All releases of Ant prior to Ant 1.6 do not support XML namespaces. No support basically implies two things here:
<prefix>'
are not treated specially, which means that custom tasks and types have
actually been able to use such attributes as parameter names. Again,
such tasks/types are going to break when namespace support is enabled
on the parser.
Use of colons in element names has been discouraged in the past, and using any attribute starting with "xml" is actually strongly discouraged by the XML spec to reserve such names for future use.
In build files using a lot of custom and third-party tasks, it is
easy to get into name conflicts. When individual types are defined, the
build file writer can do some namespacing manually (for example, using
"tomcat-deploy" instead of just "deploy"). But when defining whole
libraries of types using the <typedef> 'resource' attribute, the
build file writer has no chance to override or even prefix the names
supplied by the library.
Adding a 'prefix' attribute to <typedef> might have been enough,
but XML already has a well-known method for namespacing. Thus, instead
of adding a 'prefix' attribute, the <typedef> and <taskdef>
tasks get a 'uri' attribute, which stores the URI of the XML namespace
with which the type should be associated:
<typedef resource="org/example/tasks.properties" uri="http://example.org/tasks"/>
<my:task xmlns:my="http://example.org/tasks">
...
</my:task>
As the above example demonstrates, the namespace URI needs to be specified at least twice: one time as the value of the 'uri' attribute, and another time to actually map the namespace to occurrences of elements from that namespace, by using the 'xmlns' attribute. This mapping can happen at any level in the build file:
<project name="test" xmlns:my="http://example.org/tasks">
<typedef resource="org/example/tasks.properties" uri="http://example.org/tasks"/>
<my:task>
...
</my:task>
</project>
Use of a namespace prefix is of course optional. Therefore the example could also look like this:
<project name="test">
<typedef resource="org/example/tasks.properties" uri="http://example.org/tasks"/>
<task xmlns="http://example.org/tasks">
...
</task>
</project>
Here, the namespace is set as the default namespace for the <task>
element and all its descendants.
The default namespace used by Ant is "antlib:org.apache.tools.ant".
<typedef resource="org/example/tasks.properties" uri="antlib:org.apache.tools.ant"/>
<task>
....
</task>
Almost always in Ant 1.6, elements nested inside a namespaced element have the same namespace as their parent. So if 'task' in the example above allowed a nested 'config' element, the build file snippet would look like this:
<typedef resource="org/example/tasks.properties" uri="http://example.org/tasks"/> <my:task xmlns:my="http://example.org/tasks"> <my:config a="foo" b="bar"/> ... </my:task>
If the element allows or requires a lot of nested elements, the prefix needs to be used for every nested element. Making the namespace the default can reduce the verbosity of the script:
<typedef resource="org/example/tasks.properties" uri="http://example.org/tasks"/>
<task xmlns="http://example.org/tasks">
<config a="foo" b="bar"/>
...
</task>
From Ant 1.6.2, elements nested inside a namespaced element may also be in Ant's default namespace. This means that the following is now allowed:
<typedef resource="org/example/tasks.properties" uri="http://example.org/tasks"/> <my:task xmlns:my="http://example.org/tasks"> <config a="foo" b="bar"/> ... </my:task>
Attributes are only used to configure the element they belong to if:
Other attributes are simply ignored.
This means that both:
<my:task xmlns:my="http://example.org/tasks"> <my:config a="foo" b="bar"/> ... </my:task>
and
<my:task xmlns:my="http://example.org/tasks"> <my:config my:a="foo" my:b="bar"/> ... </my:task>
result in the parameters "a" and "b" being used as parameters to configure the nested "config" element.
It also means that you can use attributes from other namespaces to markup the build file with extra metadata, such as RDF and XML-Schema (whether that's a good thing or not). The same is not true for elements from unknown namespaces, which result in a error.
Now comes the difficult part: elements from different namespaces can
be woven together under certain circumstances. This has a lot to do
with the Ant 1.6
add type introspection rules:
Ant types and tasks are now free to accept arbritrary named types as
nested elements, as long as the concrete type implements the interface
expected by the task/type. The most obvious example for this is the
<condition> task, which supports various nested conditions, all
of which extend the interface Condition. To integrate a
custom condition in Ant, you can now simply <typedef> the
condition, and then use it anywhere nested conditions are allowed
(assuming the containing element has a generic add(Condition) or addConfigured(Condition) method):
<typedef resource="org/example/conditions.properties" uri="http://example.org/conditions"/>
<condition property="prop" xmlns="http://example.org/conditions">
<and>
<available file="bla.txt"/>
<my:condition a="foo"/>
</and>
</condition>
In Ant 1.6, this feature cannot be used as much as we'd all like to: a lot of code has not yet been adapted to the new introspection rules, and elements like Ant's built-in conditions and selectors are not really types in 1.6. This is expected to change in Ant 1.7.
The new AntLib feature is also very much integrated with the namespace support in Ant 1.6. Basically, you can "import" Antlibs simply by using a special scheme for the namespace URI: the antlib scheme, which expects the package name in which a special antlib.xml file is located.