XML namespace: Difference between revisions

Revision as of 19:54, 18 March 2008

Revision as of 22:00, 20 March 2008

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'''XML namespaces''' are a [[W3C]] recommendationused for providing uniquely named elements and attributes in an [[XML]] instance. They are defined by a [[W3C]] recommendation called Namespaces in XML. 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.

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.

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<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, the page at [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 URL (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.

It is also possible to map namespaces to prefixes in namespace declarations. For example:

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

In this case, any element or attribute names that start with the prefix "xhtml:" are considered to be in the XHTML namespace.

== External links ==

*[http://www.w3.org/TR/REC-xml-names/ W3C Recommendation: Namespaces in XML, namespaceversion documentation1.0]

*[http://www.w3.org/TR/xml-names11/ W3C Recommendation: Namespaces in XML, version 1.1]

*[http://xml.coverpages.org/namespaces.html Cover Pages: Namespaces in XML]

*[http://www.service-architecture.com/xml/articles/common_xml_vocabularies.html Common XML Vocabularies]