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

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

<p>
The <code>ProjectHelper</code> in Apache 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's default <code>ProjectHelper</code>
(<code>org.apache.tools.ant.helper.ProjectHelper2</code>) parses the
usual <code>build.xml</code> 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>

<p>
<em>Since Ant 1.8.2</em>, the <a href="Tasks/import.html">import</a> task will also
try to use the proper helper to parse the imported file. So it is possible to write
different build files in different languages and have them import each other.
</p>

<h2 id="repository">How is Ant is selecting the proper ProjectHelper</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':
</p>
<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 <samp>META-INF</samp>: it searches in the classpath for a
    file <samp>META-INF/services/org.apache.tools.ant.ProjectHelper</samp>.
    This file will just contain the fully qualified name of the
    implementation of <code>ProjectHelper</code> to instantiate;</li>
  <li>it will also search with the system class loader for
    <code>ProjectHelper</code> service declarations in the <samp>META-INF</samp>;</li>
  <li>last but not least it will add its default <code>ProjectHelper</code>
    that can parse classical <samp>build.xml</samp> files.</li>
</ul>
<p>
In case of an error while trying to instantiate 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 <strong>system</strong>
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 id="writing">Writing your own ProjectHelper</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
instantiated 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:
</p>
<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>
Now that you have your implementation ready, you have to declare it to Ant. Three
solutions here:
</p>
<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 <samp>META-INF</samp>: in the jar you will build with your
    implementation, add a file
    <samp>META-INF/services/org.apache.tools.ant.ProjectHelper</samp>.
    And then in this file just put the fully qualified name of your
    implementation</li>
  <li>use the <a href="Tasks/projecthelper.html">projecthelper</a> task (<em>since
    Ant 1.8.2</em>) which will install dynamically a helper in the internal helper
    'repository'. Then your helper can be used on the next call to the
    <a href="Tasks/import.html">import</a> task.</li>
</ul>

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