XML Reference Guide

Mar 14, 2003

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A cousin of Hypertext Markup Language (HTML), Extensible Markup Language (XML) provides a human-readable, platform-independent way to represent data, whether it's content destined for a web page or brochure, or data destined for an enterprise database.

Like HTML and their mutual parent, Standard Generalized Markup Language (SGML), XML works on the principle of tags, which delineate elements and content. Consider, for example, the following XML document:

<?xml version="1.0"?>
<candy>
  <product>Chocolate Bar</product>
</candy>

This simple document has two elements: candy and product. These elements are indicated by tags, or constructions that start with the less-than sign (<) and end with the greater-than sign (>) surrounding the name of the element. In other words, the candy element starts with the candy opening tag:

<candy>

and ends with the candy closing tag:

</candy>

The slash (/) at the beginning indicates that it's a closing tag.

Because the product element is between the candy opening and closing tags of the candy element, it's said to be nested within the candy element.

XML elements frequently have attributes (name/value pairs that appear within the opening tag of the element). For example, in the following document, the product element is said to have a type attribute with a value of chocolate and an itemid attribute with a value of C33:

<?xml version="1.0"?>
<candy>
  <product type="chocolate" itemid="C33">Harvey Bar</product>
</candy>

If you're familiar with the specific set of elements and attributes you can play with in HTML, you may be a little surprised to find out that there are no such restrictions in XML itself. That's what makes it so powerful. Application developers and industries are free to create their own vocabularies, such as these:

Extensible Hypertext Markup Language (XHTML) is an XML version of HTML.
Scalable Vector Graphics (SVG) is a means for describing graphics via tags rather than binary information.
Business Process Markup Language (BPML) provides a way to describe the flow of information within an enterprise.

You might design an application that outputs order information like this:

<?xml version="1.0"?>
<order orderid="THX1138" customerNumber="3263827">
  <lineitem itemid="C33">
    <quantity>36</quantity>
    <unitprice currency="dollars">.35</unitprice>
  </lineitem>
  <lineitem itemid="M48">
    <quantity>1</quantity>
    <unitprice currency="dollars">2200</unitprice>
  </lineitem>
</order>

This information could be destined for any number of systems. It might be transformed into a web page that acts as a receipt, or imported into a database of orders, or sent using a web service to a fulfillment house.

XML 1.0 is defined by the XML 1.0 Recommendation from the World Wide Web Consortium (W3C), and is used as the basis for a number of related specifications and recommendations.

XML documents must adhere to several "rules" in order to be considered well-formed:

They must have a single root element, to which all other elements and text belong.
All opening tags must have a matching closing tag.

NOTE

Unlike HTML, XML is case-sensitive; the case of the tags must match exactly.

All attribute values must be enclosed in single (') or double quotes (").
Elements must be properly nested. In other words, an element that starts within another element must end within that element.

An XML document must be well-formed, or it can't be processed.

Working programmatically with XML data requires the use of a parser—an application that reads the document and breaks it into its component parts for interpretation according to a particular model.

XML also enables you to combine different types of data through the use of namespaces. Namespaces are a way to indicate, via a prefix in the element name, that an element or attribute belongs to a particular type of data. For example, consider the following simple style sheet, with transformation instructions shown in bold:

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

<xsl:template match="/">

<html>
<head><title>Message of the Day</title></head>
<body>
  <p><xsl:value-of select="message"/></p>
</body>
</html>

</xsl:template>
</xsl:stylesheet>

The elements prefixed by xsl: give instructions to the XSLT processor (as discussed in the section on XSL Transformations), and those without the prefix should be passed through to the resulting document.

It's important to note that the namespace is defined by the xmlns: attribute. In this case, the namespace is actually http://www.w3.org/1999/XSL/Transform, and not xsl:. The xsl: notation is simply the prefix associated with the namespace. The document could just as easily have been done this way:

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

<bogus:template match="/">

<html>
<head><title>Message of the Day</title></head>
<body>
  <p><bogus:value-of select="message"/></p>
</body>
</html>

</bogus:template>
</bogus:stylesheet>

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