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<title>The Ant frontend: ProjectHelper</title>
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<h1>The Ant frontend: ProjectHelper</h1>

<h2><a name="definition">What is a ProjectHelper?</a></h2>

<p>
The <code>ProjectHelper</code> in Ant is responsible for parsing the build file
and creating java instances representing the build workflow. It also signals which
kind of file it can parse, and which file name it expects as default input file.
</p>

<p>
Ant' default <code>ProjectHelper</code>
(<code>org.apache.tools.ant.helper.ProjectHelper2</code>) parses the
usual build.xml files. And if no build file is specified on the command line, it
will expect to find a file named <code>build.xml</code>.
</p>

<p>
The immediate benefit of a such abstraction it that it is possible to make Ant
understand other kind of descriptive languages than XML. Some experiments have
been done around a pure java frontend, and a groovy one too (ask the dev mailing
list for further info about these).
</p>

<h2><a name="repository">How is Ant is selecting the proper ProjectHelper</a></h2>

<p>
Ant knows about several implementations of <code>ProjectHelper</code>
and has to decide which to use for each build file.
</p>

<p>At startup Ant lists the all implementations found and keeps them
in the same order they've been found in an internal 'repository':
<ul>
    <li>the first to be searched for is the one declared by the system property
        <code>org.apache.tools.ant.ProjectHelper</code> (see
        <a href="running.html#sysprops">Java System Properties</a>);</li>
    <li>then it searches with its class loader for a <code>ProjectHelper</code>
        service declarations in the META-INF: it searches in the classpath for a
        file <code>META-INF/services/org.apache.tools.ant.ProjectHelper</code>.
        This file will just contain the fully qualified name of the
        implementation of <code>ProjectHelper</code> to instanciate;</li>
    <li>it will also search with the system class loader for
        <code>ProjectHelper</code> service declarations in the META-INF;</li>
    <li>last but not least it will add its default <code>ProjectHelper</code>
        that can parse classical build.xml files.</li>
</ul>
In case of an error while trying to instanciate a <code>ProjectHelper</code>, Ant
will log an error but won't stop.  If you want further debugging
info about the <code>ProjectHelper</code> internal 'repository', use the <b>system</b>
property <code>ant.project-helper-repo.debug</code> and set it to
<code>true</code>; the full stack trace will then also be printed.
</p>

<p>
When Ant is expected to parse a file, it will ask the
<code>ProjectHelper</code> repository to find an implementation that will be
able to parse the input file. Actually it will just iterate over the ordered list
and the first implementation that returns <code>true</code> to
<code>supportsBuildFile(File buildFile)</code> will be selected.
</p>

<p>
When Ant is started and no input file has been specified, it will search for
a default input file. It will iterate over list of <code>ProjectHelper</code>s
and will select the first one that expects a default file that actually exist.
</p>

<h2><a name="writing">Writing your own ProjectHelper</a></h2>

<p>
The class <code>org.apache.tools.ant.ProjectHelper</code> is the API expected to
be implemented. So write your own <code>ProjectHelper</code> by extending that
abstract class. You are then expected to implement at least the function
<code>parse(Project project, Object source)</code>. Note also that your
implementation will be instanciated by Ant, and it is expecting a default
constructor with no arguments.
</p>

<p>
There are some functions that will help you define what your helper is
capable of and what is is expecting:
<ul>
    <li><code>getDefaultBuildFile()</code>: defines which file name is expected if
    none provided</li>
    <li><code>supportsBuildFile(File buildFile)</code>: defines if your parser
    can parse the input file</li>

    <li><code>canParseAntlibDescriptor(URL url)</code>: whether your
      implementation is capable of parsing a given Antlib
      descriptor.  The base class returns <code>false</code></li>
    <li><code>parseAntlibDescriptor(Project containingProject, URL
        source)</code>: invoked to actually parse the Antlib
      descriptor if your implementation returned <code>true</code>
      for the previous method.</li>
</ul>
</p>

<p>
Now that you have your implementation ready, you have to declare it to Ant. Two
solutions here:
<ul>
    <li>use the system property <code>org.apache.tools.ant.ProjectHelper</code>
        (see also the <a href="running.html#sysprops">Java System Properties</a>);</li>
    <li>use the service file in META-INF: in the jar you will build with your
        implementation, add a file
        <code>META-INF/services/org.apache.tools.ant.ProjectHelper</code>.
        And then in this file just put the fully qualified name of your
        implementation</li>
</ul>
</p>

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