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AddThis Social Bookmark Button O'Reilly Book Excerpts: Learning Java, 2nd Edition

XML Basics for Java Developers, Part 5

by Patrick Niemeyer and Jonathan Knudsen

In this final in a series of XML basics for Java developers book excerpts from Learning Java, 2nd Edition, get an introduction to XSL/XSLT and Web services.

Related Reading

Learning Java
By Patrick Niemeyer, Jonathan Knudsen

XSL/XSLT

Earlier in this chapter, we used a Transformer object to copy a DOM representation of an example back to XML text. We mentioned then that we were not really tapping the potential of the Transformer. Now we'll give you the full story.

The javax.xml.transform package is the API for using the XSL/XSLT transformation language. XSL stands for Extensible Stylesheet Language. Like Cascading Stylesheets for HTML, XSL allows us to "mark up" XML documents by adding tags that provide presentation information. XSL Transformation (XSLT) takes this further by adding the ability to completely restructure the XML and produce arbitrary output. XSL and XSLT together comprise their own programming language for processing an XML document as input and producing another (usually XML) document as output. (From here on in we'll refer to them collectively as XSL.)

XSL is extremely powerful, and new applications for its use arise every day. For example, consider a web portal that is frequently updated and which must provide access to a variety of mobile devices, from PDAs to cell phones to traditional browsers. Rather than recreating the site for these and additional platforms, XSL can transform the content to an appropriate format for each platform. Multilingual sites also benefit from XSL.

You can probably guess the caveat that we're going to issue next: XSL is a big topic worthy of its own books (see, for example, O'Reilly's Java and XSLT by Eric Burke, a fellow St. Louis author), and we can only give you a taste of it here. Furthermore, some people find XSL difficult to understand at first glance because it requires thinking in terms of recursively processing document tags. Don't be put off if you have trouble following this example; just file it away and return to it when you need it. At some point, you will be interested in the power transformation can offer you.

XSL Basics

XSL is an XML-based standard, so it should come as no surprise that the language is based on XML. An XSL stylesheet is an XML document using special tags defined by the XSL namespace to describe the transformation. The most basic XSL operations include matching parts of the input XML document and generating output based on their contents. One or more XSL templates live within the stylesheet and are called in response to tags appearing in the input. XSL is often used in a purely input-driven way, where input XML tags trigger output in the order that they appear, using only the information they contain. But more generally, the output can be constructed from arbitrary parts of the input, drawing from it like a database, composing elements and attributes. The XSLT transformation part of XSL adds things like conditionals and for loops to this mix, enabling arbitrary output to be generated based on the input.

An XSL stylesheet contains as its root element a stylesheet tag. By convention, the stylesheet defines a namespace prefix xsl for the XSL namespace. Within the stylesheet are one or more template tags containing a match attribute describing the element upon which they operate.

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

   <xsl:template match="/">
     I found the root of the document!
   </xsl:template>

</xsl:stylesheet>

When a template matches an element, it has an opportunity to handle all the children of the element. The simple stylesheet above has one template that matches the root of the input document and simply outputs some plain text. By default, input not matched is simply copied to the output with its tags stripped (HTML convention). But here we match the root so we consume the entire input.

In This Series

XML Basics for Java Developers, Part 4
In part four in a series of XML basics for Java developers book excerpts from Learning Java, 2nd Edition, learn about validating documents.

XML Basics for Java Developers, Part 3
In part three in this series of book excerpts on XML basics for Java developers from Learning Java, 2nd Edition, learn about the Document Object Model (DOM).

XML Basics for Java Developers, Part 2
In this second part in a several part series on XML for Java developers from Learning Java, 2nd Edition, learn about SAX and the SAX API.

XML Basics for Java Developers, Part 1
This is the first in a series of book excerpts on XML for Java developers from Learning Java, 2nd Edition. This excerpt covers XML fundamentals.

The match attribute can refer to elements in a hierarchical path fashion starting with the root. For example, match="/Inventory/Animal" would match only the Animal elements from our zooinventory.xml file. The path may be absolute (starting with "/") or relative, in which case the template detects whenever that element appears in any context. The match attribute actually uses an expression format called XPath that allows you to describe element names using a syntax somewhat similar to a regular expression. XPath is a powerful syntax for describing sets of nodes in XML, and it includes notation for describing sets of child nodes based on path and even attributes.

Within the template, we can put whatever we want, as long as it is well-formed XML (if not, we can use a CDATA section). But the real power comes when we use parts of the input to generate output. The XSL value-of tag is used to output the content of an element or a child of the element. For example, the following template would match an Animal element and output the value of its Name child:

<xsl:template match="Animal">
   Name: <xsl:value-of select="Name"/>
</xsl:template>

The select attribute uses a similar expression format to match. Here we tell it to print the value of the Name element within Animal. We could have used a relative path to a more deeply nested element within Animal or even an absolute path to another part of the document. To refer to its own element, we can simply use "." as the path. The select expression can also retrieve attributes from the elements it refers to.

Now if we try to add the Animal template to our simple example, it won't generate any output. What's the problem? Well, if you recall, we said that a template matching an element has the opportunity to process all its children. We already have a template matching the root ("/"), so it is consuming all the input. The answer to our dilemma--and this is where things get a little tricky--is to delegate the matching to other templates using the apply-templates tag. The following example correctly prints the names of all the animals in our document:

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

   <xsl:template match="/">
      Found the root!
      <xsl:apply-templates/>
   </xsl:template>

   <xsl:template match="Animal">
      Name: <xsl:value-of select="Name"/>
   </xsl:template>

</xsl:stylesheet>

Note that we still have the opportunity to add output before and after the apply-templates tag. But upon invoking it, the template matching continues from the current node. Next we'll use what we have so far and add a few bells and whistles.

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