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See the License for the specific language governing permissions and limitations under the License. --> <html lang="en"> <head> <link rel="stylesheet" type="text/css" href="stylesheets/style.css"> <title>Writing Your Own Task</title> </head> <body> <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> <ol> <li>Create a Java class that extends <code class="code">org.apache.tools.ant.Task</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">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 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> </ol> <hr/> <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> <ol> <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> <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> <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> variable.</li> <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> </ol> <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 is</p> <ul> <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> otherwise.</li> <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">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 <var>basedir</var>.</li> <li><code class="code">org.apache.tools.ant.types.Resource</code>, Ant will resolve the string as a <code class="code">java.io.File</code> as above, then pass in as a <code class="code">org.apache.tools.ant.types.resources.FileResource</code>. <em>Since Ant 1.8</em></li> <li><code class="code">org.apache.tools.ant.types.Path</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 file.</li> <li>A subclass of <code class="code">org.apache.tools.ant.types.EnumeratedAttribute</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/FixCRLF.java</code> and the inner <code class="code">AddAsisRemove</code> class used in <code class="code">setCr</code> for an example.</li> <li>A (Java 5) 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, but of course your task will not run on JDK 1.4 or earlier. 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> </ul> <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">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. <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><inner></code> elements, you have three options:</p> <ol> <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> </ol> <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><typedef></code> you have two options.</p> <ol> <li><code class="code">public void add(Type type)</code></li> <li><code class="code">public void addConfigured(Type type)</code></li> </ol> <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">org.apache.tools.ant.taskdefs.condition.Condition</code>, one may have a class:</p> <pre> public class MyTask extends Task { private List conditions = new ArrayList(); public void add(Condition c) { conditions.add(c); } public void execute() { // iterator over the conditions } }</pre> <p>One may define and use this class like this:</p> <pre> <taskdef name="mytask" classname="MyTask" classpath="classes"/> <typedef name="condition.equals" classname="org.apache.tools.ant.taskdefs.conditions.Equals"/> <mytask> <condition.equals arg1="${debug}" arg2="true"/> </mytask></pre> <p>A more complicated example follows:</p> <pre> public class Sample { public static class MyFileSelector implements FileSelector { public void setAttrA(int a) {} public void setAttrB(int b) {} public void add(Path path) {} public boolean isSelected(File basedir, String filename, File file) { return true; } } interface MyInterface { void setVerbose(boolean val); } public static class BuildPath extends Path { public BuildPath(Project project) { super(project); } public void add(MyInterface inter) {} public void setUrl(String url) {} } public static class XInterface implements MyInterface { public void setVerbose(boolean x) {} public void setCount(int c) {} } }</pre> <p>This class defines a number of static classes that implement/extend <code class="code">Path</code>, <code class="code">MyFileSelector</code> and <code class="code">MyInterface</code>. These may be defined and used as follows:</p> <pre> <typedef name="myfileselector" classname="Sample$MyFileSelector" classpath="classes" loaderref="classes"/> <typedef name="buildpath" classname="Sample$BuildPath" classpath="classes" loaderref="classes"/> <typedef name="xinterface" classname="Sample$XInterface" classpath="classes" loaderref="classes"/> <copy todir="copy-classes"> <fileset dir="classes"> <myfileselector attra="10" attrB="-10"> <buildpath path="." url="abc"> <xinterface count="4"/> </buildpath> </myfileselector> </fileset> </copy></pre> <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">org.apache.tools.ant.Task</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> <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> <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 "message" attribute public void setMessage(String msg) { this.msg = msg; } }</pre> <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><taskdef></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> <pre> <?xml version="1.0"?> <project name="OwnTaskExample" default="main" basedir="."> <taskdef name="mytask" classname="com.mydomain.MyVeryOwnTask"/> <target name="main"> <mytask message="Hello World! MyVeryOwnTask works!"/> </target> </project></pre> <h3>Example 2</h3> <p>To use a task directly from the buildfile which created it, place the <code><taskdef></code> declaration inside a target <em>after the compilation</em>. Use the <var>classpath</var> attribute of <code><taskdef></code> to point to where the code has just been compiled.</p> <pre> <?xml version="1.0"?> <project name="OwnTaskExample2" default="main" basedir="."> <target name="build" > <mkdir dir="build"/> <javac srcdir="source" destdir="build"/> </target> <target name="declare" depends="build"> <taskdef name="mytask" classname="com.mydomain.MyVeryOwnTask" classpath="build"/> </target> <target name="main" depends="declare"> <mytask message="Hello World! MyVeryOwnTask works!"/> </target> </project></pre> <p>Another way to add a task (more permanently) is to add the task name and implementing class name to the <samp>default.properties</samp> file in the <code class="code">org.apache.tools.ant.taskdefs</code> package. Then you can use it as if it were a built-in task.</p> <hr/> <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">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 the build file invokes another build file via <a href="Tasks/ant.html"><code><ant></code></a> or <a href="Tasks/subant.html"><code><subant></code></a> or uses <a href="Tasks/antcall.html"><code><antcall></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 for</p> <ul> <li>SubBuild started</li> <li>SubBuild finished</li> </ul> <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 org.apache.tools.ant.XmlLogger</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 output.</p> <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 configured.</p> <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—for example while Ant is executing a <code><parallel></code> task.</p> <h3>Example</h3> <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> <pre> public class MyLogAdapter implements BuildListener { private MyLogger getLogger() { final MyLogger log = MyLoggerFactory.getLogger(Project.class.getName()); return log; } @Override public void buildStarted(final BuildEvent event) { final MyLogger log = getLogger(); log.info("Build started."); } @Override 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 }</pre> <hr/> <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="https://www.apache.org/foundation/getinvolved.html" 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="https://ant.apache.org/ant_task_guidelines.html" 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> </body> </html>