XML namespace: Difference between revisions

Revision as of 19:10, 28 February 2007

Revision as of 18:06, 3 April 2007

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An '''XML Namespace''' is a [[W3C]] Recommendationrecommendation for providing uniquely named elements and attributes in an [[XML]] instance. An XML instance may contain element or attribute names from more than one XML vocabulary. If each vocabulary is given a [[Namespace (computer science)|namespace]] then the ambiguity between identically named elements or attributes can be resolved.

All element names within a namespace must be unique.

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A simple example would be to consider an XML instance that contained references to a customer and an ordered product. Both the customer element and the product element could have a child element "ID_number". References to the element ID_number would therefore be ambiguous unless the two identically named but semantically different elements were brought under namespaces that would differentiate them.

== Namespace declaration ==

A namespace is declared using the reserved XML attribute <code>xmlns</code>, the value of which must be a [[Uniform Resource Identifier|URI]] (Uniform Resource Identifier) reference, e.g.:

For example:

<pre><nowiki>xmlns="http://www.w3.org/1999/xhtml"</nowiki></pre>

Note, however, that the URI is not actually read as an online address; it is simply treated by an XML parser as a string. For example, [http://www.w3.org/1999/xhtml http://www.w3.org/1999/xhtml] itself does not contain any code, it simply describes the [[xhtml]] namespace to human readers. Using a URI (such as <nowiki>"http://www.w3.org/1999/xhtml"</nowiki>) to identify a namespace, rather than a simple string (such as "xhtml"), reduces the possibility of different namespaces using duplicate identifiers. Namespace identifiers need not follow the conventions of web addresses, though they often do.