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<title>Writing Your Own Task</title>
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<h1>Developing with Ant</h1>

<h2><a name="writingowntask">Writing Your Own Task</a></h2>
<p>It is very easy to write your own task:</p>
<ol>
  <li>Create a Java class that extends <code>org.apache.tools.ant.Task</code>
      or <a href="base_task_classes.html">another class</a> that was desgined to be extended.</li>
  <li>For each attribute, write a <i>setter</i> method. The setter method must be a
    <code>public void</code> method that takes a single argument. The
    name of the method must begin with <code>set</code>, followed by the
    attribute name, with the first character of the name in uppercase, and the rest in

    lowercase.  That is, to support an attribute named
    <code>file</code> you create a method <code>setFile</code>.
    Depending on the type of the argument, Ant will perform some
    conversions for you, see <a href="#set-magic">below</a>.</li>

  <li>If your task shall contain other tasks as nested elements (like
  <a href="CoreTasks/parallel.html"><code>parallel</code></a>), your
  class must implement the interface
  <code>org.apache.tools.ant.TaskContainer</code>.  If you do so, your
  task can not support any other nested elements.  See 
  <a href="#taskcontainer">below</a>.</li>

<li>If the task should support character data (text nested between the
start end end tags), write a <code>public void addText(String)</code>
method.  Note that Ant does <strong>not</strong> expand properties on
the text it passes to the task.</li>

<li>For each nested element, write a <i>create</i>, <i>add</i> or
<i>addConfigured</i> method.  A create method must be a
<code>public</code> method that takes no arguments and returns an
<code>Object</code> type. The name of the create method must begin
with <code>create</code>, followed by the element name. An add (or
addConfigured) method must be a <code>public void</code> method that
takes a single argument of an <code>Object</code> type with a
no-argument constructor.  The name of the add (addConfigured) method
must begin with <code>add</code> (<code>addConfigured</code>),
followed by the element name.  For a more complete discussion see 
<a href="#nested-elements">below</a>.</li>

<li>Write a <code>public void execute</code> method, with no arguments, that
throws a <code>BuildException</code>. This method implements the task
itself.</li>
</ol>

<h3>The Life-cycle of a Task</h3>
<ol>
  <li>The task gets instantiated using a no-argument constructor, at parser
    time. This means even tasks that are never executed get
    instantiated.</li>

  <li>The task gets references to its project and location inside the
    buildfile via its inherited <code>project</code> and
    <code>location</code> variables.</li>

  <li>If the user specified an <code>id</code> attribute to this task,
    the project
    registers a reference to this newly created task, at parser
    time.</li>

  <li>The task gets a reference to the target it belongs to via its
    inherited <code>target</code> variable.</li>

  <li><code>init()</code> is called at parser time.</li>

  <li>All child elements of the XML element corresponding to this task
    are created via this task's <code>createXXX()</code> methods or
    instantiated and added to this task via its <code>addXXX()</code>
    methods, at parser time.</li>

  <li>All attributes of this task get set via their corresponding
    <code>setXXX</code> methods, at runtime.</li>

  <li>The content character data sections inside the XML element
    corresponding to this task is added to the task via its
    <code>addText</code> method, at runtime.</li>

  <li>All attributes of all child elements get set via their corresponding
    <code>setXXX</code> methods, at runtime.</li>

  <li><a name="execute"><code>execute()</code></a> is called at runtime. While the above initialization
    steps only occur once, the execute() method may be
    called more than once, if the task is invoked more than once. For example,
    if <code>target1</code> and <code>target2</code> both depend
    on <code>target3</code>, then running
    <code>'ant target1 target2'</code> will run all tasks in
    <code>target3</code> twice.</li>
</ol>

<h3><a name="set-magic">Conversions Ant will perform for attributes</a></h3>

<p>Ant will always expand properties before it passes the value of an
attribute to the corresponding setter method.</p>

<p>The most common way to write an attribute setter is to use a
<code>java.lang.String</code> argument.  In this case Ant will pass
the literal value (after property expansion) to your task.  But there
is more!  If the argument of you setter method is</p>

<ul>

  <li><code>boolean</code>, your method will be passed the value
  <i>true</i> if the value specified in the build file is one of
  <code>true</code>, <code>yes</code>, or <code>on</code> and
  <i>false</i> otherwise.</li>

  <li><code>char</code> or <code>java.lang.Character</code>, your
  method will be passed the first character of the value specified in
  the build file.</li>

  <li>any other primitive type (<code>int</code>, <code>short</code>
  and so on), Ant will convert the value of the attribute into this
  type, thus making sure that you'll never receive input that is not a
  number for that attribute.</li>

  <li><code>java.io.File</code>, Ant will first determine whether the
  value given in the build file represents an absolute path name.  If
  not, Ant will interpret the value as a path name relative to the
  project's basedir.</li>

  <li><code>org.apache.tools.ant.types.Path</code>, Ant will tokenize
  the value specified in the build file, accepting <code>:</code> and
  <code>;</code> as path separators.  Relative path names will be
  interpreted as relative to the project's basedir.</li>

  <li><code>java.lang.Class</code>, Ant will interpret the value
  given in the build file as a Java class name and load the named
  class from the system class loader.</li>

  <li>any other type that has a constructor with a single
  <code>String</code> argument, Ant will use this constructor to
  create a new instance from the value given in the build file.</li>

  <li>A subclass of
  <code>org.apache.tools.ant.types.EnumeratedAttribute</code>, Ant
  will invoke this classes <code>setValue</code> method.  Use this if
  your task should support enumerated attributes (attributes with
  values that must be part of a predefined set of values).  See
  <code>org/apache/tools/ant/taskdefs/FixCRLF.java</code> and the
  inner <code>AddAsisRemove</code> class used in <code>setCr</code>
  for an example.</li>

</ul>

<p>What happens if more than one setter method is present for a given
attribute?  A method taking a <code>String</code> argument will always
lose against the more specific methods.  If there are still more
setters Ant could chose from, only one of them will be called, but we
don't know which, this depends on the implementation of your Java
virtual machine.</p>

<h3><a name="nested-elements">Supporting nested elements</a></h3>

<p>Let's assume your task shall support nested elements with the name
<code>inner</code>.  First of all, you need a class that represents
this nested element.  Often you simply want to use one of Ant's
classes like <code>org.apache.tools.ant.types.FileSet</code> to
support nested <code>fileset</code> elements.</p>

<p>Attributes of the nested elements or nested child elements of them
will be handled using the same mechanism used for tasks (i.e. setter
methods for attributes, addText for nested text and
create/add/addConfigured methods for child elements).</p>

<p>Now you have a class <code>NestedElement</code> that is supposed to
be used for your nested <code>&lt;inner&gt;</code> elements, you have
three options:</p>

<ol>
  <li><code>public NestedElement createInner()</code></li>
  <li><code>public void addInner(NestedElement anInner)</code></li>
  <li><code>public void addConfiguredInner(NestedElement anInner)</code></li>
</ol>

<p>What is the difference?</p>

<p>Option 1 makes the task create the instance of
<code>NestedElement</code>, there are no restrictions on the type.
For the options 2 and 3, Ant has to create an instance of
<code>NestedInner</code> before it can pass it to the task, this
means, <code>NestedInner</code> must have a <code>public</code> no-arg
constructor.  This is the only difference between options 1 and 2.</p>

<p>The difference between 2 and 3 is what Ant has done to the object
before it passes it to the method.  <code>addInner</code> will receive
an object directly after the constructor has been called, while
<code>addConfiguredInner</code> gets the object <em>after</em> the
attributes and nested children for this new object have been
handled.</p>

<p>What happens if you use more than one of the options?  Only one of
the methods will be called, but we don't know which, this depends on
the implementation of your Java virtual machine.</p>

<h3><a name="taskcontainer">TaskContainer</a></h3>

<p>The <code>TaskContainer</code> consists of a single method,
<code>addTask</code> that basically is the same as an <a
href="#nested-elements">add method</a> for nested elements.  The task
instances will be configured (their attributes and nested elements
have been handled) when your task's <code>execute</code> method gets
invoked, but not before that.</p>

<p>When we <a href="#execute">said</a> <code>execute</code> would be
called, we lied ;-).  In fact, Ant will call the <code>perform</code>
method in <code>org.apache.tools.ant.Task</code>, which in turn calls
<code>execute</code>.  This method makes sure that <a
href="#buildevents">Build Events</a> will be triggered.  If you
execute the task instances nested into your task, you should also
invoke <code>perform</code> on these instances instead of
<code>execute</code>.</p>

<h3>Example</h3>
<p>Let's write our own task, which prints a message on the
<code>System.out</code> stream.
The
task has one attribute, called <code>message</code>.</p>
<blockquote>
<pre>
package com.mydomain;

import org.apache.tools.ant.BuildException;
import org.apache.tools.ant.Task;

public class MyVeryOwnTask extends Task {
    private String msg;

    // The method executing the task
    public void execute() throws BuildException {
        System.out.println(msg);
    }

    // The setter for the &quot;message&quot; attribute
    public void setMessage(String msg) {
        this.msg = msg;
    }
}
</pre>
</blockquote>
<p>It's really this simple ;-)</p>
<p>Adding your task to the system is rather simple too:</p>
<ol>
  <li>Make sure the class that implements your task is in the classpath when
    starting Ant.</li>
  <li>Add a <code>&lt;taskdef&gt;</code> element to your project.
    This actually adds your task to the system.</li>
  <li>Use your task in the rest of the buildfile.</li>
</ol>


<h3>Example</h3>
<blockquote>
<pre>
&lt;?xml version=&quot;1.0&quot;?&gt;

&lt;project name=&quot;OwnTaskExample&quot; default=&quot;main&quot; basedir=&quot;.&quot;&gt;
  &lt;taskdef name=&quot;mytask&quot; classname=&quot;com.mydomain.MyVeryOwnTask&quot;/&gt;

  &lt;target name=&quot;main&quot;&gt;
    &lt;mytask message=&quot;Hello World! MyVeryOwnTask works!&quot;/&gt;
  &lt;/target&gt;
&lt;/project&gt;
</pre>
</blockquote>
<h3>Example 2</h3>
To use a task directly from the buildfile which created it, place the
<code>&lt;taskdef&gt;</code> declaration inside a target
<i>after the compilation</i>. Use the <code>classpath</code> attribute of
<code>&lt;taskdef&gt;</code> to point to where the code has just been
compiled.
<blockquote>
<pre>
&lt;?xml version=&quot;1.0&quot;?&gt;

&lt;project name=&quot;OwnTaskExample2&quot; default=&quot;main&quot; basedir=&quot;.&quot;&gt;

  &lt;target name=&quot;build&quot; &gt;
    &lt;mkdir dir=&quot;build&quot;/&gt;
    &lt;javac srcdir=&quot;source&quot; destdir=&quot;build&quot;/&gt;
  &lt;/target&gt;

  &lt;target name=&quot;declare&quot; depends=&quot;build&quot;&gt;
    &lt;taskdef name=&quot;mytask&quot;
        classname=&quot;com.mydomain.MyVeryOwnTask&quot;
        classpath=&quot;build&quot;/&gt;
  &lt;/target&gt;

  &lt;target name=&quot;main&quot; depends=&quot;declare&quot;&gt;
    &lt;mytask message=&quot;Hello World! MyVeryOwnTask works!&quot;/&gt;
  &lt;/target&gt;
&lt;/project&gt;
</pre>
</blockquote>


<p>Another way to add a task (more permanently), is to add the task name and
implementing class name to the <code>default.properties</code> file in the
<code>org.apache.tools.ant.taskdefs</code>
package. Then you can use it as if it were a built-in task.</p>

<hr>
<h2><a name="buildevents">Build Events</a></h2>
<P>Ant is capable of generating build events as it performs the tasks necessary to build a project.
Listeners can be attached to Ant to receive these events. This capability could be used, for example,
to connect Ant to a GUI or to integrate Ant with an IDE.
</P>
<p>To use build events you need to create an ant <code>Project</code> object. You can then call the
<code>addBuildListener</code> method to add your listener to the project. Your listener must implement
the <code>org.apache.tools.antBuildListener</code> interface. The listener will receive BuildEvents
for the following events</P>
<ul>
<li>Build started</li>
<li>Build finished</li>
<li>Target started</li>
<li>Target finished</li>
<li>Task started</li>
<li>Task finished</li>
<li>Message logged</li>
</ul>

<p>
If you wish to attach a listener from the command line you may use the
<code>-listener</code> option. For example:</p>
<blockquote>
  <pre>ant -listener org.apache.tools.ant.XmlLogger</pre>
</blockquote>
<p>will run Ant with a listener that generates an XML representation of the build progress. This
listener is included with Ant, as is the default listener, which generates the logging to standard output.</p>

<hr>
<h2><a name="integration">Source code integration</a></h2>


The other way to extend Ant through Java is to make changes to existing tasks, which is positively encouraged.
Both changes to the existing source and new tasks can be incorporated back into the Ant codebase, which
benefits all users and spreads the maintenance load around.
<p>

Please consult the
<a href="http://jakarta.apache.org/site/getinvolved.html">Getting Involved</a> pages on the Jakarta web site
for details on how to fetch the latest source and how to submit changes for reincorporation into the
source tree.
<p>
Ant also has some
<a href="http://jakarta.apache.org/ant/ant_task_guidelines.html">task guidelines</a>
which provides some advice to people developing and testing tasks. Even if you intend to
keep your tasks to yourself, you should still read this as it should be informative.

<hr>
<p align="center">Copyright &copy; 2000-2002 Apache Software Foundation. All rights
Reserved.</p>

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