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JSTL XML flow control tags 

Fang
Joined:
08/13/2009
Posts:
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August 23, 2010 22:55:58    Last update: August 24, 2010 15:45:04
The tags
XML flow control tags are exactly the same as their Core flow control equivalents, except that the test condition with a boolean EL expression is replaced by the select condition with an XPath expression. In the case of the forEach tag, the items attribute is replaced with the select attribute.

In a test condition, the XPath expression is evaluated to a boolean value by the rules of the XPath boolean() function, which converts its argument to a boolean as follows:
  • a number is true if and only if it is neither positive or negative zero nor NaN.
  • a node-set is true if and only if it is non-empty.
  • a string is true if and only if its length is non-zero.
  • an object of a type other than the four basic types is converted to a boolean in a way that is dependent on that type.


<x:if>
Display "UK based customer" if the customer's location attribute equals "UK": 
<x:if select="$customer/self::node()[location='UK']">
UK based customer
</x:if>


<x:choose>,<x:when>,<x:otherwise>
Display hello message with name if the customer's firstName is not empty, display generic message otherwise: 
<x:choose>
<x:when select="$customer/firstName">
Hello <x:out select="$customer/firstName"/>
</x:when>
<x:otherwise>
Hello my friend
</x:otherwise>
</x:choose>


<x:forEach>
Write out the name of each author for a list of authors matched by the XPath: 
<x:forEach select="$doc//author">
    <x:out select="@name"/><br>
</x:forEach>


Test it
Make these additions to the expanded test application:
  1. Create a new Java class XMLFlowControl: 
    package jstl.demo.handler;

import java.io.IOException;
import java.util.*;
import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.jsp.jstl.core.Config;

import jstl.demo.DemoHandler;

public class XMLFlowControl implements DemoHandler {
    public void handleRequest(HttpServletRequest req,
			      HttpServletResponse resp)
		throws IOException, ServletException {
	RequestDispatcher d = req.getRequestDispatcher("/xmlflowcontrol.jsp");
	d.forward(req, resp);
    }
}


  2. Create a new JSP (xmlflowcontrol.jsp) under webapp: 
    <%@taglib uri="http://java.sun.com/jsp/jstl/xml" prefix="x"%>
<html>
<head>
<title>JSTL XML Flow Control Tags</title>
</head>

<body>
<x:parse var="customers">
    <customers>
	<customer>
	    <location>US</location>
	    <firstName>Daniel</firstName>
	    <lastName>Smith</lastName>
	</customer>

	<customer>
	    <location>UK</location>
	    <firstName>Oliver</firstName>
	    <lastName>Jones</lastName>
	</customer>
    </customers>
</x:parse>

<x:set var="customer" select="$customers//customer[2]"/>

<p>
<b>Testing &lt;x:if&gt;:</b><br>
<x:if select="$customer/self::node()[location='UK']">
    <x:out select='$customer/firstName'/> is located in UK.
</x:if>
</p>

<p>
<b>Testing &lt;x:choose&gt;, &lt;x:when&gt;, &lt;x:otherwise&gt;:</b><br>
<x:choose>
<x:when select='$customer/firstName'>
    Hello <x:out select='$customer/firstName'/>!
</x:when>
<x:otherwise>
    Hello my friend!
</x:otherwise>
</x:choose>
</p>

<p>
<b>Testing &lt;x:forEach&gt;:</b><br>
<ul>
<x:forEach select="$customers//customer">
    <li><x:out select="firstName"/> <x:out select="lastName"/></li>
</x:forEach>
</ul>
</p>

</body>
</html>


  3. Compile and package the WAR with: mvn package

  4. Deploy the WAR to a Tomcat or JBoss server of your choice (for example, JBoss 5.1.0 GA).

  5. Test the page with this URL (JBoss 5.1.0 GA or Tomcat running on port 8080):
    http://localhost:8080/jstl-demo/demo/XMLFlowControl

    You may adjust the URL if your servlet container runs on a different port or the web app is bound to a different context root.

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