Chapter 8. XSL Transformations (XSLT)

The Extensible Stylesheet Language (XSL) is divided into two parts: XSL Transformations (XSLT) and XSL Formatting Objects (XSL-FO). This chapter describes XSLT. Chapter 14 covers XSL-FO.

XSLT is an XML application for specifying rules by which one XML document is transformed into another XML document. An XSLT document—that is, an XSLT stylesheet—contains template rules. Each template rule has a pattern and a template. An XSLT processor compares the elements and other nodes in an input XML document to the template-rule patterns in a stylesheet. When one matches, it writes the template from that rule into the output tree. When it's done, it may further serialize the output tree into an XML document or some other format like plain text or HTML.

This chapter describes the template rules and a few other elements that appear in an XSLT stylesheet. XSLT uses the XPath syntax to identify matching nodes. We'll introduce a few pieces of XPath here, but most of it will be covered in Chapter 9.

An Example Input Document

To demonstrate XSL Transformations, we first need a document to transform. Example 8-1 shows the document used in this chapter. The root element is people, which contains two person elements. The person elements have roughly the same structure (a name followed by professions and hobbies) with some differences. For instance, Alan Turing has three professions, but Richard Feynman only has one. Feynman has a middle_initial and a hobby, but Turing doesn't. Still, these are clearly variations on the same basic structure. A DTD that permitted both of these would be easy to write.

Example 8-1. An XML document describing two people

<?xml version="1.0"?>
<people>
  <person born="1912" died="1954">
    <name>
      <first_name>Alan</first_name>
      <last_name>Turing</last_name>
    </name>
    <profession>computer scientist</profession>
    <profession>mathematician</profession>
    <profession>cryptographer</profession>
  </person>
  <person born="1918" died="1988">
    <name>
      <first_name>Richard</first_name>
      <middle_initial>P</middle_initial>
      <last_name>Feynman</last_name>
    </name>
    <profession>physicist</profession>
    <hobby>Playing the bongoes</hobby>
  </person>
</people>

Example 8-1 is an XML document. For purposes of this example, it will be stored in a file called people.xml. It doesn't have a DTD; however, this is tangential. XSLT works equally well with valid and invalid (but well-formed) documents. This document doesn't use namespaces either, although it could. XSLT works just fine with namespaces. Unlike DTDs, XSLT does pay attention to the namespace URIs instead of the prefixes. Thus, it's possible to use one prefix for an element in the input document and different prefixes for the same namespace in the stylesheet and output documents.