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<title>Tutorial: Writing Tasks</title>
<meta name="author" content="Jan Matèrne">
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<h1>Tutorial: Writing Tasks</h1>
<p>This document provides a step by step tutorial for writing
tasks.</p>
<h2>Content</h2>
<p><ul>
<li><a href="#buildenvironment">Set up the build environment</a></li>
<li><a href="#write1">Write the Task</a></li>
<li><a href="#use1">Use the Task</a></li>
<li><a href="#TaskAdapter">Integration with TaskAdapter</a></li>
<li><a href="#derivingFromTask">Deriving from Ant´s Task</a></li>
<li><a href="#attributes">Attributes</a></li>
<li><a href="#NestedText">Nested Text</a></li>
<li><a href="#NestedElements">Nested Elements</a></li>
<li><a href="#complex">Our task in a little more complex version</a></li>
<li><a href="#TestingTasks">Test the Task</a></li>
<li><a href="#resources">Resources</a></li>
</ul></p>
<a name="buildenvironment"/>
<h2>Set up the build environment</h2>
<p>Ant builds itself, we are using Ant too (why we would write
a task if not? :-) therefore we should use Ant for our build.<p>
<p>We choose a directory as root directory. All things will be done
here if I say nothing different. I will reference this directory
as <i>root-directory</i> of our project. In this root-directory we
create a text file names <i>build.xml</i>. What should Ant do for us?
<ul>
<li>compiles my stuff</li>
<li>make the jar, so that I can deploy it</li>
<li>clean up everything</li>
</ul>
So the buildfile contains three targets.
<pre class="code">
<?xml version="1.0" encoding="ISO-8859-1"?>
<project name="MyTask" basedir="." default="jar">
<target name="clean" description="Delete all generated files">
<delete dir="classes"/>
<delete file="MyTasks.jar"/>
</target>
<target name="compile" description="Compiles the Task">
<javac srcdir="src" destdir="classes"/>
</target>
<target name="jar" description="JARs the Task">
<jar destfile="MyTask.jar" basedir="classes"/>
</target>
</project>
</pre>
This buildfile uses often the same value (src, classes, MyTask.jar), so we should rewrite that
using <property>s. On second there are some handicaps: <javac> requires that the destination
directory exists; a call of "clean" with a non existing classes directory will fail; "jar" requires
the execution of some steps bofore. So the refactored code is:
<pre class="code">
<?xml version="1.0" encoding="ISO-8859-1"?>
<project name="MyTask" basedir="." default="jar">
<b><property name="src.dir" value="src"/></b>
<b><property name="classes.dir" value="classes"/></b>
<target name="clean" description="Delete all generated files">
<delete dir="<b>${classes.dir}</b>" <b>failonerror="false"</b>/>
<delete file="<b>${ant.project.name}.jar</b>"/>
</target>
<target name="compile" description="Compiles the Task">
<b><mkdir dir="${classes.dir}"/></b>
<javac srcdir="<b>${src.dir}</b>" destdir="${classes.dir}"/>
</target>
<target name="jar" description="JARs the Task" <b>depends="compile"</b>>
<jar destfile="${ant.project.name}.jar" basedir="${classes.dir}"/>
</target>
</project>
</pre>
<i>ant.project.name</i> is one of the
<a href="http://ant.apache.org/manual/using.html#built-in-props" target="_blank">
build-in properties [1]</a> of Ant.
<a name="write1"/>
<h2>Write the Task</h2>
Now we write the simplest Task - a HelloWorld-Task (what else?). Create a text file
<i>HelloWorld.java</i> in the src-directory with:
<pre class="code">
public class HelloWorld {
public void execute() {
System.out.println("Hello World");
}
}
</pre>
and we can compile and jar it with <tt>ant</tt> (default target is "jar" and via
its <i>depends</i>-clause the "compile" is executed before).
<a name="use1"/>
<h2>Use the Task</h2>
<p>But after creating the jar we want to use our new Task. Therefore we need a
new target "use". Before we can use our new task we have to declare it with
<a href="http://ant.apache.org/manual/CoreTasks/taskdef.html" target="_blank">
<taskdef> [2]</a>. And for easier process we change the default clause:
<pre class="code">
<?xml version="1.0" encoding="ISO-8859-1"?>
<project name="MyTask" basedir="." default="<b>use</b>">
...
<b><target name="use" description="Use the Task" depends="jar">
<taskdef name="helloworld" classname="HelloWorld" classpath="${ant.project.name}.jar"/>
<helloworld/>
</target></b>
</project>
</pre>
Important is the <i>classpath</i>-attribute. Ant searches in its /lib directory for
tasks and our task isn´t there. So we have to provide the right location. </p>
<p>Now we can type in <tt>ant</tt> and all should work ...
<pre class="output">
Buildfile: build.xml
compile:
[mkdir] Created dir: C:\tmp\anttests\MyFirstTask\classes
[javac] Compiling 1 source file to C:\tmp\anttests\MyFirstTask\classes
jar:
[jar] Building jar: C:\tmp\anttests\MyFirstTask\MyTask.jar
use:
[helloworld] Hello World
BUILD SUCCESSFUL
Total time: 3 seconds
</pre>
<a name="TaskAdapter"/>
<h2>Integration with TaskAdapter</h2>
<p>Our class has nothing to do with Ant. It extends no superclass and implements
no interface. How does Ant know to integrate? Via name convention: our class provides
a method with signature <tt>public void execute()</tt>. This class is wrapped by Ant´s
<tt>org.apache.tools.ant.TaskAdapter</tt> which is a task and uses reflection for
setting a reference to the project and calling the <i>execute()</i> method.</p>
<p><i>Setting a reference to the project</i>? Could be interesting. The Project class
gives us some nice abilities: access to Ant´s logging facilities getting and setting
properties and much more. So we try to use that class:
<pre class="code">
import org.apache.tools.ant.Project;
public class HelloWorld {
private Project project;
public void setProject(Project proj) {
project = proj;
}
public void execute() {
String message = project.getProperty("ant.project.name");
project.log("Here is project '" + message + "'.", Project.MSG_INFO);
}
}
</pre>
and the execution with <tt>ant</tt> will show us the expected
<pre class="output">
use:
Here is project 'MyTask'.
</pre></p>
<a name="derivingFromTask"/>
<h2>Deriving from Ant´s Task</h2>
<p>Ok, that works ... But usually you will extend <tt>org.apache.tools.ant.Task</tt>.
That class is integrated in Ant, get´s the project-reference, provides documentation
fiels, provides easier access to the logging facility and (very useful) gives you
the exact location where <i>in the buildfile</i> this task instance is used.</p>
<p>Oki-doki - let´s us use some of these:
<pre class="code">
import org.apache.tools.ant.Task;
public class HelloWorld extends Task {
public void execute() {
// use of the reference to Project-instance
String message = getProject().getProperty("ant.project.name");
// Task´s log method
log("Here is project '" + message + "'.");
// where this task is used?
log("I am used in: " + getLocation() );
}
}
</pre>
which gives us when running
<pre class="output">
use:
[helloworld] Here is project 'MyTask'.
[helloworld] I am used in: C:\tmp\anttests\MyFirstTask\build.xml:23:
</pre>
<a name="attributes">
<h2>Attributes</h2>
<p>Now we want to specify the text of our message (it seems that we are
rewriting the <echo/> task :-). First we well do that with an attribute.
It is very easy - for each attribute provide a <tt>public void set<attributename>(<type>
newValue)</tt> method and Ant will do the rest via reflection.</p>
<pre class="code">
import org.apache.tools.ant.Task;
import org.apache.tools.ant.BuildException;
public class HelloWorld extends Task {
String message;
public void setMessage(String msg) {
message = msg;
}
public void execute() {
if (message==null) {
throw new BuildException("No message set.");
}
log(message);
}
}
</pre>
<p>Oh, what´s that in execute()? Throw a <i>BuildException</i>? Yes, that´s the usual
way to show Ant that something important is missed and complete build should fail. The
string provided there is written as build-failes-message. Here it´s necessary because
the log() method can´t handle a <i>null</i> value as parameter and throws a NullPointerException.
(Of course you can initialize the <i>message</i> with a default string.)</p>
<p>After that we have to modify our buildfile:
<pre class="code">
<target name="use" description="Use the Task" depends="jar">
<taskdef name="helloworld"
classname="HelloWorld"
classpath="${ant.project.name}.jar"/>
<helloworld <b>message="Hello World"</b>/>
</target>
</pre>
That´s all.</p>
<p>Some background for working with attributes: Ant supports any of these datatypes as
arguments of the set-method:<ul>
<li>elementary data type like <i>int</i>, <i>long</i>, ...</li>
<li>its wrapper classes like <i>java.lang.Integer</i>, <i>java.lang.Long</i>, ...</li>
<li><i>java.lang.String</i></li>
<li>some more classes (e.g. <i>java.io.File</i>; see
<a href="http://ant.apache.org/manual/develop.html#set-magic">Manual
'Writing Your Own Task' [3]</a>)</li>
</ul>
Before calling the set-method all properties are resolved. So a <tt><helloworld message="${msg}"/></tt>
would not set the message string to "${msg}" if there is a property "msg" with a set value.
<a name="NestedText"/>
<h2>Nested Text</h2>
<p>Maybe you have used the <echo> task in a way like <tt><echo>Hello World</echo></tt>.
For that you have to provide a <tt>public void addText(String text)</tt> method.
<pre class="code">
...
public class HelloWorld extends Task {
...
public void addText(String text) {
message = text;
}
...
}
</pre>
But here properties are <b>not</b> resolved! For resolving properties we have to use
Project´s <tt>replaceProperties(String propname) : String</tt> method which takes the
property name as argument and returns its value (or ${propname} if not set).</p>
<a name="NestedElements"/>
<h2>Nested Elements</h2>
<p>There are several ways for inserting the ability of handling nested elements. See
the <a href="http://ant.apache.org/manual/develop.html#nested-elements">Manual [4]</a> for other.
We use the first way of the three described ways. There are several steps for that:<ol>
<li>We create a class for collecting all the infos the nested element should contain.
This class is created by the same rules for attributes and nested elements
as for the task (set<attributename>() methods). </li>
<li>The task holds multiple instances of this class in a list.</li>
<li>A factory method instantiates an object, saves the reference in the list
and returns it to Ant Core.</li>
<li>The execute() method iterates over the list and evaluates its values.</li>
</li></p>
<pre class="code">
import java.util.Vector;
import java.util.Iterator;
...
public void execute() {
if (message!=null) log(message);
for (Iterator it=messages.iterator(); it.hasNext(); ) { <b>// 4</b>
Message msg = (Message)it.next();
log(msg.getMsg());
}
}
Vector messages = new Vector(); <b>// 2</b>
public Message createMessage() { <b>// 3</b>
Message msg = new Message();
messages.add(msg);
return msg;
}
public class Message { <b>// 1</b>
public Message() {}
String msg;
public void setMsg(String msg) { this.msg = msg; }
public String getMsg() { return msg; }
}
...
</pre>
<p>Then we can use the new nested element. But where is xml-name for that defined?
The mapping XML-name : classname is defined in the factory method:
<tt>public <i>classname</i> create<i>XML-name</i>()</tt>. Therefore we write in
the buildfile
<pre class="code">
<helloworld>
<message msg="Nested Element 1"/>
<message msg="Nested Element 2"/>
</helloworld>
</pre>
<a name="complex"/>
<h2>Our task in a little more complex version</h2>
<p>For recapitulation now a little refactored buildfile:
<pre class="code">
<?xml version="1.0" encoding="ISO-8859-1"?>
<project name="MyTask" basedir="." default="use">
<property name="src.dir" value="src"/>
<property name="classes.dir" value="classes"/>
<target name="clean" description="Delete all generated files">
<delete dir="${classes.dir}" failonerror="false"/>
<delete file="${ant.project.name}.jar"/>
</target>
<target name="compile" description="Compiles the Task">
<mkdir dir="${classes.dir}"/>
<javac srcdir="${src.dir}" destdir="${classes.dir}"/>
</target>
<target name="jar" description="JARs the Task" depends="compile">
<jar destfile="${ant.project.name}.jar" basedir="${classes.dir}"/>
</target>
<target name="use.init"
description="Taskdef the HelloWorld-Task"
depends="jar">
<taskdef name="helloworld"
classname="HelloWorld"
classpath="${ant.project.name}.jar"/>
</target>
<target name="use.without"
description="Use without any"
depends="use.init">
<helloworld/>
</target>
<target name="use.message"
description="Use with attribute 'message'"
depends="use.init">
<helloworld message="attribute-text"/>
</target>
<target name="use.fail"
description="Use with attribute 'fail'"
depends="use.init">
<helloworld fail="true"/>
</target>
<target name="use.nestedText"
description="Use with nested text"
depends="use.init">
<helloworld>nested-text</helloworld>
</target>
<target name="use.nestedElement"
description="Use with nested 'message'"
depends="use.init">
<helloworld>
<message msg="Nested Element 1"/>
<message msg="Nested Element 2"/>
</helloworld>
</target>
<target name="use"
description="Try all (w/out use.fail)"
depends="use.without,use.message,use.nestedText,use.nestedElement"
/>
</project>
</pre>
And the code of the task:
<pre class="code">
import org.apache.tools.ant.Task;
import org.apache.tools.ant.BuildException;
import java.util.Vector;
import java.util.Iterator;
/**
* The task of the tutorial.
* Print´s a message or let the build fail.
* @author Jan Matèrne
* @since 2003-08-19
*/
public class HelloWorld extends Task {
/** The message to print. As attribute. */
String message;
public void setMessage(String msg) {
message = msg;
}
/** Should the build fail? Defaults to <i>false</i>. As attribute. */
boolean fail = false;
public void setFail(boolean b) {
fail = b;
}
/** Support for nested text. */
public void addText(String text) {
message = text;
}
/** Do the work. */
public void execute() {
// handle attribute 'fail'
if (fail) throw new BuildException("Fail requested.");
// handle attribute 'message' and nested text
if (message!=null) log(message);
// handle nested elements
for (Iterator it=messages.iterator(); it.hasNext(); ) {
Message msg = (Message)it.next();
log(msg.getMsg());
}
}
/** Store nested 'message's. */
Vector messages = new Vector();
/** Factory method for creating nested 'message's. */
public Message createMessage() {
Message msg = new Message();
messages.add(msg);
return msg;
}
/** A nested 'message'. */
public class Message {
// Bean constructor
public Message() {}
/** Message to print. */
String msg;
public void setMsg(String msg) { this.msg = msg; }
public String getMsg() { return msg; }
}
}
</pre>
And it works:
<pre class="output">
C:\tmp\anttests\MyFirstTask>ant
Buildfile: build.xml
compile:
[mkdir] Created dir: C:\tmp\anttests\MyFirstTask\classes
[javac] Compiling 1 source file to C:\tmp\anttests\MyFirstTask\classes
jar:
[jar] Building jar: C:\tmp\anttests\MyFirstTask\MyTask.jar
use.init:
use.without:
use.message:
[helloworld] attribute-text
use.nestedText:
[helloworld] nested-text
use.nestedElement:
[helloworld]
[helloworld]
[helloworld]
[helloworld]
[helloworld] Nested Element 1
[helloworld] Nested Element 2
use:
BUILD SUCCESSFUL
Total time: 3 seconds
C:\tmp\anttests\MyFirstTask>ant use.fail
Buildfile: build.xml
compile:
jar:
use.init:
use.fail:
BUILD FAILED
C:\tmp\anttests\MyFirstTask\build.xml:36: Fail requested.
Total time: 1 second
C:\tmp\anttests\MyFirstTask>
</pre>
Next step: test ...
<a name="TestingTasks"/>
<h2>Test the Task</h2>
<p>We have written a test already: the use.* tasks in the buildfile. But its
difficult to test that automatically. Common (and in Ant) used is JUnit for
that. For testing tasks Ant provides a baseclass <tt>org.apache.tools.ant.BuildFileTest</tt>.
This class extends <tt>junit.framework.TestCase</tt> and can therefore be integrated
into the unit tests. But this class provides some for testing tasks useful methods:
initialize Ant, load a buildfile, execute targets,
expecting BuildExceptions with a specified text, expect a special text
in the output log ... </p>
<p>In Ant it is usual that the testcase has the same name as the task with a prepending
<i>Test</i>, therefore we will create a file <i>HelloWorldTest.java</i>. Because we
have a very small project we can put this file into <i>src</i> directory (Ant´s own
testclasses are in /src/testcases/...). Because we have already written our tests
for "hand-test" we can use that for automatic tests, too. But there is one little
problem we have to solve: all test supporting classes are not part of the binary
distribution of Ant. So you can build the special jar file from source distro with
target "test-jar" or you can download a nightly build from
<a href="http://gump.covalent.net/jars/latest/ant/ant-testutil.jar">
http://gump.covalent.net/jars/latest/ant/ant-testutil.jar [5]</a>.</p>
<p>For executing the test and creating a report we need the optional tasks <junit>
and <junitreport>. So we add to the buildfile:
<pre class="code">
...
<font color="#9F9F9F"><project name="MyTask" basedir="." </font>default="test"<font color="#9F9F9F">></font>
...
<property name="ant.test.lib" value="ant-testutil.jar"/>
<property name="report.dir" value="report"/>
<property name="junit.out.dir.xml" value="${report.dir}/junit/xml"/>
<property name="junit.out.dir.html" value="${report.dir}/junit/html"/>
<path id="classpath.run">
<path path="${java.class.path}"/>
<path location="${ant.project.name}.jar"/>
</path>
<path id="classpath.test">
<path refid="classpath.run"/>
<path location="${ant.test.lib}"/>
</path>
<target name="clean" description="Delete all generated files">
<delete failonerror="false" includeEmptyDirs="true">
<fileset dir="." includes="${ant.project.name}.jar"/>
<fileset dir="${classes.dir}"/>
<fileset dir="${report.dir}"/>
</delete>
</target>
<font color="#9F9F9F"><target name="compile" description="Compiles the Task">
<mkdir dir="${classes.dir}"/>
<javac srcdir="${src.dir}" destdir="${classes.dir}" </font>classpath="${ant.test.lib}"<font color="#9F9F9F">/>
</target></font>
...
<target name="junit" description="Runs the unit tests" depends="jar">
<delete dir="${junit.out.dir.xml}" />
<mkdir dir="${junit.out.dir.xml}" />
<junit printsummary="yes" haltonfailure="no">
<classpath refid="classpath.test"/>
<formatter type="xml"/>
<batchtest fork="yes" todir="${junit.out.dir.xml}">
<fileset dir="${src.dir}" includes="**/*Test.java"/>
</batchtest>
</junit>
</target>
<target name="junitreport" description="Create a report for the rest result">
<mkdir dir="${junit.out.dir.html}" />
<junitreport todir="${junit.out.dir.html}">
<fileset dir="${junit.out.dir.xml}">
<include name="*.xml"/>
</fileset>
<report format="frames" todir="${junit.out.dir.html}"/>
</junitreport>
</target>
<target name="test"
depends="junit,junitreport"
description="Runs unit tests and creates a report"
/>
...
</pre></p>
<p>Back to the <i>src/HelloWorldTest.java</i>. We create a class extending
<i>BuildFileTest</i> with String-constructor (JUnit-standard), a <i>setUp()</i>
method initializing Ant and for each testcase (targets use.*) a <i>testXX()</i>
method invoking that target.
<pre class="code">
import org.apache.tools.ant.BuildFileTest;
public class HelloWorldTest extends BuildFileTest {
public HelloWorldTest(String s) {
super(s);
}
public void setUp() {
// initialize Ant
configureProject("build.xml");
}
public void testWithout() {
executeTarget("use.without");
assertEquals("Message was logged but should not.", getLog(), "");
}
public void testMessage() {
// execute target 'use.nestedText' and expect a message
// 'attribute-text' in the log
expectLog("use.message", "attribute-text");
}
public void testFail() {
// execute target 'use.fail' and expect a BuildException
// with text 'Fail requested.'
expectBuildException("use.fail", "Fail requested.");
}
public void testNestedText() {
expectLog("use.nestedText", "nested-text");
}
public void testNestedElement() {
executeTarget("use.nestedElement");
assertLogContaining("Nested Element 1");
assertLogContaining("Nested Element 2");
}
}
</pre></p>
<p>When starting <tt>ant</tt> we´ll get a short message to STDOUT and
a nice HTML-report.
<pre class="output">
C:\tmp\anttests\MyFirstTask>ant
Buildfile: build.xml
compile:
[mkdir] Created dir: C:\tmp\anttests\MyFirstTask\classes
[javac] Compiling 2 source files to C:\tmp\anttests\MyFirstTask\classes
jar:
[jar] Building jar: C:\tmp\anttests\MyFirstTask\MyTask.jar
junit:
[mkdir] Created dir: C:\tmp\anttests\MyFirstTask\report\junit\xml
[junit] Running HelloWorldTest
[junit] Tests run: 5, Failures: 0, Errors: 0, Time elapsed: 2,334 sec
junitreport:
[mkdir] Created dir: C:\tmp\anttests\MyFirstTask\report\junit\html
[junitreport] Using Xalan version: Xalan Java 2.4.1
[junitreport] Transform time: 661ms
test:
BUILD SUCCESSFUL
Total time: 7 seconds
C:\tmp\anttests\MyFirstTask>
</pre></p>
<a name="resources"/>
<h2>Resources</h2>
<p>This tutorial and its resources are available via
<a href="http://nagoya.apache.org/bugzilla/show_bug.cgi?id=22570">BugZilla [6]</a>.
The ZIP provided there contains<ul>
<li>this tutorial</li>
<li>the buildfile (last version)</li>
<li>the source of the task (last version)</li>
<li>the source of the unit test (last version)</li>
<li>the ant-testutil.jar (nightly build of 2003-08-18)</li>
<li>generated classes</li>
<li>generated jar</li>
<li>generated reports</li>
</ul>
The last sources and the buildfile are also available
<a href="tutorial-writing-tasks-src.zip">here [7]</a> inside the manual.
</p>
Used Links:<br/>
[1] <a href="http://ant.apache.org/manual/using.html#built-in-props">http://ant.apache.org/manual/using.html#built-in-props</a><br/>
[2] <a href="http://ant.apache.org/manual/CoreTasks/taskdef.html">http://ant.apache.org/manual/CoreTasks/taskdef.html</a><br/>
[3] <a href="http://ant.apache.org/manual/develop.html#set-magic">http://ant.apache.org/manual/develop.html#set-magic</a><br/>
[4] <a href="http://ant.apache.org/manual/develop.html#nested-elements">http://ant.apache.org/manual/develop.html#nested-elements</a><br/>
[5] <a href="http://gump.covalent.net/jars/latest/ant/ant-testutil.jar">http://gump.covalent.net/jars/latest/ant/ant-testutil.jar</a><br/>
[6] <a href="http://nagoya.apache.org/bugzilla/show_bug.cgi?id=22570">http://nagoya.apache.org/bugzilla/show_bug.cgi?id=22570</a><br/>
[7] <a href="tutorial-writing-tasks-src.zip">tutorial-writing-tasks-src.zip</a><br/>
<hr>
<p align="center">Copyright © 2003 Apache Software Foundation. All rights
Reserved.</p>
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