wangwei
/
antliscence
forked from youys/ant

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    <link rel="stylesheet" type="text/css" href="../stylesheets/style.css">
<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 'xmlns' and 'xmlns:<code>&lt;prefix&gt;</code>'
        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 "xml" 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
      "tomcat-deploy" instead of just "deploy"). But when defining whole
      libraries of types using the <code>&lt;typedef&gt;</code> 'resource' 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 'prefix' 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 'prefix' attribute, the <code>&lt;typedef&gt;</code> and <code>&lt;taskdef&gt;</code>
      tasks get a 'uri' 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="<a href="http://example.org/tasks">http://example.org/tasks</a>"/&gt;
 &lt;my:task xmlns:my="<a href="http://example.org/tasks">http://example.org/tasks</a>"&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 '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:
    </p><pre> &lt;project name="test" xmlns:my="<a href="http://example.org/tasks">http://example.org/tasks</a>"&gt;
   &lt;typedef resource="org/example/tasks.properties" uri="<a href="http://example.org/tasks">http://example.org/tasks</a>"/&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="<a href="http://example.org/tasks">http://example.org/tasks</a>"/&gt;
   &lt;task xmlns="<a href="http://example.org/tasks">http://example.org/tasks</a>"&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 "antlib:org.apache.tools.ant".
    </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 'task' in the
      example above allowed a nested 'config' element, the build file snippet
      would look like this:
    </p><pre> &lt;typedef resource="org/example/tasks.properties" uri="<a href="http://example.org/tasks">http://example.org/tasks</a>"/&gt;
 &lt;my:task xmlns:my="<a href="http://example.org/tasks">http://example.org/tasks</a>"&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="<a href="http://example.org/tasks">http://example.org/tasks</a>"/&gt;
          &lt;task xmlns="<a href="http://example.org/tasks">http://example.org/tasks</a>"&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="<a href="http://example.org/tasks">http://example.org/tasks</a>"/&gt;
 &lt;my:task xmlns:my="<a href="http://example.org/tasks">http://example.org/tasks</a>"&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 *not* apply to attributes)
      </li>
      <li> they are in the same namespace as the element they belong to
      </li>
    </ul>
    <p>
      In Ant 1.9.1 two attribute namespaces <code>ant:if</code> and <code>ant:unless</code> were added
      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="<a href="http://example.org/tasks">http://example.org/tasks</a>"&gt;
   &lt;my:config a="foo" b="bar"/&gt;
   ...
 &lt;/my:task&gt;
</pre>
    <p>
      and
    </p>
    <pre> &lt;my:task xmlns:my="<a href="http://example.org/tasks">http://example.org/tasks</a>"&gt;
   &lt;my:config my:a="foo" my:b="bar"/&gt;
   ...
 &lt;/my:task&gt;
</pre>
    <p>
      result in the parameters "a" and "b" being used as parameters to configure the nested "config" 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 <tt>Condition</tt>. 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 <tt>add(Condition)</tt> or <tt>addConfigured(Condition)</tt> method):
</p><pre> &lt;typedef resource="org/example/conditions.properties" uri="<a href="http://example.org/conditions">http://example.org/conditions</a>"/&gt;
 &lt;condition property="prop" xmlns="<a href="http://example.org/conditions">http://example.org/conditions</a>"&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 <tt>antlib</tt> scheme, which expects the package name in which a special <tt>antlib.xml</tt> file is located.
    </p>

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