XML namespace: Difference between revisions

Revision as of 18:49, 13 February 2016

Revision as of 00:50, 17 March 2016

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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.

Following publication of the Namespaces recommendation, there was an intensive debate about how a relative URI should be handled, with some intensely arguing that it should simply be treated as a character string, and others arguing with conviction that it should be turned into an absolute URI by resolving it against the base URI of the document.<ref>{{cite web|title=News from the trenches|first=Leigh|last=Dodds|date=24 May 2000|url=http://www.xml.com/pub/a/2000/05/24/deviant/index.html |publisher=O'Reilly Media, Inc.}}</ref> The result of the debate was a ruling from W3C that relative URIs were deprecated.<ref>{{citecitation|title=W3C XML Plenary decision on relative URI references in namespace declarations|first=Dan|last=Connolly|date=11 Sep 2000}}</ref>

The use of URIs taking the form of URLs in the http scheme (such as ''http://www.w3.org/1999/xhtml'') is common, despite the absence of any formal relationship with the HTTP protocol. The Namespaces specification does not say what should happen if such a URL is dereferenced (that is, if software attempts to retrieve a document from this ___location). One convention adopted by some users is to place an [[RDDL]] document at the ___location.<ref>{{citecitation|first=Elliotte Rusty|last=Harold|title=RDDL Me This: What Does a Namespace URL Locate?|date=20 Feb 2001|url=http://www.oreillynet.com/pub/a/oreilly/xml/news/xmlnut2_0201.html}}</ref> In general, however, users should assume that the namespace URI is simply a name, not the address of a document on the Web.

== Namespace declaration ==

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* As attribute nodes named "xmlns" or "xmlns:xxx", exactly as the namespaces are written in the source XML document. This is the model presented by [[Document Object Model|DOM]].

* As namespace declarations: distinguished from attributes, but corresponding one-to-one with the relevant attributes in the source XML document. This is the model presented by [[JDOM]].

* As in-scope namespace bindings: in this model, the application is able to determine which namespaces are in scope for any given element, but is not able to determine which elements contain the actual declarations. This is the model used in [[XPath]], [[XSLT]], and [[XQuery]].

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{{W3C standards}}

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