For a start, an XSL processor uses namespaces to indicate which elements to process itself and which to put directly into the result tree. Let's start with a quick review of namespaces in general, and then look at the three main uses of namespaces in XSL.

Because we can define our own element and attribute names in XML, there is always the chance that we will want to use the same name with different meanings in a document. If I am processing XML that is describing metalworking, I might want to create an element in my output called <template>. As we have seen, XSLT has an element with the same name, so I might try something like this:

<template match="pattern">
<template><value-of select="."/></template>
</template>

The purpose of XML namespaces is to make our element names unique. In this case, it would disambiguate my two <template> elements, so that the XSL processor knows that it must process the first, but that the second should be copied directly to the output tree.

XML namespaces use a Uniform Resource Identifier (URI) to ensure uniqueness of names. Why a URI? Simply that a key aspect of a URI is that it is unique. When we use the URI such as http://www.wrox.com /namespaces, a registration authority allocates the domain name wrox.com to Wrox Press and to nobody else. It is then up to Wrox to ensure that any URIs within this domain are unique.

So we can make our template elements unique by associating them with a namespace. One way to do this is with a namespace prefix, and that is what we have been doing with the XSLT namespace in our examples so far. We have associated the prefix xsl with the URI http://www.w3.org/1999/XSL/Transform using the reserved xmlns attribute within our stylesheet element:

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

Since the stylesheet element itself is part of the XSLT namespace, this also uses the prefix. As you can see, the prefix is separated from the element name by a colon.

In the terminology of the W3C recommendation (http://www.w3.org/TR/REC-xml-names), xsl:stylesheet is a qualified name (or QName), xsl is a prefix and stylesheet is a local part.

So we could modify our stylesheet to look like this:

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

<xsl:template match="pattern">
 <template><value-of select="."/></template>
</xsl:template>
</xsl:stylesheet>

Now the XSL processor knows to process the elements starting xsl: and to place other elements directly into the result tree.

Let's also declare a namespace for our <pattern> and <template> elements:

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

<xsl:template match="wrox:pattern">
 <wrox:template><value-of select="."/></wrox:template>
</xsl:template>
</xsl:stylesheet>

Now that we have done this, we can make either of these namespaces the default namespace for the document by removing the colon and prefix in the namespace declaration and the prefix and colon in element and attribute names that use that namespace. Note that we can do this with either namespace, so the following two documents are equivalent.

Here is the document with the Wrox namespace as default:

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

<xsl:template match="pattern">
 <template><value-of select="."/></template>
</xsl:template>
</xsl:stylesheet>

This is the document with the XSLT namespace as default:

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

<template match="wrox:pattern">
 <wrox:template><value-of select="."/></wrox:template>
</template>
</stylesheet>

In the example above, we have used namespaces for three distinct purposes, although we used the Wrox namespace for two of these:

• to indicate which elements were part of the XSLT namespace and so should be processed by the XSL processor
• to match elements, such as the <pattern> element, in the source tree only if they belong to a certain namespace
• to ensure that elements in the result tree, such as <template>, belong to the namespace we want

Let's just look a little more at matching namespace-qualified nodes in the source tree. Take the case where we have a stylesheet that is operating on an XML Schema document. We might have a line in our stylesheet such as:

<xsl:apply-templates select="xsd:complexType"/>

If we have not specified the namespace we are using for the xsd prefix in our stylesheet, our XSLT processor will match on the complete name xsd:complexType (since the colon is a valid part of an XML element name). If we then have another schema using xs as a qualifier rather than xsd, or using the XML Schema namespace as the default, we will no longer get matches in our stylesheet. This is unlikely to be what we want. By specifying the XML Schema namespace in our stylesheet, we can be certain that we will match XML Schema elements whatever prefix they are using, or even if they have no prefix because they belong in the default namespace of our source document.

In general in this book, we will use the xsl prefix for XSLT elements and suitable qualifiers for other namespaces.