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XML Basics for Java Developers, Part 5
Pages: 1, 2

Transforming the Zoo Inventory

Your boss just called, and it's now imperative that your zoo clients have access to the zoo inventory through the Web, today! Well, after reading Chapter 14, you should be thoroughly prepared to build a nice "zoo portal." Let's get you started by creating an XSL stylesheet to turn our zooinventory.xml into HTML:



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

   <xsl:template match="/Inventory">
      <html><head><title>Zoo Inventory</title></head>
      <body><h1>Zoo Inventory</h2>
      <table border="1">
      <tr><td><b>Name</b></td><td><b>Species</b></td>
      <td><b>Habitat</b></td><td><b>Temperament</b></td>
      <td><b>Diet</b></td></tr>
         <xsl:apply-templates/>
           <!-- Process Inventory -->
      </table>
      </body>
      </html>
   </xsl:template>
   <xsl:template match="Inventory/Animal">
      <tr><td><xsl:value-of select="Name"/></td>
          <td><xsl:value-of select="Species"/></td>
         <td><xsl:value-of select="Habitat"/></td>
         <td><xsl:value-of select="Temperament"/></td> 
         <td><xsl:apply-templates select="Food|FoodRecipe"/>
            <!-- Process Food,FoodRecipe--></td></tr>
   </xsl:template>

   <xsl:template match="FoodRecipe">
      <table>
      <tr><td><em><xsl:value-of select="Name"/></em></td></tr>
      <xsl:for-each select="Ingredient">
         <tr><td><xsl:value-of select="."/></td></tr>
      </xsl:for-each>
      </table>
   </xsl:template>

</xsl:stylesheet>

The stylesheet contains three templates. The first matches /Inventory and outputs the beginning of our HTML document (the header) along with the start of a table for the animals. It then delegates using apply-templates before closing the table and adding the HTML footer. The next template matches Inventory/Animal, printing one row of an HTML table for each animal. Although there are no other Animal elements in the document, it still doesn't hurt to specify that we will match an Animal only in the context of an Inventory, because in this case we are relying on Animal to start and end our table. (This template makes sense only in the context of an Inventory.) Finally, we provide a template that matches FoodRecipe and prints a small (nested) table for that information. FoodRecipe makes use of the for-each operation to loop over child nodes with a select specifying that we are only interested in Ingredient children. For each Ingredient, we output its value in a row.

There is one more thing to note in the Animal template. Our apply-templates element has a select attribute that limits the elements affected. In this case, we are using the "|" regular expression-like syntax to say that we want to apply templates for only the Food or FoodRecipe child elements. Why do we do this? Because we didn't match the root of the document (only Inventory), we still have the default stylesheet behavior of outputting the plain text of nodes that aren't matched. We want this behavior for the Food element in the event that a FoodRecipe isn't there. But we don't want it for all the other elements of Animal that we've handled explicitly. Alternatively, we could have been more verbose, adding a template matching the root and another template just for the Food element. That would also mean that new tags added to our XML would be ignored and not change the output. This may or may not be the behavior you want, and there are other options as well. As with all powerful tools, there is usually more than one way to do something.

XSLTransform

Now that we have a stylesheet, let's apply it! The following simple program, XSLTransform, uses the javax.xml.transform package to apply the stylesheet to an XML document and print the result. You can use it to experiment with XSL and our example code.

import javax.xml.transform.*;
import javax.xml.transform.stream.*;
  
public class XSLTransform 
{
   public static void main( String [] args ) throws Exception
   {
      if ( args.length < 2 || !args[0].endsWith(".xsl") ) {
         System.err.println("usage: XSLTransform file.xsl file.xml");
         System.exit(1);
      }
      TransformerFactory factory = TransformerFactory.newInstance(  );
      Transformer transformer = 
         factory.newTransformer( new StreamSource( args[0] ) );
      StreamSource xmlsource = new StreamSource( args[1] );
      StreamResult output = new StreamResult( System.out );
      transformer.transform( xmlsource, output );
   }
}

Run XSLTransform, passing the XSL stylesheet and XML input, as in the following command:

% java XSLTransform zooinventory.xsl zooinventory.xml > zooinventory.html

The output should look like Figure 23-2.


Figure 23-2. Image of the zoo inventory table

Constructing the transform is a similar process to that of getting a SAX or DOM parser. The difference from our earlier use of the TransformerFactory is that this time we construct the transformer, passing it the XSL stylesheet source. The resulting Transformer object is then a dedicated machine that knows how to take input XML and generate output according to its rules.

One important thing to note about XSLTransform is that it is not guaranteed thread-safe. If you must make concurrent transformations in many threads, they must either coordinate their use of the transformer or have their own instances.

XSL in the Browser

With our XSLTransform example, you can see how you'd go about rendering XML to an HTML document on the server side. But as mentioned in the introduction, modern web browsers support XSL on the client side as well. Internet Explorer 5.x and above, Netscape 6.x, and Mozilla can automatically download an XSL stylesheet and use it to transform an XML document. To make this happen, just add a standard XSL stylesheet reference in your XML. You can put the stylesheet directive next to your DOCTYPE declaration in the zooinventory.xml file:

<?xml-stylesheet type="text/xsl" href="zooinventory.xsl"?>

Now, as long as the zooinventory.xsl file is available at the same location (base URL) as the zooinventory.xml file, the browser will use it to render HTML on the client side.

Web Services

One of the most interesting directions for XML is web services. A web service is simply an application service supplied over the network, making use of XML to describe the request and response. Normally, web services run over HTTP and use an XML-based protocol called SOAP. SOAP stands for Simple Object Access Protocol and is an evolving W3C standard. The combination of XML and HTTP provides a universally accessible interface for services.

SOAP and other XML-based remote procedure call mechanisms can be used in place of Java RMI for cross-platform communications and as an alternative to CORBA. There is a lot of excitement surrounding web services, and it is likely that they will grow in importance in coming years. To learn more about SOAP, see http://www.w3.org/TR/SOAP/. To learn more about Java APIs related to web services, keep an eye on http://java.sun.com/webservices/.

Well, that's it for our brief introduction to XML. There is a lot more to learn about this exciting new area, and many of the APIs are evolving rapidly. We hope we've given you a good start.

With this chapter we also wrap up the main part of our book. We hope that you've enjoyed Learning Java. We welcome your feedback to help us keep making this book better in the future.


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