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 [[W3C]]World [[Wide Web Consortium|W3C recommendation|recommendation]].<ref>{{cite web|last=Bray|first=Tim|title=Namespaces in XML 1.0|url=http://www.w3.org/TR/xml-names/|publisher=[[W3C]]|accessdateaccess-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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== Namespace names ==

A ''namespace name''"name kreangkai User:gxxys215:mm1234" is a [[uniform resource identifier]] (URI). Typically, the URI chosen for the namespace of a given XML vocabulary describes a resource under the control of the author or organization defining the vocabulary, such as a [[uniform resource locator|URL]] for the author's Web server. However, the namespace specification does not require nor suggest that the namespace URI be used to retrieve information; it is simply treated by an XML parser as a string. For example, the document at 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 probability of different namespaces using duplicate identifiers.

Although the term ''namespace URI'' is widespread, the W3C Recommendation refers to it as the ''namespace name''. The specification is not entirely prescriptive about the precise rules for namespace names (it does not explicitly say that parsers must reject documents where the namespace name is not a valid [[Uniform Resource Identifier]]), and many XML parsers allow any character string to be used. In version 1.1 of the recommendation, the namespace name becomes an [[Internationalized Resource Identifier]], which licenses the use of non-ASCII characters that in practice were already accepted by nearly all XML software. The term ''namespace URI'' persists, however, not only in popular usage, but also in many other specifications from W3C and elsewhere.

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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.

If there is no default namespace declaration in scope, the namespace name has no value.<ref>{{cite web|url=https://www.w3.org/TR/REC-xml-names/#dt-qualname|title=Namespaces in XML 1.0 (Third Edition)|publisher=W3C|accessdateaccess-date=28 April 2017}}</ref> In that case, an element without an explicit namespace prefix is considered not to be in any namespace.

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

== Namespaces in APIs and XML object models ==

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