Namespaces

Remarks#

Element and attribute names in XML are called QNames (qualified names).

A QName is made of:

a namespace (a URI)
a prefix (an NCName, NC because it contains no colon)
a local name (an NCName)

Only the namespace and the local name are relevant for comparing two QNames. The prefix is only a proxy to the namespace.

The namespace and prefix are optional, but the namespace is always present if the prefix is present (this is ensured at the syntactic level, so this cannot be done wrong).

The lexical representation of a QName is prefix:local-name. The namespace is bound separately using the special xmlns:... attributes (reminder: attributes beginning with xml are reserved in XML).

If the prefix is empty, no colon is used in the lexical representation of the QName, which only contains the local-name. QNames with an empty prefix either have no namespace (if no default namespace is in scope) or are in the default namespace.

Bind a prefix to a namespace

A namespace is a URI, but to avoid verbosity, prefixes are used as a proxy.

In the following example, the prefix my-prefix is bound to the namespace https://www.example.com/my-namespace by using the special attribute xmlns:my-prefix (my-prefix can be replaced with any other prefix):

<?xml version="1.0"?>
<my-prefix:foo xmlns:my-prefix="https://www.example.com/my-namespace">
  <!-- the element my-prefix:foo
       lives in the namespace https://www.example.com/my-namespace -->
</my-prefix:foo>

Absence of namespace

In XML, element and attribute names live in namespaces.

By default, they are in no namespace:

<?xml version="1.0"?>
<foo attr="value">
  <!-- the foo element is in no namespace, neither is the attr attribute -->
</foo>

Irrelevance of prefixes

These two documents are semantically equivalement, as namespaces matter, not prefixes.

<?xml version="1.0"?>
<myns:foo xmlns:myns="https://www.example.com/my-namespace">
</myns:foo>

<?xml version="1.0"?>
<ns:foo xmlns:ns="https://www.example.com/my-namespace">
</ns:foo>

Default namespace

The default namespace is the namespace corresponding to the absence of any prefix. It can be declared with the special xmlns attribute.

<?xml version="1.0"?>
<foo xmlns="https://www.example.com/my-namespace">
  <!-- the element foo is in the namespace
       https://www.example.com/my-namespace -->
</foo>

If no default namespace is declared, then names with no prefix are in no namespace.

Attribute names with no prefix

Elements and attributes behave differently with respect to default namespaces. This is often the source of confusion.

An attribute whose name has no prefix lives in no namespace, also when a default namespace is in scope.

<?xml version="1.0"?>
<foo attr="value" xmlns="https://www.example.com/my-namespace">
  <!-- The attribute attr is in no namespace, even though
       a default namespace is in scope. The element foo,
       however, is in the default namespace. -->
</foo>

Scope of namespace bindings

A namespace binding (special xmlns or xmlns:... attribute) is in scope for all the descendants of the enclosing element, including this element.

<?xml version="1.0"?>
<root>
  <my:element xmlns:my="https://www.example.com/ns1">
    <!-- here, the prefix my is bound to https://www.example.com/ns1 -->
  </my:element>
  <my:element xmlns:my="https://www.example.com/ns2">
    <!-- here, the prefix my is bound to https://www.example.com/ns2 -->
  </my:element>
</root>

The binding can be overriden in a nested element (this affects readability though):

<?xml version="1.0"?>
<my:element xmlns:my="https://www.example.com/ns1">
  <!-- here, the prefix my is bound to https://www.example.com/ns1 -->
  <my:first-child-element/>

  <my:child-element xmlns:my="https://www.example.com/ns2">
    <!-- here, the prefix my is bound to https://www.example.com/ns2,
         including for the element my:child-element -->
  </my:child-element>

  <!-- here, the prefix my is bound to https://www.example.com/ns1 -->
  <my:last-child-element/>

</my:element>

It is very common to declare all namespace bindings in the root element, which improves readability.

<?xml version="1.0"?>
<root
  xmlns="https://www.example.com/default-namespace"
  xmlns:ns1="https://www.example.com/ns1"
  xmlns:ns2="https://www.example.com/ns2">
  
  <ns1:element>
    <ns2:other-element/>
  </ns1:element>

</root>