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<title>Writing Your Own Task</title>
<h1>Developing with Apache Ant</h1>
<h2 id="writingowntask">Writing Your Own Task</h2>
<p>It is very easy to write your own task:</p>
<li>Create a Java class that extends <code class="code"></code>
or <a href="base_task_classes.html">another class</a> that was designed to be extended.</li>
<li id="footnote-1-back">For each attribute, write a <em>setter</em> 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<a href="#footnote-1">*</a>. That is, to
support an attribute named <var>file</var> you create a
method <code class="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="Tasks/parallel.html"><code>parallel</code></a>), your class must implement the
interface <code class="code"></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 and end tags), write
a <code class="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 <em>create</em>, <em>add</em> or <em>addConfigured</em>
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 class="code">public void execute()</code> method, with no arguments, that throws
a <code>BuildException</code>. This method implements the task itself.</li>
<p id="footnote-1"><a href="#footnote-1-back">*</a> Actually the case of the letters after the first
one doesn't really matter to Ant, using all lower case is a good convention, though.</p>
<h3>The Life-cycle of a Task</h3>
<li>The xml element that contains the tag corresponding to the task gets converted to
an <code class="code">UnknownElement</code> at parse time.
This <code class="code">UnknownElement</code> gets placed in a list within a target object, or
recursively within another <code class="code">UnknownElement</code>.
<li>When the target is executed, each <code class="code">UnknownElement</code> is invoked using
an <code class="code">perform()</code> method. This instantiates the task. This means that tasks
only gets instantiated at run time.
<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 <var>id</var> attribute to this task, the project registers a
reference to this newly created task, at run time.</li>
<li>The task gets a reference to the target it belongs to via its inherited <code>target</code>
<li><code class="code">init()</code> is called at run time.</li>
<li>All child elements of the XML element corresponding to this task are created via this
task's <code class="code">createXXX()</code> methods or instantiated and added to this task via
its <code class="code">addXXX()</code> methods, at run time. Child elements corresponding
to <code class="code">addConfiguredXXX()</code> are created at this point but the
actual <em>addConfigured</em> method is not called.</li>
<li>All attributes of this task get set via their corresponding <code class="code">setXXX()</code>
methods, at run time.</li>
<li>The content character data sections inside the XML element corresponding to this task is added
to the task via its <code class="code">addText()</code> method, at run time.</li>
<li>All attributes of all child elements get set via their
corresponding <code class="code">setXXX()</code> methods, at run time.</li>
<li>If child elements of the XML element corresponding to this task have been created
for <code class="code">addConfiguredXXX()</code> methods, those methods get invoked now.</li>
<li id="execute"><code class="code">execute()</code> is called at run time. If <q>target1</q>
and <q>target2</q> both depend on <q>target3</q>, then running <kbd>ant target1 target2</kbd>
will run all tasks in <q>target3</q> twice.</li>
<h3 id="set-magic">Conversions Ant will perform for attributes</h3>
<p>Ant will always expand properties before it passes the value of an attribute to the corresponding
setter method. <em>Since Ant 1.8</em>, it is possible to <a href="Tasks/propertyhelper.html">extend
Ant's property handling</a> such that a non-string Object may be the result of the evaluation of a
string containing a single property reference. These will be assigned directly via setter methods of
matching type. Since it requires some beyond-the-basics intervention to enable this behavior, it may
be a good idea to flag attributes intended to permit this usage paradigm.</p>
<p>The most common way to write an attribute setter is to use
a <code class="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
<li><code>boolean</code>, your method will be passed the value <q>true</q> if the value specified
in the build file is one of <q>true</q>, <q>yes</q>, or <q>on</q> and <q>false</q>
<li><code>char</code> or <code class="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 class="code">int</code>, <code class="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 class="code"></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 <var>basedir</var>.</li>
<li><code class="code"></code>, Ant will resolve the string as
a <code class="code"></code> as above, then pass in as
a <code class="code"></code>. <em>Since Ant
<li><code class="code"></code>, Ant will tokenize the value
specified in the build file, accepting <q>:</q> and <q>;</q> as path separators. Relative path
names will be interpreted as relative to the project's <var>basedir</var>.</li>
<li><code class="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 class="code">String</code> argument,
Ant will use this constructor to create a new instance from the value given in the build
<li>A subclass of <code class="code"></code>, Ant
will invoke this class's <code class="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/</code> and the
inner <code class="code">AddAsisRemove</code> class used in <code class="code">setCr</code> for
an example.</li>
<li>An enumeration, Ant will call the setter with the enum constant matching the value
given in the build file. This is easier than using <code class="code">EnumeratedAttribute</code>
and can result in cleaner code. Note
that any override of <code class="code">toString()</code> in the enumeration is ignored; the
build file must use the declared name (see <code>Enum.getName()</code>). You may wish to use
lowercase enum constant names, in contrast to usual Java style, to look better in build
files. <em>Since Ant 1.7.0</em></li>
<p>What happens if more than one setter method is present for a given attribute? A method taking
a <code class="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 id="nested-elements">Supporting nested elements</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 class="code"></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. <em>setter</em> methods for
attributes, <code class="code">addText()</code> for nested text
and <em>create</em>/<em>add</em>/<em>addConfigured</em> methods for child elements).</p>
<p>Now you have a class <code class="code">NestedElement</code> that is supposed to be used for your
nested <code>&lt;inner&gt;</code> elements, you have three options:</p>
<li><code class="code">public NestedElement createInner()</code></li>
<li><code class="code">public void addInner(NestedElement anInner)</code></li>
<li><code class="code">public void addConfiguredInner(NestedElement anInner)</code></li>
<p>What is the difference?</p>
<p>Option 1 makes the task create the instance of <code class="code">NestedElement</code>, there are
no restrictions on the type. For the options 2 and 3, Ant has to create an instance
of <code class="code">NestedInner</code> before it can pass it to the task, this
means, <code class="code">NestedInner</code> must have a <code>public</code> no-arg constructor or
a <code>public</code> one-arg constructor taking a <code class="code">Project</code> class as a
parameter. 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 class="code">addInner()</code> will receive an object directly after the constructor
has been called, while <code class="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 JVM.</p>
<h3 id="nestedtype">Nested Types</h3>
<p>If your task needs to nest an arbitrary type that has been defined
using <code>&lt;typedef&gt;</code> you have two options.</p>
<li><code class="code">public void add(Type type)</code></li>
<li><code class="code">public void addConfigured(Type type)</code></li>
<p>The difference between 1 and 2 is the same as between 2 and 3 in the previous section.</p>
<p>For example suppose one wanted to handle objects object of
type <code class="code"></code>, one may have a
public class MyTask extends Task {
private List conditions = new ArrayList();
public void add(Condition c) {
public void execute() {
// iterator over the conditions
<p>One may define and use this class like this:</p>
&lt;taskdef name="mytask" classname="MyTask" classpath="classes"/&gt;
&lt;typedef name="condition.equals"
&lt;condition.equals arg1="${debug}" arg2="true"/&gt;
<p>A more complicated example follows:</p>
public class Sample {
public static class MyFileSelector implements FileSelector {
public void setAttrA(int a) {}
public void setAttrB(int b) {}
public void add(Path path) {}
public boolean isSelected(File basedir, String filename, File file) {
return true;
interface MyInterface {
void setVerbose(boolean val);
public static class BuildPath extends Path {
public BuildPath(Project project) {
public void add(MyInterface inter) {}
public void setUrl(String url) {}
public static class XInterface implements MyInterface {
public void setVerbose(boolean x) {}
public void setCount(int c) {}
<p>This class defines a number of static classes that
implement/extend <code class="code">Path</code>, <code class="code">MyFileSelector</code>
and <code class="code">MyInterface</code>. These may be defined and used as follows:</p>
&lt;typedef name="myfileselector" classname="Sample$MyFileSelector"
classpath="classes" loaderref="classes"/&gt;
&lt;typedef name="buildpath" classname="Sample$BuildPath"
classpath="classes" loaderref="classes"/&gt;
&lt;typedef name="xinterface" classname="Sample$XInterface"
classpath="classes" loaderref="classes"/&gt;
&lt;copy todir="copy-classes"&gt;
&lt;fileset dir="classes"&gt;
&lt;myfileselector attra="10" attrB="-10"&gt;
&lt;buildpath path="." url="abc"&gt;
&lt;xinterface count="4"/&gt;
<h3 id="taskcontainer">TaskContainer</h3>
<p>The <code class="code">TaskContainer</code> consists of a single
method, <code class="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 class="code">execute</code> method gets invoked, but not before that.</p>
<p>When we <a href="#execute">said</a> <code class="code">execute</code> would be called, we lied
;-). In fact, Ant will call the <code class="code">perform</code> method
in <code class="code"></code>, which in turn
calls <code class="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 class="code">perform</code> on these instances instead
of <code class="code">execute</code>.</p>
<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>
package com.mydomain;
public class MyVeryOwnTask extends Task {
private String msg;
// The method executing the task
public void execute() throws BuildException {
// The setter for the &quot;message&quot; attribute
public void setMessage(String msg) {
this.msg = msg;
<p>It's really this simple ;-)</p>
<p>Adding your task to the system is rather simple too:</p>
<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>
&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;
<h3>Example 2</h3>
<p>To use a task directly from the buildfile which created it, place
the <code>&lt;taskdef&gt;</code> declaration inside a target <em>after the compilation</em>. Use
the <var>classpath</var> attribute of <code>&lt;taskdef&gt;</code> to point to where the code has
just been compiled.</p>
&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 name=&quot;declare&quot; depends=&quot;build&quot;&gt;
&lt;taskdef name=&quot;mytask&quot;
&lt;target name=&quot;main&quot; depends=&quot;declare&quot;&gt;
&lt;mytask message=&quot;Hello World! MyVeryOwnTask works!&quot;/&gt;
<p>Another way to add a task (more permanently) is to add the task name and implementing class name
to the <samp></samp> file in
the <code class="code"></code> package. Then you can use it as if it
were a built-in task.</p>
<h2 id="buildevents">Build Events</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 class="code">Project</code> object. You can
then call the <code class="code">addBuildListener</code> method to add your listener to the
project. Your listener must implement
the <code class="code"></code> interface. The listener will receive
BuildEvents for the following events</p>
<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>
<p>If the build file invokes another build file
via <a href="Tasks/ant.html"><code>&lt;ant&gt;</code></a>
or <a href="Tasks/subant.html"><code>&lt;subant&gt;</code></a> or
uses <a href="Tasks/antcall.html"><code>&lt;antcall&gt;</code></a>, you are creating a new Ant
"project" that will send target and task level events of its own but never sends build
started/finished events. <em>Since Ant 1.6.2</em>, <code class="code">BuildListener</code> interface
has an extension named <code class="code">SubBuildListener</code> that will receive two new events
<li>SubBuild started</li>
<li>SubBuild finished</li>
<p>If you are interested in those events, all you need to do is to implement the new interface
instead of <code class="code">BuildListener</code> (and register the listener, of course).</p>
<p>If you wish to attach a listener from the command line you may use the <kbd>-listener</kbd>
option. For example:</p>
<pre class="input">ant -listener</pre>
<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
<p><strong>Note</strong>: A listener must not access <code class="code">System.out</code>
and <code class="code">System.err</code> directly since output on these streams is redirected by
Ant's core to the build event system. Accessing these streams can cause an infinite loop in
Ant. Depending on the version of Ant, this will either cause the build to terminate or the JVM to
run out of Stack space. A logger, also, may not access <code class="code">System.out</code>
and <code class="code">System.err</code> directly. It must use the streams with which it has been
<p><strong>Note</strong>: All methods of a <code class="code">BuildListener</code> except for the
"Build Started" and "Build Finished" events may occur on several threads simultaneously&mdash;for
example while Ant is executing a <code>&lt;parallel&gt;</code> task.</p>
<p>Writing an adapter to your favourite log library is very easy. Just implement
the <code class="code">BuildListener</code> interface, instantiate your logger and delegate the
message to that instance.</p>
<p>When starting your build provide your adapter class and the log library to the build classpath
and activate your logger via <kbd>-listener</kbd> option as described above.</p>
public class MyLogAdapter implements BuildListener {
private MyLogger getLogger() {
final MyLogger log = MyLoggerFactory.getLogger(Project.class.getName());
return log;
public void buildStarted(final BuildEvent event) {
final MyLogger log = getLogger();"Build started.");
public void buildFinished(final BuildEvent event) {
final MyLogger logger = getLogger();
MyLogLevelEnum loglevel = ... // map event.getPriority() to enum via Project.MSG_* constants
boolean allOK = event.getException() == null;
String logmessage = ... // create log message using data of the event and the message invoked
logger.log(loglevel, logmessage);
// implement all methods in that way
<h2 id="integration">Source code integration</h2>
<p>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>
<p>Please consult the <a href=""
target="_top">Getting Involved</a> pages on the Apache web site for details on how to fetch the
latest source and how to submit changes for reincorporation into the source tree.</p>
<p>Ant also has some <a href="" target="_top">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.</p>