Em adição a este documento, consulte Namespaces Crash Course.
Espaços de nomes XML fornecem uma maneira para distinguir nomes duplicados de elementos e atributos. Os nomes de elementos e atributos duplicados podem ocorrer quando um documento XML contém elementos e atributos de dois ou mais esquemas XML diferentes (ou DTDs). para citar Wikipédia: "Em geral, um espaço de nome é um recipiente abstrato que fornece contexto para os itens... este contém e permite a desambiguação de itens com o mesmo nome."
If you are familiar with C++ namespaces, Java packages, perl packages, or Python module importing, you are already familiar with the namespace concept.
An XML namespace is identified by an unique name (called a URI, not a URL, even though it can look like a URL). An URI is any string, although most people choose a URL-based URI because URLs are an easy way to hope for uniqueness. Although there's nothing preventing someone else from using the namespace http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul, it's fairly unlikely anyone would choose that accidentally. Even if they did accidentally choose it, they may not define the same elements as XUL anyway (e.g., <textbox/>) in their schema/DTD.
Any element type or attribute name in an XML namespace can be uniquely identified by its XML namespace and its "local name". Together, these two items define a qualified name, or QName.
For example, <xul:textbox/> uses a namespace named "xul" and a local name "textbox". This distinguishes it from, for example, <foobar:textbox/> which might occur in the same document. The xul and foobar namespaces must be defined at the top of the XML document in which they are used, like so:
<foobar:some-element
xmlns:xul="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
xmlns:foobar="the-foobar-namespace">
<xul:textbox id="foo" value="bar"/>
<foobar:textbox favorite-food="pancakes"/>
</foobar:some-element>
Notice I've mixed two <textboxes/> in the same document. The only way to distinguish that they have different meanings is with namespaces.
There's only one other thing to know: "default namespace". Every XML element has a "default namespace", and this is used with XUL elements all the time. In XUL documents, you'll usually see this:
<window
id="foo"
xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
...
...
</window>
and in XHTML documents, you'll see this:
<html xmlns="http://www.w3.org/1999/xhtml"> ... ... </html>
There is a very subtle difference here than before. Before I wrote xmlns:xul="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul" but here the :xul piece is omitted. This signifies to the XML parser that http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul is the default namespace for the element and its descendant elements (unless further overridden by a default namespace on a descendant element), and that any element without a namespace (i.e., no prefix and colon) belongs to the default namespace. That's why we can write the shorthand <textbox/> instead of <xul:textbox/> in XUL (although the latter is just as correct when not using http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul as the default namespace) -- the XUL namespace is defined as the default on the topmost element. In other words, a default namespace permits a kind of short-hand to be used for all descendants of an element.
Here's a question: what namespace contains the element foo in the XML document below?
<foo/>
The answer is that it's in no namespace, or alternately, it's in the namespace denoted by the empty string:
<foo xmlns=""/>
This second example is semantically equivalent to the first.
Now, a second question: what namespaces are the bar, baz, and quux attributes in?
<foo bar="value">
<element xmlns="namespace!" baz="value">
<element quux="value"/>
</element>
</foo>
bar is obviously not in a namespace. What about baz and quux? The answer is that they aren't in a namespace either. In fact no unprefixed attribute is ever in a namespace, primarily because XML originally didn't have namespaces, and all XML from that time had to stay in no namespace. This is a source of perennial confusion for XML namespaces.