XML namespaces are used for providing uniquely named
elements and attributes in an
XML
Extensible Markup Language (XML) is a markup language and file format for storing, transmitting, and reconstructing arbitrary data. It defines a set of rules for encoding documents in a format that is both human-readable and machine-readable. T ...
document. They are defined in a
W3C recommendation
The World Wide Web Consortium (W3C) is the main international standards organization for the World Wide Web. Founded in 1994 and led by Tim Berners-Lee, the consortium is made up of member organizations that maintain full-time staff working to ...
.
An XML instance may contain element or attribute names from more than one XML vocabulary. If each vocabulary is given a
namespace
In computing, a namespace is a set of signs (''names'') that are used to identify and refer to objects of various kinds. A namespace ensures that all of a given set of objects have unique names so that they can be easily identified.
Namespaces ...
, 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.
Namespace names
A ''namespace name'' is a
uniform resource identifier
A Uniform Resource Identifier (URI) is a unique sequence of characters that identifies a logical or physical resource used by web technologies. URIs may be used to identify anything, including real-world objects, such as people and places, conc ...
(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
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
Extensible HyperText Markup Language (XHTML) is part of the family of XML markup languages. It mirrors or extends versions of the widely used HyperText Markup Language (HTML), the language in which Web pages are formulated.
While HTML, prior ...
namespace to human readers. Using a URI (such as
"http://www.w3.org/1999/xhtml") 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
A Uniform Resource Identifier (URI) is a unique sequence of characters that identifies a logical or physical resource used by web technologies. URIs may be used to identify anything, including real-world objects, such as people and places, conc ...
), 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. The result of the debate was a ruling from W3C that relative URIs were deprecated.
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 In computing, Resource Directory Description Language (RDDL) is an extension of XHTML Basic 1.0. An RDDL document, called a Resource Directory, provides a package of information about some target. The targets which RDDL was designed to describe are ...
document at the location. 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
An XML namespace is declared using the reserved XML attribute
xmlns
or
xmlns:''prefix''
, the value of which must be a valid
namespace name.
For example, the following declaration maps the "xhtml:" prefix to the XHTML namespace:
xmlns:xhtml="http://www.w3.org/1999/xhtml"
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.
It is also possible to declare a default namespace. For example:
xmlns="http://www.w3.org/1999/xhtml"
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.
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.
Nearly all programming models allow the name of an element or attribute node to be retrieved as a three-part name: the local name, the namespace prefix, and the namespace URI. Applications should avoid attaching any significance to the choice of prefix, but the information is provided because it can be helpful to human readers. Names are considered equal if the namespace URI and local name match.
In addition, most models provide some way of determining which namespaces have been declared for a given element. This information is needed because some XML vocabularies allow qualified names (containing namespace prefixes) to appear in the content of elements or attributes, as well as in their names. There are three main ways this information can be provided:
* 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
DOM Dom or DOM may refer to:
People and fictional characters
* Dom (given name), including fictional characters
* Dom (surname)
* Dom La Nena (born 1989), stage name of Brazilian-born cellist, singer and songwriter Dominique Pinto
* Dom people, an et ...
.
* 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
JDOM is an open-source Java-based document object model for XML that was designed specifically for the Java platform so that it can take advantage of its language features. JDOM integrates with Document Object Model (DOM) and Simple API for X ...
.
* 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
XPath (XML Path Language) is an expression language designed to support the query or transformation of XML documents. It was defined by the World Wide Web Consortium (W3C) and can be used to compute values (e.g., strings, numbers, or Boolean v ...
,
XSLT
XSLT (Extensible Stylesheet Language Transformations) is a language originally designed for transforming XML documents into other XML documents, or other formats such as HTML for web pages, plain text or XSL Formatting Objects, which may subseque ...
, and
XQuery
XQuery (XML Query) is a query and functional programming language that queries and transforms collections of structured and unstructured data, usually in the form of XML, text and with vendor-specific extensions for other data formats (JSON, bin ...
.
See also
*
Namespace
In computing, a namespace is a set of signs (''names'') that are used to identify and refer to objects of various kinds. A namespace ensures that all of a given set of objects have unique names so that they can be easily identified.
Namespaces ...
References
External links
Namespaces in XML 1.0 (Third Edition)Namespaces in XML 1.1 (Second Edition)Namespaces Crash Course (Mozilla Developer Network)
{{DEFAULTSORT:Xml Namespace
XML
ja:Extensible Markup Language#XML名前空間