XML Namespace and QName Explained

The XML Namespaces specification defines a way to group element and attribute names so that schemas created by one organization will not conflict with those created by another. Just as two Java classes can have the same name as long as they are defined in separate packages, two XML elements can have the same name as long as they belong to different namespaces.

Each namespace defined in an XML document must be associated with a distinct uniform resource identifier (URI), which is usually a URL. These URIs have no semantic meaning and do not refer to actual web resources. You should define namespace URIs using domains that you control to prevent naming conflicts for the same reason that you should follow the URL naming convention for Java packages.

Two URIs are considered distinct if they are distinct character strings, regardless of whether they would resolve to the same physical resource (i.e. http://localhost and http://george are distinct URIs in the context of XML namespaces even on the host george).

Namespaces are associated with a prefix when they are declared and this prefix is used along with a local name to represent an element in an XML document. A namespace declaration looks like this:

...

The namespace http://url1 is bound to the prefix "a" and the namespace http://url2 is bound to the prefix "b" in this example. Three child elements of : , , and , would have no namespace, a namespace of "http://url1", and a namespace of "http://url2", respectively, and all would have the local name "child".

Namespaces have a scope associated with them. A namespace declared in a parent element is bound to a given prefix for that element as well as for all of its child elements, unless that prefix is "overridden" in a child element by being assigned to a different namespace. The association between the namespace and prefix declared in an element do not apply to the siblings of that element. This is equivalent to the scope of variable names within the Java programming language.

A default namespace can be defined by omitting the prefix mapping in the declaration as in "xmlns='http://url3'". At most one default namespace is in effect at any given point in an XML document. The default namespace is scoped just as the prefix mappings are. If a default namespace is in scope and an element appears with no prefix then it is associated with that default namespace.

Attribute names never inherit the default namespace and must be explicitly mapped to a namespace.

The "qName", or qualified name, argument contains the element name exactly as it appears in the XML document, including the prefix and colon, if appropriate