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                      <meta name="author" value="Peter Donald">
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      <title>Apache Myrmidon - On ClassLoaders in Ant 2</title>
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                <p><strong>Myrmidon</strong></p>
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      <p><strong>Container Design</strong></p>
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                    <a name="ClassLoader Management"><strong>ClassLoader Management</strong></a>
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      <blockquote>
                        <p>In many ways Ant 2 needs to follow rules similar to a number of
            different application servers with respect to ClassLoader management.
            Ant 2 will create a number of different ClassLoaders that have access
            to different sets of resources (and thus Classes). The main reason
            for this arrangment is to partition different sections of the
            application such as the Container, the Task API, task/type libraries
            and support libraries.</p>
                                <p>The recomended structure for ClassLoader relationships is a hierarchy.
            When a ClassLoader is asked for a resource (or a class) it first delegates
            to it's parent to ask for the resource. If the resource is not present in
            its parent ClassLoader then the ClassLoader attempts to locate the resource
            in it's own store. In practice this means that all the classes (and static
            variables defined by said classes) in a parent ClassLoader are shared with
            the child ClassLoaders.</p>
                                <p>Using kooky ascii art, the specific ClassLoader structure for Ant 2 is as
            follows:</p>
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        <td bgcolor="#ffffff"><pre>
                  Bootstrap
                      |
                   System
                      |
                   Common
                  /      \
             Container  Shared
                         /   \
                    Antlib1  Antlib2 ...
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                                <ul>
                <li>
                  The
                    <strong>Bootstrap</strong> ClassLoader contains the classes and resources
                  provided by the Java runtime.

                </li>
                <li>
                  The
                    <strong>System</strong> ClassLoader contains the classes that were made accessible
                  via the CLASSPATH environment variable. If the standard ant script was used then this
                  should only contain the classes that are used to bootstrap the ant runtime. ie

                    <code>$ANT_HOME/bin/ant-launcher.jar</code>
                </li>
                <li>
                  The
                    <strong>Common</strong> ClassLoader contains the classes and resources
                  that are made visible to both the Container and to all the ant type libraries. This
                  contains all the classes that the Container uses to communicate with tasks and other
                  supporting infrastructure. In particular it contains the following APIs;

                    <ul>
                        <li>
                            <em>Task API</em> - Contains the classes that are part of the API used
                          to define tasks.

                        </li>
                        <li>
                            <em>ProjectListener API</em> - Contains the classes necessary to define new
                          ProjectListeners.

                        </li>
                        <li>
                            <em>Aspect API</em> - Contains the classes that are used to define Aspects
                          of the container.

                        </li>
                        <li>
                            <em>Container API</em> - Contains the interfaces that are required to communicate
                          with the objects deep within the container.
                            <strong>NOTE</strong>: These interfaces
                          are not to be used by user tasks but are made available so that certain tasks (such
                          as &lt;antcall/&gt;) can be implemented. However they are subject to change without
                          notice between between different Ant 2 versions.

                        </li>
                    </ul>
                    <p>
                      These classes are loaded from all the jars present in the
                        <code>$ANT_HOME/lib</code>
                      directory.

                    </p>
                </li>
                <li>
                  The
                    <strong>Container</strong> ClassLoader contains all the classes and resources
                  that are part of the actual implementation of the Container. These classes are not
                  directly accessible to any Ant library or task. Some of the classes are indirectly
                  accessible to tasks and other elements defined in the ant libraries as they implement
                  interfaces defined in the
                    <strong>Common</strong> ClassLoader. The classes that are
                  stored in jars in the
                    <code>$ANT_HOME/bin/lib/</code> directory.

                </li>
                <li>
                  The
                    <strong>Shared</strong> ClassLoader contains all the classes and resources
                  that are shared across all of the ant libraries (unless they are als needed by the
                  container in which case they should be placed int the
                    <strong>Container</strong>
                  ClassLoader). This ClassLoader is populated by all the jars that are contained in
                  the
                    <code>$ANT_HOME/shared/</code> directory.

                </li>
                <li>
                  The
                    <strong>AntLib</strong> ClassLoaders each contain the classes and resources
                  that required by that particular library. Note that in some cases a single Ant
                  Library will manifest as a single ClassLoader containing a single jar. However
                  in some cases it is possible for one Ant Library to have multiple jars in its
                  ClassLoader or even have multiple ClassLoaders. See XXXX for further details.

                </li>
            </ul>
                    </blockquote>
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