XML and Namespaces
This article explains namespaces in XML: when to use namespaces, and how to declare a namespace.
When you see a piece of XML like this:
<foo:bar xmlns:foo="http://foo.example.com/2008/foo"> <foo:baz mumble="1"/> </foo:bar>
it is using a namespace.
XML 1.0 itself doesn’t define a namespace mechanism, although it does already reserve the colon for future namespace use. It also reserves names starting with the letters X, M and L in any case for official use in future specifications. The minimal conformance requirement for an XML document is that it is well-formed. The other side of the coin is that anything that’s not well-formed is not an XML document, whatever the sender may tell you.
Namespaces in XML became a W3C recommendation in 1999, a year after XML 1.0. It is an optional addition to XML.
The purpose of namespaces is to separate different uses of the same
names. For example, you may have a list of people with element names
<address>, and I can have a list of computers with
element names like
<address> (for IP-addresses). To
unambiguously point to my meaning of
<address> I can introduce a
Even if the names don’t overlap exactly, namespaces serve to separate different vocabularies.
An XML namespace is a URI
reference (I’ll use URI for short) and you can make them up any way you
want. However, because we wanted to separate my element’s names from
yours, it makes sense to use URIs that point to your own organisation. A
first stab at my namespace URI could be
but notice that it contains very little information about what it
represents. It is also hard to find out what it means, except perhaps by
sending a mail to the address. The same problem exists for URIs with the
urn: schemes. As a best practice, use
http: URLs and try
to maintain some documentation at that address. So, we’ll go for
URI references can get pretty long, so if every element and attribute in a namespace would suddenly be preceded by the URI, then the size of your XML documents would explode. That is why the elements themselves use something shorter called a namespace prefix, which is declared using a namespace declaration. The idea is that URI references ensure global uniqueness, while prefixes are enough to ensure local uniqueness within a document, or even a part of a document. Here is an example of a namespace declaration:
Notice that it looks like it is just an attribute with the funny name
xmlns:foo, but since it starts with the letters xml and it contains a
colon, it can’t be a normal attribute. What it does is make
http://foo.example.com/2008/foo inside the current
element and its children. This shorthand can be used on element names,
<foo:bar/> and on attribute names, like so
bar element and the
baz attribute are said to be in the
http://foo.example.com/2008/foo namespace, but you’ll mostly
abbreviate that to just the foo namespace.
This is part one in a series of articles about XML for programmers. In the next parts, we will look at the available tools for working with XML and namespaces.