Tutorial: Writing Tasks

This document provides a step by step tutorial for writing tasks.

This tutorial is part of the Ant-Manual

Set up the build environment

Ant builds itself, we are using Ant too (why we would write a task if not? :-) therefore we should use Ant for our build.

We choose a directory as root directory. All things will be done here if I say nothing different. I will reference this directory as root-directory of our project. In this root-directory we create a text file named build.xml. What should Ant do for us?

So the buildfile contains three targets.

{{{<?xml version="1.0" encoding="ISO-8859-1"?> <project name="MyTask" basedir="." default="jar">

</project> }}}

This buildfile often uses the same value (src, classes, MyTask.jar), so we should rewrite that using <property>s. On second look 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 before. So the refactored code is:

{{{<?xml version="1.0" encoding="ISO-8859-1"?> <project name="MyTask" basedir="." default="jar">

</project>}}}

ant.project.name is one of the build-in properties [1] of Ant.

Write the Task

Now we write the simplest Task - a HelloWorld-Task (what else?). Create a text file HelloWorld.java in the src-directory with:

{{{public class HelloWorld {

} }}}

and we can compile and jar it with  ant  (default target is "jar" and via its depends-clause the "compile" is executed before).

Use the Task

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 <taskdef> [2]. And for easier process we change the default clause:

<?xml version="1.0" encoding="ISO-8859-1"?>
<project name="MyTask" basedir="." default="use">

    ...

    <target name="use" description="Use the Task" depends="jar">
        <taskdef name="helloworld" classname="HelloWorld" classpath="${ant.project.name}.jar"/>
        <helloworld/>
    </target>

</project>

Important is the classpath-attribute. Ant searches in its /lib directory for tasks and our task isn´t there. So we have to provide the right location.

Now we can type in  ant  and all should work ...

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

Integration with TaskAdapter

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 public void execute(). This class is wrapped by Ant´s  org.apache.tools.ant.TaskAdapter  which is a task and uses reflection for setting a reference to the project and calling the execute() method.

Setting a reference to the project? 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:

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);
    }
}

and the execution with ant will show us the expected

use:
Here is project 'MyTask'.

Deriving from Ant´s Task

Ok, that works ... But usually you will extend  org.apache.tools.ant.Task . 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 in the buildfile this task instance is used.

Oki-doki - let´s us use some of these:

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() );
    }
}

which gives us when running

use:
[helloworld] Here is project 'MyTask'.
[helloworld] I am used in: C:\tmp\anttests\MyFirstTask\build.xml:23:

Attributes

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 public void  set<attributename>(<type> newValue)  method and Ant will do the rest via reflection.

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);
    }

}

Oh, what´s that in execute()? Throw a BuildException? 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 null value as parameter and throws a NullPointerException. (Of course you can initialize the message with a default string.)

After that we have to modify our buildfile:

    <target name="use" description="Use the Task" depends="jar">
        <taskdef name="helloworld"
                 classname="HelloWorld"
                 classpath="${ant.project.name}.jar"/>
        <helloworld message="Hello World"/>
    </target>

That´s all.

Some background for working with attributes: Ant supports any of these datatypes as arguments of the set-method:

Before calling the set-method all properties are resolved. So a <helloworld message="${msg}"/> would not set the message string to "${msg}" if there is a property "msg" with a set value.

Nested Text

Maybe you have used the <echo> task in a way like <echo>Hello World</echo>. For that you have to provide a public void addText(String text) method.

...
public class HelloWorld extends Task {
    ...
    public void addText(String text) {
        message = text;
    }
    ...
}

But here properties are not resolved! For resolving properties we have to use Project´s  replaceProperties(String propname) : String  method which takes the property name as argument and returns its value (or ${propname} if not set).

Nested Elements

There are several ways for inserting the ability of handling nested elements. See the Manual [4] for other. We use the first way of the three described ways. There are several steps for that:

import java.util.Vector;
import java.util.Iterator;
...
    public void execute() {
        if (message!=null) log(message);
        for (Iterator it=messages.iterator(); it.hasNext(); ) {      // 4
            Message msg = (Message)it.next();
            log(msg.getMsg());
        }
    }


    Vector messages = new Vector();                                  // 2

    public Message createMessage() {                                 // 3
        Message msg = new Message();
        messages.add(msg);
        return msg;
    }

    public class Message {                                           // 1
        public Message() {}

        String msg;
        public void setMsg(String msg) { this.msg = msg; }
        public String getMsg() { return msg; }
    }
...

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:  public classname createXML-name() . Therefore we write in the buildfile

        <helloworld>
            <message msg="Nested Element 1"/>
            <message msg="Nested Element 2"/>
        </helloworld>

Our task in a little more complex version

For recapitulation now a little refactored buildfile:

<?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>

And the code of the task:

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 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 false. 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; }
    }

}

And it works:

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>

Next step: test ...

Test the Task

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  org.apache.tools.ant.BuildFileTest . This class extends  junit.framework.TestCase  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 ...

In Ant it is usual that the testcase has the same name as the task with a prepending Test, therefore we will create a file HelloWorldTest.java. Because we have a very small project we can put this file into src 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 http://gump.covalent.net/jars/latest/ant/ant-testutil.jar [5].

For executing the test and creating a report we need the optional tasks <junit> and <junitreport>. So we add to the buildfile:

...
<project name="MyTask" basedir="." default="test">
...
    <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>

    <target name="compile" description="Compiles the Task">
        <mkdir dir="${classes.dir}"/>
        <javac srcdir="${src.dir}" destdir="${classes.dir}" classpath="${ant.test.lib}"/>
    </target>
...
    <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"
    />
...

Back to the src/HelloWorldTest.java. We create a class extending BuildFileTest with String-constructor (JUnit-standard), a setUp() method initializing Ant and for each testcase (targets use.*) a testXX() method invoking that target.

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");
    }
}

When starting  ant  we´ll get a short message to STDOUT and a nice HTML-report.

:\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>

Resources

This tutorial and its resources are available via BugZilla [6]. The ZIP provided there contains

{{{Used Links:

[1] http://ant.apache.org/manual/using.html#built-in-props [2] http://ant.apache.org/manual/CoreTasks/taskdef.html [3] http://ant.apache.org/manual/develop.html#set-magic [4] http://ant.apache.org/manual/develop.html#nested-elements [5] http://gump.covalent.net/jars/latest/ant/ant-testutil.jar [6] http://issues.apache.org/bugzilla/show_bug.cgi?id=22570 }}}

AntTutorialWritingTasks (last edited 2009-09-20 22:07:42 by localhost)