XML Namespaces Tutorial

XML | Namespaces

Namespaces
Default Namespace
Using URIs to Define Namespaces

Namespaces
A namespace allows you to identify elements and attributes in an XML document. Namespaces are useful when the XML document contains data from different domains or languages. In particular, namespaces can be used to distinguish between tags with the same name that belong to different domains or languages.

For example, an XML document may contain tags from the XSLT language (stylesheet, template) as well as tags from HTML (html, body):
```
<stylesheet>
    <template match="/">
        <html>
            <body>
                <apply-templates />
            </body>
        </html>
    </template>
</stylesheet>
```
A namespace is identified by a prefix (alias). This alias can be used to qualify a tag or attribute and mark it as belonging to a specific namespace. Tags and attributes are associated with a namespace by prefixing their names with the alias, followed by a colon ":":
```
<xsl:stylesheet>
    <xsl:template match="/">
        <html>
            <body>
                <xsl:apply-templates />
            </body>
        </html>
    </xsl:template>
</xsl:stylesheet>
```
But before using a namespace alias, it must be declared. For example, to declare a namespace for the alias "xsl", you associate this alias with a value (by prefixing it with the keyword "xmlns" followed by a colon):
```
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
    <xsl:template match="/">
        <html>
            <body>
                <xsl:apply-templates />
            </body>
        </html>
    </xsl:template>
</xsl:stylesheet>
```
The alias name can be any valid string; there is no strict naming convention, except that it must be a valid name (see tag naming rules).
So the alias "xsl" can be replaced with "myxsl" or any other valid name.

If an alias is used to qualify an element, then it must be used in both the opening and closing tags of that element.

If an element has no alias (for example, "<html>"), then it does not belong to any namespace (unless a default namespace is defined — see the next section).

Note that a namespace only applies to the element where it is defined and its descendant elements.
Default Namespace
A default namespace allows you to associate all unprefixed elements in an XML document with a namespace. Only one default namespace can be defined for a given element. Similarly, an XML document may not define any default namespace, in which case unprefixed elements do not belong to any namespace.

To define a default namespace, use the same syntax shown above but without specifying an alias after the "xmlns" keyword.
```
<xsl:stylesheet xmlns="http://www.w3.org/1999/xhtml" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
    <xsl:template match="/">
        <html>
            <body>
                <xsl:apply-templates />
            </body>
        </html>
    </xsl:template>
</xsl:stylesheet>
```
In this case, the "<html>" and "<body>" elements (since they are not prefixed) belong to the default namespace defined by the value "http://www.w3.org/1999/xhtml".

It is also possible to declare a namespace directly within a child element:
```
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
    <xsl:template match="/">
        <html xmlns="http://www.w3.org/1999/xhtml">
            <body>
                <xsl:apply-templates />
            </body>
        </html>
    </xsl:template>
</xsl:stylesheet>
```
You can also associate an attribute with a namespace:
```
<h:html xmlns:h="http://www.w3.org/1999/xhtml">
    <h:body h:class="body1">
    </h:body>
</h:html>
```
Using URIs to Define Namespaces
A URI (Uniform Resource Identifier) can take the form of either:
- URL (Uniform Resource Locator)
  The first part of a URL specifies the protocol, such as http, followed by the path to the resource.
```
<h:html xmlns:h="http://www.w3.org/1999/xhtml">
    
</h:html>
```
- URN (Uniform Resource Name)
  A URN is composed of words separated by colons.
```
<i:book xmlns:i="urn:isbn:123">
    
</i:book>
```
  The first part is the keyword: "urn"
  The second part is the Namespace Identifier (NID): "isbn"
  The final part is the Namespace Specific String (NSS): "123"