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  <title>XmlNamespaceSupport</title>
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  <body>
    <h2 id="namespace">XML Namespace Support</h2>
    Apache Ant 1.6 introduces support for XML namespaces.
    <h3>History</h3>

    <p>
      All releases of Ant prior to Ant 1.6 do not support XML namespaces.  No support basically
      implies two things here:
    </p>
    <ul>
      <li>Element names correspond to the "qname" of the tags, which is usually the same as the
        local name. But if the build file writer uses colons in names of defined tasks/types,
        those become part of the element name. Turning on namespace support gives
        colon-separated prefixes in tag names a special meaning, and thus build files using
        colons in user-defined tasks and types will break.
      </li>
      <li>Attributes with the names <q>xmlns</q> and <q>xmlns:<em>prefix</em></q> 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.
      </li>
    </ul>
    <p>Use of colons in element names has been discouraged in the past, and using any attribute
      starting with <q>xml</q> is actually strongly discouraged by the XML spec to reserve such
      names for future use.
    </p>

    <h3>Motivation</h3>
    <p>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 <q>tomcat-deploy</q> instead of
      just <q>deploy</q>). But when defining whole libraries of types using
      the <code>&lt;typedef&gt;</code> <var>resource</var> attribute, the build file writer has
      no chance to override or even prefix the names supplied by the library.</p>

    <h3>Assigning Namespaces</h3>
    <p>
      Adding a <var>prefix</var> attribute to <code>&lt;typedef&gt;</code> might have been
      enough, but XML already has a well-known method for namespacing. Thus, instead of adding
      a <var>prefix</var> attribute, the <code>&lt;typedef&gt;</code>
      and <code>&lt;taskdef&gt;</code> tasks get a <var>uri</var> attribute, which stores the
      URI of the XML namespace with which the type should be associated:
    </p>
    <pre>
&lt;typedef resource="org/example/tasks.properties" uri="http://example.org/tasks"/&gt;
&lt;my:task xmlns:my="http://example.org/tasks"&gt;
    ...
&lt;/my:task&gt;</pre>
    <p>As the above example demonstrates, the namespace URI needs to be specified at least
      twice: one time as the value of the <var>uri</var> attribute, and another time to actually
      map the namespace to occurrences of elements from that namespace, by using
      the <var>xmlns</var> attribute. This mapping can happen at any level in the build file:
    </p>
    <pre>
&lt;project name="test" xmlns:my="http://example.org/tasks"&gt;
    &lt;typedef resource="org/example/tasks.properties" uri="http://example.org/tasks"/&gt;
    &lt;my:task&gt;
        ...
    &lt;/my:task&gt;
&lt;/project&gt;</pre>
    <p>
      Use of a namespace prefix is of course optional. Therefore the example could also look
      like this:
    </p>
    <pre>
&lt;project name="test"&gt;
    &lt;typedef resource="org/example/tasks.properties" uri="http://example.org/tasks"/&gt;
    &lt;task xmlns="http://example.org/tasks"&gt;
        ...
    &lt;/task&gt;
&lt;/project&gt;</pre>
    <p>
      Here, the namespace is set as the default namespace for the <code>&lt;task&gt;</code>
      element and all its descendants.
    </p>

    <h3>Default Namespace</h3>
    <p>
      The default namespace used by Ant is <code>antlib:org.apache.tools.ant</code>.
    </p>
    <pre>
&lt;typedef resource="org/example/tasks.properties" uri="antlib:org.apache.tools.ant"/&gt;
&lt;task&gt;
    ...
&lt;/task&gt;</pre>

    <h3>Namespaces and Nested Elements</h3>
    <p>
      Almost always in Ant 1.6, elements nested inside a namespaced element have the same
      namespace as their parent. So if <code>task</code> in the example above allowed a
      nested <code>config</code> element, the build file snippet would look like this:
    </p>
    <pre>
&lt;typedef resource="org/example/tasks.properties" uri="http://example.org/tasks"/&gt;
&lt;my:task xmlns:my="http://example.org/tasks"&gt;
    &lt;my:config a="foo" b="bar"/&gt;
    ...
&lt;/my:task&gt;</pre>
    <p>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:
    </p>
    <pre>
&lt;typedef resource="org/example/tasks.properties" uri="http://example.org/tasks"/&gt;
&lt;task xmlns="http://example.org/tasks"&gt;
    &lt;config a="foo" b="bar"/&gt;
    ...
&lt;/task&gt;</pre>
    <p>
      <em>Since Ant 1.6.2</em>, elements nested inside a namespaced element may also be in Ant's
      default namespace. This means that the following is now allowed:
    </p>
    <pre>
&lt;typedef resource="org/example/tasks.properties"
    uri="http://example.org/tasks"/&gt;
&lt;my:task xmlns:my="http://example.org/tasks"&gt;
    &lt;config a="foo" b="bar"/&gt;
    ...
&lt;/my:task&gt;</pre>

    <h3>Namespaces and Attributes</h3>
    <p>
      Attributes are only used to configure the element they belong to if:
    </p>
    <ul>
      <li>they have no namespace (note that the default namespace does <strong>not</strong> apply to attributes)</li>
      <li>they are in the same namespace as the element they belong to</li>
    </ul>
    <p>
      <em>Since Ant 1.9.1</em> two attribute namespaces <code>ant:if</code>
      and <code>ant:unless</code> are available to allow you to insert elements conditionally.
    </p>
    <p>
      Other attributes are simply ignored.
    </p>
    <p>
      This means that both:
    </p>
    <p>
    </p>
    <pre>
&lt;my:task xmlns:my="http://example.org/tasks"&gt;
    &lt;my:config a="foo" b="bar"/&gt;
    ...
&lt;/my:task&gt;</pre>
    <p>
      and
    </p>
    <pre>
&lt;my:task xmlns:my="http://example.org/tasks"&gt;
    &lt;my:config my:a="foo" my:b="bar"/&gt;
    ...
&lt;/my:task&gt;</pre>
    <p>
      result in the parameters <var>a</var> and <var>b</var> being used as parameters to
      configure the nested <code>config</code> element.
    </p>
    <p>
      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.
    </p>
    <h3>Mixing Elements from Different Namespaces</h3>

    <p>
      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 <a href="../develop.html#nestedtype">add type introspection rules</a>: Ant types and tasks
      are now free to accept arbitrary 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 <code>&lt;condition&gt;</code> task, which supports various nested conditions, all of
      which extend the interface <code class="code">Condition</code>. To integrate a custom
      condition in Ant, you can now simply <code>&lt;typedef&gt;</code> the condition, and then use
      it anywhere nested conditions are allowed (assuming the containing element has a
      generic <code class="code">add(Condition)</code>
      or <code class="code">addConfigured(Condition)</code> method):
    </p>
    <pre>
&lt;typedef resource="org/example/conditions.properties" uri="http://example.org/conditions"/&gt;
&lt;condition property="prop" xmlns="http://example.org/conditions"&gt;
    &lt;and&gt;
        &lt;available file="bla.txt"/&gt;
        &lt;my:condition a="foo"/&gt;
    &lt;/and&gt;
&lt;/condition&gt;</pre>
    <p>
      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.
    </p>
    <h3>Namespaces and Antlib</h3>

    <p>
      The new <a href="antlib.html">AntLib</a> 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 <code>antlib</code> scheme, which expects the
      package name in which a special <samp>antlib.xml</samp> file is located.
    </p>

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