XML Namespaces

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XML Namespaces provide a method to avoid element name conflicts.

Name Conflicts

In XML, element names are defined by the developer. This often results in a conflict when trying to mix XML documents from different XML applications.

This XML carries HTML table information:

<table>
  <tr>
    <td>Speakers</td>
    <td>Laptops</td>
  </tr>
</table>

This XML carries information about Bench

<table>
  <name>A School Bench</name>
  <width>80</width>
  <length>120</length>
</table>

If these XML fragments were added together, there would be a name conflict. Both contain a <table> element, but the elements have different content and meaning.

An XML parser will not know how to handle these differences.

Solving the Name Conflict Using a Prefix

Name conflicts in XML can easily be avoided using a name prefix.

This XML carries information about an HTML table, and a piece of furniture:

<h:table>
  <h:tr>
    <h:td>Speakers</h:td>
    <h:td>Laptops</h:td>
  </h:tr>
</h:table>

<f:table>
  <f:name>A School Bench</f:name>
  <f:width>80</f:width>
  <f:length>120</f:length>
</f:table>

In the example above, there will be no conflict because the two <table> elements have different names.

XML Namespaces - The xmlns Attribute

When using prefixes in XML, a so-called namespace for the prefix must be defined.

The namespace is defined by the xmlns attribute in the start tag of an element.

The namespace declaration has the following syntax. xmlns:prefix="URI".

<root>

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

<f:table xmlns:f="http://www.qctutorials.com/furniture">
  <f:name>A School Bench</f:name>
  <f:width>80</f:width>
  <f:length>120</f:length>
</f:table>

</root>

In the example above, the xmlns attribute in the <table> tag give the h: and f: prefixes a qualified namespace.

When a namespace is defined for an element, all child elements with the same prefix are associated with the same namespace.

Namespaces can be declared in the elements where they are used or in the XML root element:

<root
xmlns:h="http://www.w3.org/TR/html4/"
xmlns:f="http://www.qctutorials.com/furniture">

<h:table>
  <h:tr>
    <h:td>Speakers</h:td>
    <h:td>Laptops</h:td>
  </h:tr>
</h:table>

<f:table>
  <f:name>A School Bench</f:name>
  <f:width>80</f:width>
  <f:length>120</f:length>
</f:table>

</root>

Note: The namespace URI is not used by the parser to look up information.

Uniform Resource Identifier (URI)

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).

In our examples we will only use URLs.

Default Namespaces

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

xmlns="namespaceURI"

This XML carries HTML table information:

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

This XML carries information about a piece of furniture:

<table xmlns="http://www.qctutorials.com/furniture">
  <name>A School Bench</name>
  <width>80</width>
  <length>120</length>
</table>

Namespaces in Real Use

XSLT is an XML language that can be used to transform XML documents into other formats, like HTML.

In the XSLT document below, you can see that most of the tags are HTML tags.

The tags that are not HTML tags have the prefix xsl, identified by the namespace xmlns:xsl="http://www.w3.org/1999/XSL/Transform":

<?xml version="1.0" encoding="ISO-8859-1"?>

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

<xsl:template match="/">
<html>
<body>
  <h2>Friends List</h2>
  <table border="1">
    <tr>
      <th align="left">Title</th>
      <th align="left">Artist</th>
    </tr>
    <xsl:for-each select="list/friends">
    <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>

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