XML namespace: Difference between revisions

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{{short description|Method of providing unique elements and attributes in an XML document}}

'''XML namespaces''' are used for providing uniquely named [[Data element|elements]] and attributes in an [[XML]] document. They are defined in a [[World Wide Web Consortium|W3C recommendation]].<ref>{{cite web|last=Bray|first=Tim|title=Namespaces in XML 1.0|url=http://www.w3.org/TR/xml-names/|publisher=[[W3C]]|access-date=9 October 2010|author2=Dave Hollander |author3=Andrew Layman |author4=Richard Tobin |author5=Henry S. Thompson |date=December 2009}}</ref><ref name="timelinehistory">{{cite web |url=http://www.dblab.ntua.gr/~bikakis/XML%20and%20Semantic%20Web%20W3C%20Standards%20Timeline-HistoryXMLSemanticWebW3CTimeline1.2.pdf |title=XML and Semantic Web W3C Standards Timeline (v.1.2) |access-date=2022-04-02 |archive-date=2022-12-29 |archive-url=https://web.archive.org/web/20221229130856/http://www.dblab.ntua.gr/~bikakis/XMLSemanticWebW3CTimeline1.2.pdf |url-status=dead }}</ref> An XML instance may contain element or attribute names from more than one XML vocabulary. If each vocabulary is given a [[namespace]], the ambiguity between identically named elements or attributes can be resolved.

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 named '''id'''. References to the '''id''' element would therefore be ambiguous; placing them in different namespaces would remove the ambiguity.

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For example, the following declaration maps the "xhtml:" prefix to the XHTML namespace:

<pre><nowiki>xmlns:xhtml="http://www.w3.org/20211999/xhtml"</nowiki></pre>

Any element or attribute whose name starts with the prefix "xhtml:" is considered to be in the XHTML namespace, if it or an ancestor has the above namespace declaration.

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It is also possible to declare a default namespace. For example:

<pre><nowiki>xmlns="http://www.w3.org/20211999/xhtml"</nowiki></pre>

In this case, any element without a namespace prefix is considered to be in the XHTML namespace, if it or an ancestor has the above default namespace declaration.

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Attributes are never subject to the default namespace. An attribute without an explicit namespace prefix is considered not to be in any namespace.

==<span lang="en" dir="ltr">Namespaces</span> in APIs and XML object models ==

Different specifications have taken different approaches on how namespace information is presented to applications.