XML Reference Guide

Mar 14, 2003

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Using XSLT to transform XML into another form has a lot of advantages, not the least of which is flexibility. When you want to change the output, you don't have to go back to the code, you simply have to change the XSLT style sheet. But it's easy to mistake this simplicity for shallowness. XSLT can enable you to do much more than simply reformatting text.

Consider a situation in which not all of the information destined for the final output is in the XML file. How can you incorporate it without complex pre- or post-processing? For example, say we have a file of location information:

<places>
    <place>
        <city>New York</city>
        <state>NY</state>
        <zip>10001</zip>
    </place>
    <place>
        <city>Boston</city>
        <state>MA</state>
        <zip>02134</zip>
     </place>
     <place>
        <city>Denver</city>
        <state>CO</state>
        <zip>80002</zip>
     </place>
</places>

What if we wanted to display not just the city, state, and ZIP code, but also the temperature? And to make things more difficult, what if we wanted the live temperature at the moment of transformation?

One option is to make a call to another application, such as a Java class, from within the stylesheet.

For example, consider this basic style sheet:

<?xml version="1.0" ?>
<xsl:stylesheet version="1.0" 
       xmlns:xsl="http://www.w3.org/1999/XSL/Transform">

   <xsl:template match="/">
       <html>
           <head><title>Weather</title></head>
           <body>
              <table>
                 <th>City</th><th>State</th><th>ZIP code</th>
                 <th>Current Temp</th>

                 <xsl:apply-templates />
              </table>
           </body>
       </html>
   </xsl:template>

   <xsl:template match="place">
       <tr>
          <td><xsl:value-of select="city"/></td>
          <td><xsl:value-of select="state"/></td>
          <td><xsl:value-of select="zip"/></td>
          <td></td>
       </tr>
   </xsl:template>
	
</xsl:stylesheet>

This stylesheet creates an HTML page that displays the data, but doesn't yet include the actual temperature. To get the actual temperature, we can create a simple Java class:

package com.informit.chase;

public class Weather extends Object {

    public static String getTemp(String zip){
        String returnTemp = "";

        //GET TEMPERATURE HERE

        return returnTemp;
    }

}

I've left out the implementation of the getTemp() method because it's not crucial to the discussion; the temperature could be coming from a web service, a thermometer, or some other source. The important part is that this method returns the temperature based on the ZIP code. Now we just have to integrate it into the style sheet.

To call a Java class from an XSLT style sheet, we first have to let the processor know where to find the code. We can do that by defining a new namespace:

<?xml version="1.0" ?>
<xsl:stylesheet version="1.0" 
      xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
      xmlns:java="com.informit.chase.Weather"
      exclude-result-prefixes="java">

   <xsl:template match="/">
       <html>
           <head><title>Weather</title></head>
           <body>
...

Here I've defined the java: namespace to represent the com.informit.chase.Weather class. (Note that this class must be on the CLASSPATH when you perform the transformation.) Now I just have to call it from within the stylesheet itself:

...
   <xsl:template match="place">
       <tr>
          <td><xsl:value-of select="city"/></td>
          <td><xsl:value-of select="state"/></td>
          <td><xsl:value-of select="zip"/></td>

          <xsl:variable name="theZip" select="zip"/>
          <xsl:variable name="theTemp" select="java:getTemp($theZip)"/>
          <td><xsl:value-of select="$theTemp"/>></td>
       </tr>
   </xsl:template>
...

First I'm creating a simple variable so I can pass the value of the ZIP code to the method. Next I'm creating a variable and populating it with the results of the method. Note that because the java: namespace represents the class, we just need to note the actual method. Finally, all we need to do is drop the value into the template.

The result is the final page (with spaces added for intelligibility):

<?xml version="1.0" encoding="UTF-8"?>
<html>
   <head><title>Weather</title></head>
   <body>
      <table><th>City</th><th>State</th><th>ZIP code</th>
             <th>Current Temp</th>
        <tr>
           <td>New York</td><td>NY</td><td>10001</td><td>45</td>
        </tr>
        <tr>
           <td>Boston</td><td>MA</td><td>02134</td><td>44</td>
        </tr>
        <tr>
           <td>Denver</td><td>CO</td><td>80002</td><td>76</td>
        </tr>
      </table>
   </body>
</html>

Extension methods are completely implementation dependent. For example, MSXML doesn't support Java at all. This version covers calling Java from the XSLT engine built in to Java 1.4. Check the documentation for your XSLT engine to find out how to extend it by calling another application.

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