Tutorial: Writing Tasks

Jan Matèrne, 2003-09-01

This document provides a tutorial for writing tasks.

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 names build.xml. What should Ant do for us?

• compiles my stuff
• make the jar, so that I can deploy it
• clean up everything

So the buildfile contains three targets.

{{{ <?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> }}}

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 before. So the refactored code is:

{{{ <?xml version="1.0" encoding="ISO-8859-1"?>

• <project name="MyTask" basedir="." default="jar">

  • <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>

  </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 {

• public void execute() {
  • System.out.println("Hello World");
  }

• } }}}

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:

• elementary data type like int, long, ...
• its wrapper classes like java.lang.Integer, java.lang.Long, ...
• java.lang.String

• some more classes (e.g. java.io.File; see Manual 'Writing Your Own Task' [3])

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:

• 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).

• The task holds multiple instances of this class in a list.
• A factory method instantiates an object, saves the reference in the list and returns it to Ant Core.
• The execute() method iterates over the list and evaluates its values.

{{{ 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´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 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.

{{{ 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> }}}

Resources

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

• this tutorial
• the buildfile (last version)
• the source of the task (last version)
• the source of the unit test (last version)
• the ant-testutil.jar (nightly build of 2003-08-18)
• generated classes
• generated jar
• generated reports

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://nagoya.apache.org/bugzilla/show_bug.cgi?id=22570 }}}