Namespaces support in W3C XML Schema is flexible yet straightforward. It not only allows the use of any prefix in instance documents (unlike DTDs) but also lets you open your schemas to accept unknown elements and attributes from known or unknown namespaces.
Each W3C XML Schema document is bound to a specific namespace through the
targetNamespace attribute, or to the absence of namespace through the lack of such an attribute. We need at least one schema document per namespace we want to define (elements and attributes without namespaces can be defined in any schema, though).
Until now we have omitted the
targetNamespac attribute, which means that we were working without namespaces. To get into namespaces, let's first imagine that our example belongs to a single namespace:
<book isbn="0836217462" xmlns="http://example.org/ns/books/"> .../... </book>
The least intrusive way to adapt our schema is to add some more attributes to our
<xs:schema targetNamespace="http://example.org/ns/books/" xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:bk="http://example.org/ns/books/" elementFormDefault="qualified" attributeFormDefault="unqualified"> .../... </xs:schema>
The namespace declarations play an important role. The first one (
says not only that we've chosen to use the prefix
xs to identify the elements that will be
W3C XML Schema instructions, but also that we will prefix the W3C XML Schema predefined datatypes
xs as we have done all over the examples thus far. Understand that we
could have chosen any prefix instead of
xs. We could even make http://www.w3.org/2001/XMLSchema
our default namespace and in this case, we wouldn't have prefixed the W3C XML Schema elements nor its
Since we are working with the http://example.org/ns/books/
namespace, we define it (with a
bk prefix). This means that we
will now prefix the references to "objects" (datatypes, elements, attributes,
...) belonging to this namespace with
bk:. Again, we could have
chosen any prefix to identify this namespace or even have made it our default
namespaces (note that the XPath expressions used in
xs:keyref do not use a default namespace,
targetNamespace attribute lets you define, independently of the namespace declarations, which namespace
is described in this schema. If you need to reference objects belonging to this namespace, which
is usually the case except when using a pure "Russian doll" design, you need to provide a
namespace declaration in addition to the
The final two attributes (
attributeFormDefault) are a
facility provided by W3C XML Schema to control, within a single schema, whether attributes and elements are considered by default to be qualified (in a namespace). This differentiation between qualified and unqualified can be indicated by specifying the default values, as above, but also when defining the elements and attributes, by adding a
attribute of value
It is important to note that only local elements and attributes can be specified as unqualified. All globally defined elements and attributes must always be qualified.
Importing definitions from external namespaces
W3C XML Schema, not unlike XSLT and XPath, uses namespace prefixes within the value of some attributes to identify the namespace of data types, elements, attributes, atc. For instance, we've used this feature all along our examples to identify the W3C XML Schema predefined datatypes. This mechanism can be extended to import definitions from any other namespace and so reuse them in our schemas.
Reusing definitions from other namespaces is done through a three-step process. This process needs to be done even for
the XML 1.0 namespace, in order to declare attributes such as
xml:lang. First, the namespace must be defined as usual.
<xs:schema targetNamespace="http://example.org/ns/books/" xmlns:xml="http://www.w3.org/XML/1998/namespace" xmlns:bk="http://example.org/ns/books/" xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified" attributeFormDefault="qualified"> .../... </xs:schema>
Then W3C XML Schema needs to be informed of the location at which it can find the schema
corresponding to the namespace. This is done using an
<xs:import namespace="http://www.w3.org/XML/1998/namespace" schemaLocation="myxml.xsd"/>
W3C XML Schema now knows that it should attempt to find any reference belonging to the XML namespace in a schema located at myxml.xsd. We can now use the external definition.
<xs:element name="title"> <xs:complexType> <xs:simpleContent> <xs:extension base="xs:string"> <xs:attribute ref="xml:lang"/> </xs:extension> </xs:simpleContent> </xs:complexType> </xs:element>
You may wonder why we have chosen to reference the
xml:lang attribute from the XML namespace,
rather than creating an attribute with a type
xml:lang. We've done so because there is an important difference between referencing an attribute (or an element) and
referencing a datatype when namespaces are concerned:
- Referencing an element or an attribute imports the whole thing with its name and namespace,
- Referencing a datatype imports only its definition, leaving you with the task of giving a name to the element and attribute you're defining and using the target namespace (or no namespace if your attribute or element is unqualified).
Including unknown elements
To finish this section about namespaces, we need to see how, as promised in
our introduction, we can open our schema to unknown elements, attributes and
namespaces. This is done using
xs:anyAttribute, allowing, respectivly, to include any elements or
For instance, if we want to extend the definition of our description type to any XHTML tag, we could declare:
<xs:complexType name="descType" mixed="true"> <xs:sequence> <xs:any namespace="http://www.w3.org/1999/xhtml" processContents="skip" minOccurs="0" maxOccurs="unbounded"/> </xs:sequence> </xs:complexType>
xs:anyAttribute gives the same functionality for attributes.
descType is now mixed content and accepts an unbounded number of any element from
the http://www.w3.org/1999/xhtml namespace. The
processContents attribute is set to
telling a W3C XML Schema processor that no validation of these elements should be attempted. The other permissible
values could are
strict asking to validate these elements or
lax asking to
validate them when possible. The
namespace attribute accepts a whitespace-separated list of URIs and the
##local (non qualified elements) and
##targetNamespace (the target namespace) that can be included in the list and
##other (any namespace other than the target) or
##any (any namespace) that can replace the list. It is not possible to specify any namespace except those from a list.