XML Namespaces

XML Namespaces provide a method to avoid element name conflicts.

Name Conflicts

Since element names in XML are not fixed, very often a name conflict will occur when two different documents use the same names describing two different types of elements.

This XML document carries information in a table:

<table>
   <tr>
   <td>Apples</td>
   <td>Bananas</td>
   </tr>
</table>

This XML document carries information about a table (a piece of furniture):

<table>
   <name>African Coffee Table</name>
   <width>80</width>
   <length>120</length>
</table>

If these two XML documents were added together, there would be an element name conflict because both documents contain a <table> element with different content and definition.

Solving Name Conflicts using a Prefix

This XML document carries information in a table:

<h:table>
   <h:tr>
   <h:td>Apples</h:td>
   <h:td>Bananas</h:td>
   </h:tr>
</h:table>

This XML document carries information about a piece of furniture:

<f:table>
   <f:name>African Coffee Table</f:name>
   <f:width>80</f:width>
   <f:length>120</f:length>
</f:table>

Now the element name conflict is gone because the two documents use a different name for their <table> element (<h:table> and <f:table>).

By using a prefix, we have created two different types of <table> elements.

Using Namespaces

This XML document carries information in a table:

<h:table xmlns:h="http://www.w3.org/TR/html4/">
   <h:tr>
   <h:td>Apples</h:td>
   <h:td>Bananas</h:td>
   </h:tr>
</h:table>

This XML document carries information about a piece of furniture:

<f:table xmlns:f="http://www.w3schools.com/furniture">
   <f:name>African Coffee Table</f:name>
   <f:width>80</f:width>
   <f:length>120</f:length>
</f:table>

Instead of using only prefixes, an xmlns attribute has been added to the <table> tag to give the element prefix a qualified name associated with a namespace.

The Namespace Attribute

The namespace attribute is placed in the start tag of an element and has the following syntax:

xmlns:namespace-prefix="namespace"

In the examples above, the namespace itself is defined using an Internet address:

xmlns:f="http://www.w3schools.com/furniture"

The W3C namespace specification states that the namespace itself should be a Uniform Resource Identifier (URI).

When a namespace is defined in the start tag of an element, all child elements with the same prefix are associated with the same namespace.

Note that the address used to identify the namespace, is not used by the parser to look up information. The only purpose is to give the namespace a unique name. However, very often companies use the namespace as a pointer to a real Web page containing information about the namespace.

Uniform Resource Identifiers

A Uniform Resource Identifier (URI) is a string of characters which identifies an Internet Resource. The most common URI is the Uniform Resource Locator (URL) which identifies an Internet domain address. Another, not so common type of URI is the Universal Resource Name (URN). 

Since the furniture example uses an internet address to identify its namespace, we can be sure that its namespace is unique.

Default Namespaces

Defining a default namespace for an element saves us from using prefixes in all the child elements. It has the following syntax:

<element xmlns="namespace">

This XML document carries information in a table:

<table xmlns="http://www.w3.org/TR/html4/">
   <tr>
   <td>Apples</td>
   <td>Bananas</td>
   </tr>
</table>

This XML document carries information about a piece of furniture:

<table xmlns="http://www.w3schools.com/furniture">
   <name>African Coffee Table</name>
   <width>80</width>
   <length>120</length>
</table>

Namespaces in Real Use

When you start using XSL, you will soon see namespaces in real use. XSL style sheets are used to transform XML documents into other formats like HTML.

If you take a close look at the XSL document below, you will see that most of the tags are HTML tags. The tags that are not HTML tags have the prefix xsl, identified by the namespace "http://www.w3.org/TR/xsl":

<?xml version="1.0" encoding="ISO-8859-1"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/TR/xsl">
<xsl:template match="/">
  <html>
  <body>
    <table border="2" bgcolor="yellow">
      <tr>
        <th>Title</th>
        <th>Artist</th>
      </tr>
      <xsl:for-each select="CATALOG/CD">
      <tr>
        <td><xsl:value-of select="TITLE"/></td>
        <td><xsl:value-of select="ARTIST"/></td>
      </tr>
      </xsl:for-each>
    </table>
  </body>
  </html>
</xsl:template>
</xsl:stylesheet>