />

If you do not give the escapeXml attribute a false value when using c:out , the character entity codes shown in Table 23-3 will display instead of the escaped characters.

Table 23-3. The c:out tag's escaped characters

c:out value attribute character

Character entity code

<

<

>

>

&

&

`

'

"

"

Figure 23-1 shows a part of the JSP using the code in Example 23-2.

Figure 23-1. A JSP using the various core tags to display time zone IDs