Namespaces distinguish between elements with different meanings but the same name by assigning each element a URI. Generally, all the elements from one XML application are assigned to one URI, and all the elements from a different XML application are assigned to a different URI. These URIs are called namespace names . Elements with the same name but different URIs are different kinds of elements. Elements with the same name and the same URI are the same kind of element. Most of the time a single XML application has a single namespace URI for all its elements, though a few applications use multiple namespaces to subdivide different parts of the application. For instance, XSL uses different namespaces for XSL Transformations (XSLT) and XSL Formatting Objects (XSL-FO).
Since URIs frequently contain characters such as /, %,
and ~ that are not legal in XML
names, short prefixes such as rdf and
xsl stand in for them in element
and attribute names. Each prefix is associated with a URI. Names whose prefixes
are associated with the same URI are in the same namespace. Names
whose prefixes are associated with different URIs are in different
namespaces. Prefixed elements and attributes in namespaces have
names that contain exactly one colon. They look like this:
rdf:description xlink:type xsl:template
Everything before the colon is called the prefix. Everything after the colon is called the local part. The complete name, including the colon, is called the qualified name , QName, or raw name. The prefix identifies the namespace to which the element or attribute belongs. The local part identifies the particular element or attribute within the namespace.
In a document that contains both SVG and MathML set
elements, one could be an svg:set
element, and the other could be a mathml:set element. Then there'd be no
confusion between them. In an XSLT stylesheet that transforms
documents into XSL formatting objects, the XSLT processor would
recognize elements with the prefix xsl as XSLT instructions and elements with
the prefix fo as literal result
elements.
Prefixes may be composed from any legal XML name character
except the colon. The three-letter prefix xml used for standard XML attributes such
as xml:space, xml:lang, and xml:base is always bound to the URI
http://www.w3.org/XML/1998/namespace and
need not be explicitly declared. Other prefixes beginning with the
three letters xml (in any
combination of case) are reserved for use by XML and its related
specifications. Otherwise, you're free to name your prefixes in any
way that's convenient. One further restriction namespaces add to XML
is that the local part may not contain any colons. In short, the only legal use of a colon in XML
is to separate a namespace prefix from the local part in a qualified
name.
Each prefix in a qualified name must be associated with a URI. For
example, all XSLT elements are associated with the http://www.w3.org/1999/XSL/Transform URI.
The customary prefix xsl is used
in place of the longer URI http://www.w3.org/1999/XSL/Transform.
You can't use the URI in the name directly. For one
thing, the slashes in most URIs aren't legal characters in XML
names. However, it's occasionally useful to refer to the full name
without assuming a particular prefix. One convention used on many
XML mailing lists and in XML documentation is to enclose the URI
in curly braces and prefix it to the name. For example, the
qualified name xsl:template
might be written as the full name {http://www.w3.org/1999/XSL/Transform}template.
Another convention is to append the local name to the namespace
name after a sharp sign so that it becomes a URI fragment
identifier. For example, http://www.w3.org/1999/XSL/Transform#template.
However, both forms are only conveniences for communication among
human beings when the URI is important but the prefix isn't.
Neither an XML parser nor an XSLT processor will accept or
understand the long forms.
Prefixes are bound to namespace URIs by attaching an xmlns:prefix
attribute to the prefixed element or one of its ancestors. (The
prefix should be replaced by the actual
prefix used.) For example, the xmlns:rdf attribute of this rdf:RDF element binds the prefix rdf to the namespace URI http://www.w3.org/TR/REC-rdf-syntax#:
<rdf:RDF xmlns:rdf="http://www.w3.org/TR/REC-rdf-syntax#">
<rdf:Description
about="http://www.cafeconleche.org/examples/impressionists.xml">
<title> Impressionist Paintings </title>
<creator> Elliotte Rusty Harold </creator>
<description>
A list of famous impressionist paintings organized
by painter and date
</description>
<date>2000-08-22</date>
</rdf:Description>
</rdf:RDF>Bindings have scope within the element where they're declared
and within its contents. The xmlns:rdf attribute declares the rdf prefix for the rdf:RDF element, as well as its descendant
elements. An RDF processor will recognize rdf:RDF and rdf:Description as RDF elements because
both have prefixes bound to the particular URI specified by the RDF
specification. It will not consider the title, creator, description, and date elements to be RDF elements because
they do not have prefixes bound to the http://www.w3.org/TR/REC-rdf-syntax#
URI.
The prefix can be declared in the topmost element that uses
the prefix or in any ancestor thereof. This may be the root element
of the document, or it may be an element at a lower level. For
instance, the Dublin Core elements could be attached to the http://purl.org/dc/ namespace by adding an
xmlns:dc attribute to the
rdf:Description element, as shown
in Example 4-3, since all
Dublin Core elements in this document appear inside a single
rdf:Description element. In other
documents that spread the elements out more, it might be more
convenient to put the namespace declaration on the root element. If
necessary, a single element can include multiple namespace
declarations for different prefixes.
Example 4-3. RDF and Dublin Core metadata identified by namespaces
<?xml version="1.0" encoding="ISO-8859-1" standalone="yes"?>
<catalog>
<rdf:RDF xmlns:rdf="http://www.w3.org/TR/REC-rdf-syntax#">
<rdf:Description xmlns:dc="http://purl.org/dc/"
about="http://www.cafeconleche.org/examples/impressionists.xml">
<dc:title> Impressionist Paintings </dc:title>
<dc:creator> Elliotte Rusty Harold </dc:creator>
<dc:description>
A list of famous impressionist paintings organized
by painter and date
</dc:description>
<dc:date>2000-08-22</dc:date>
</rdf:Description>
</rdf:RDF>
<painting>
<title>Memory of the Garden at Etten</title>
<artist>Vincent Van Gogh</artist>
<date>November, 1888</date>
<description>
Two women look to the left. A third works in her garden.
</description>
</painting>
<painting>
<title>The Swing</title>
<artist>Pierre-Auguste Renoir</artist>
<date>1876</date>
<description>
A young girl on a swing. Two men and a toddler watch.
</description>
</painting>
<!-- Many more paintings... -->
</catalog>A DTD for this document can include different content
specifications for the dc:description and description elements. A stylesheet can
attach different styles to dc:title and title. Software that sorts the catalog by
date can pay attention to the date elements and ignore the dc:date elements.
In this example, the elements without prefixes, such as catalog, painting, description, artist, and title, are not in any namespace.
Furthermore, unprefixed attributes (such as the about attribute of rdf:Description in the previous example)
are never in any namespace. Being an attribute of an element in the
http://www.w3.org/TR/REC-rdf-syntax#
namespace is not sufficient to put the attribute in the http://www.w3.org/TR/REC-rdf-syntax#
namespace. The only way an attribute belongs to a namespace is if it
has a declared prefix, like rdf:about.
In XML 1.1 there's one exception to the rule that
unprefixed attributes are never in a namespace. In XML
1.1/Namespaces 1.1, the xmlns
attribute is defined to be in the namespace http://www.w3.org/2000/xmlns/. In XML
1.0/Namespaces 1.0, the xmlns
attribute is not in any namespace.
It is possible to redefine a prefix within a document so that in one element the prefix refers to one namespace URI, while in another element it refers to a different namespace URI. In this case, the closest ancestor element that declares the prefix takes precedence. However, in most cases, redefining prefixes is a very bad idea that only leads to confusion and is not something you should actually do.
Many XML applications have customary prefixes. For example, SVG
elements often use the prefix svg, and RDF elements often have the
prefix rdf. However, these
prefixes are simply conventions and can be changed based on
necessity, convenience, or whim. Before a prefix can be used, it
must be bound to a URI like http://www.w3.org/2000/svg or http://www.w3.org/1999/02/22-rdf-syntax-ns#.
It is these URIs that are standardized, not the prefixes. The prefix
can change as long as the URI stays the same. An RDF processor looks
for the RDF URI, not any particular prefix. As long as nobody
outside the w3.org domain uses namespace URIs
in the w3.org domain, and as long as the W3C
keeps a careful eye on what its people are using for namespaces, all
conflicts can be avoided.
Namespace URIs do not necessarily point to any actual document
or page. In fact, they don't have to use the http scheme. They might even use some
other protocol like mailto in
which URIs don't even point to documents. However, if you're
defining your own namespace using an http URI, it would not be a bad idea to
place some documentation for the XML application at the namespace
URI. The W3C got tired of receiving broken-link reports for the
namespace URIs in their specifications, so they added some simple
pages at their namespace URIs. For more formal purposes that offer
some hope of automated resolution and other features, you can place
a Resource Directory Description Language (RDDL)
document at the namespace URI. This possibility will be discussed
further in Chapter 15. You are
by no means required to do this, though. Many namespace URIs lead to
404-Not Found errors when you actually plug them into a web browser.
Namespace URIs are purely formal identifiers. They are not the
addresses of a page, and they are not meant to be followed as
links.
Parsers compare namespace URIs on a character-by-character basis. If the URIs differ in even a single normally insignificant place, then they define separate namespaces. For instance, the following URLs all point to the same page:
| http://www.w3.org/1999/02/22-rdf-syntax-ns# |
| http://WWW.W3.ORG/1999/02/22-rdf-syntax-ns# |
| http://www.w3.org/1999/02/22-rdf-syntax-ns/ |
| http://www.w3.org/1999/02/22-rdf-syntax-ns/index.rdf |
However, only the first is the correct namespace name for the RDF. These four URLs identify four separate namespaces.
You often know that all the content of a particular
element will come from a particular XML application. For instance,
inside an SVG svg element, you're
only likely to find other SVG elements. You can indicate that an
unprefixed element and all its unprefixed descendant
elements belong to a particular namespace by attaching an xmlns attribute with no prefix to the top
element. For example:
<svg xmlns="http://www.w3.org/2000/svg"
width="12cm" height="10cm">
<ellipse rx="110" ry="130" />
<rect x="4cm" y="1cm" width="3cm" height="6cm" />
</svg>Here, although no elements have any prefixes, the svg, ellipse, and rect elements are in the http://www.w3.org/2000/svg
namespace.
The attributes are a different story. Default namespaces only
apply to elements, not to attributes. Thus, in the previous example,
the width, height, rx, ry,
x, and y attributes are not in any
namespace.
You can change the default namespace within a particular
element by adding an xmlns
attribute to the element. Example
4-4 is an XML document that initially sets the default
namespace to http://www.w3.org/1999/xhtml for all the
XHTML elements. This namespace declaration applies within most of
the document. However, the svg
element has an xmlns attribute
that resets the default namespace to http://www.w3.org/2000/svg for itself and
its content. The XLink information is included in attributes,
however, so these must be placed in the XLink namespace using
explicit prefixes.
Example 4-4. An XML document that uses default namespaces
<?xml version="1.0"?>
<html xmlns="http://www.w3.org/1999/xhtml"
xmlns:xlink="http://www.w3.org/1999/xlink">
<head><title>Three Namespaces</title></head>
<body>
<h1 align="center">An Ellipse and a Rectangle</h1>
<svg xmlns="http://www.w3.org/2000/svg"
width="12cm" height="10cm">
<ellipse rx="110" ry="130" />
<rect x="4cm" y="1cm" width="3cm" height="6cm" />
</svg>
<p xlink:type="simple" xlink:href="ellipses.html">
More about ellipses
</p>
<p xlink:type="simple" xlink:href="rectangles.html">
More about rectangles
</p>
<hr/>
<p>Last Modified May 13, 2000</p>
</body>
</html>The default namespace does not apply to any elements or attributes with prefixes. These still belong to whatever namespace their prefix is bound to. However, an unprefixed child element of a prefixed element still belongs to the default namespace.