Using Namespaces

Java uses packages to avoid name clashes. Programmers can use the same name for different classes as long as they aren't in the same package. XML has a similar namespace mechanism for element and attribute names.

A namespace is identified by a Uniform Resource Identifier (URI), such as

 http://www.w3.org/2001/XMLSchema uuid:1c759aed-b748-475c-ab68-10679700c4f2 urn:com:books-r-us 

The HTTP URL form is the most common. Note that the URL is just used as an identifier string, not as a locator for a document. For example, the namespace identifiers

 http://www.horstmann.com/corejava http://www.horstmann.com/corejava/index.html 

denote different namespaces, even though a web server would serve the same document for both URLs.

There need not be any document at a namespace URLthe XML parser doesn't attempt to find anything at that location. However, as a help to programmers who encounter a possibly unfamiliar namespace, it is customary to place a document explaining the purpose of the namespace at the URL location. For example, if you point your browser to the namespace URL for the XML Schema namespace (http://www.w3.org/2001/XMLSchema), you will find a document describing the XML Schema standard.

Why use HTTP URLs for namespace identifiers? It is easy to ensure that they are unique. If you choose a real URL, then the host part's uniqueness is guaranteed by the domain name system. Your organization can then arrange for the uniqueness of the remainder of the URL. This is the same rationale that underlies the use of reversed domain names in Java package names.

Of course, although you want long namespace identifiers for uniqueness, you don't want to deal with long identifiers any more than you have to. In the Java programming language, you use the import mechanism to specify the long names of packages, and then use just the short class names. In XML, there is a similar mechanism, like this:

<element xmlns="namespaceURI">
   children
</element>

The element and its children are now part of the given namespace.

A child can provide its own namespace, for example:

 <element xmlns="namespaceURI₁">
   <child xmlns="namespaceURI₂">
      grandchildren
   </child>
   more children
</element>

Then the first child and the grandchildren are part of the second namespace.

That simple mechanism works well if you need only a single namespace or if the namespaces are naturally nested. Otherwise, you will want to use a second mechanism that has no analog in Java. You can have an alias for a namespacea short identifier that you choose for a particular document. Here is a typical example:

 <xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">    <xsd:element name="gridbag" type="GridBagType"/>    . . . </xsd:schema> 

The attribute

xmlns:alias="namespaceURI"

defines a namespace and an alias. In our example, the alias is the string xsd. Thus, xsd:schema really means "schema in the namespace http://www.w3.org/2001/XMLSchema".

NOTE

Only child elements inherit the namespace of their parent. Attributes without an explicit alias prefix are never part of a namespace. Consider this contrived example:

 <configuration xmlns="http://www.horstmann.com/corejava"    xmlns:si="http://www.bipm.fr/enus/3_SI/si.html">    <size value="210" si:unit="mm"/>    . . . </configuration> 

The elements configuration and size are part of the namespace with URI http://www.horstmann.com/corejava. The attribute si:unit is part of the namespace with URI http://www.bipm.fr/enus/3_SI/si.html. However, the attribute value is not part of any namespace.

You can control how the parser deals with namespaces. By default, the Sun DOM parser is not "namespace aware."

To turn on namespace handling, call the setNamespaceAware method of the DocumentBuilderFactory:

 factory.setNamespaceAware(true); 

Then all builders the factory produces support namespaces. Each node has three properties:

• The qualified name, with an alias prefix, returned by getNodeName, getTagName, and so on

• The local name, without a prefix or a namespace, returned by the getLocalName method

• The namespace URI, returned by the getNamespaceURI method

Here is an example. Suppose the parser sees the following element:

 <xsd:schema xmlns:xsl="http://www.w3.org/2001/XMLSchema"> 

It then reports the following:

• Qualified name = xsd:schema

• Local name = schema

• Namespace URI = http://www.w3.org/2001/XMLSchema

NOTE

If namespace awareness is turned off, then getLocalName and getNamespaceURI return null.

 org.w3c.dom.Node 1.4 

• String getLocalName()

  returns the local name (without alias prefix), or null if the parser is not namespace aware.

• String getNamespaceURI()

  returns the namespace URI, or null if the node is not part of a namespace or if the parser is not namespace aware.

 javax.xml.parsers.DocumentBuilderFactory 1.4 

• boolean isNamespaceAware()

• void setNamespaceAware(boolean value)

  are the "namespaceAware" property of the factory. If set to true, the parsers that this factory generates are namespace aware.