Creating a Maven Plugin with Ant

While Maven is an improvement on Ant, Ant can still be useful when describing parts of the build process. Ant provides a set of tasks which can come in handy when you need to perform file operations or XSLT transformations or any other operation you could think of. There is a large library of available Ant tasks for everything from running JUnit tests to transforming XML to copying files to a remote server using SCP. An overview of available Ant tasks can be found online in the Apache Ant Manual. You can use these tasks as a low-level build customization language, and you can also write a Maven plugin where, instead of a Mojo written in Java, you can pass parameters to a Mojo which is an Ant build target.

To create a Maven plugin using Ant, you will need to have a pom.xml and a single Mojo implemented in Ant. To get started, create a project directory named firstant-maven-plugin. Place the following pom.xml in this directory.

<project>
  <modelVersion>4.0.0</modelVersion>
  <groupId>org.sonatype.mavenbook.plugins</groupId>
  <artifactId>firstant-maven-plugin</artifactId>
  <name>Example Ant Mojo - firstant-maven-plugin</name>
  <packaging>maven-plugin</packaging>
  <version>1.0-SNAPSHOT</version>
  <dependencies>
    <dependency>
      <groupId>org.apache.maven</groupId>
      <artifactId>maven-script-ant</artifactId>
      <version>${maven.version}</version>
    </dependency>
  </dependencies>
  <build>
    <plugins>
      <plugin>
        <artifactId>maven-plugin-plugin</artifactId>
        <version>2.4</version>
        <dependencies>
          <dependency>
            <groupId>org.apache.maven.plugin-tools</groupId>
            <artifactId>maven-plugin-tools-ant</artifactId>
            <version>2.4</version>
          </dependency>
        </dependencies>
      </plugin>
    </plugins>
  </build>
</project>

Next, you will need to create your Ant Mojo. An Ant mojo consists of two parts: the Ant tasks in an XML file, and a file which supplies Mojo descriptor information. The Ant plugin tools are going to look for both of these files in ${basedir}/src/main/scripts. One file will be named echo.build.xml and it will contain the Ant XML.

<project>
  <target name="echotarget">
    <echo>${message}</echo>
  </target>
</project>

The other file will describe the Echo Ant Mojo and will be in the echo.mojos.xml file also in ${basedir}/src/main/scripts.

<pluginMetadata>
  <mojos>
    <mojo>
      <goal>echo</goal>
      <call>echotarget</call>
      <description>Echos a Message</description>
      <parameters>
        <parameter>
          <name>message</name>
          <property>message</property>
          <required>false</required>
          <expression>${message}</expression>
          <type>java.lang.Object</type>
          <defaultValue>Hello Maven World</defaultValue>
          <description>Prints a message</description>
        </parameter>
      </parameters>
    </mojo>
  </mojos>
</pluginMetadata>

This echo.mojos.xml file configures the Mojo descriptor for this plugin. It supplies the goal name "echo", and it tells Maven what Ant task to call in the call element. In addition to configuring the description, this XML file configures the message parameter to use the expression ${message} and to have a default value of "Hello Maven World."

If you've configured your plugin groups in ~/.m2/settings.xml to include org.sonatype.mavenbook.plugins, you can install this Ant plugin by executing the following command at the command-line:

$ mvn install
[INFO] ------------------------------------------------------------------------
[INFO] Building Example Ant Mojo - firstant-maven-plugin
[INFO]    task-segment: [install]
[INFO] ------------------------------------------------------------------------
[INFO] [plugin:descriptor]
[INFO] Using 3 extractors.
[INFO] Applying extractor for language: java
[INFO] Extractor for language: java found 0 mojo descriptors.
[INFO] Applying extractor for language: bsh
[INFO] Extractor for language: bsh found 0 mojo descriptors.
[INFO] Applying extractor for language: ant
[INFO] Extractor for language: ant found 1 mojo descriptors.
...
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESSFUL
[INFO] ------------------------------------------------------------------------

Note that the plugin:descriptor goal found a single Ant mojo descriptor. To run this goal, you would execute the following command-line:

$ mvn firstant:echo
...
[INFO] [firstant:echo]
echotarget:
     [echo] Hello Maven World
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESSFUL
[INFO] ------------------------------------------------------------------------

The echo goal executed and printed out the default value of the message parameter. If you are used to Apache Ant build scripts, you will notice that Ant prints out the name of the target executed and then adds a logging prefix to the output of the echo Ant task.

While Maven is an improvement on Ant, Ant can still be useful when describing parts of the build process. Ant provides a set of tasks which can come in handy when you need to perform file operations or XSLT transformations or any other operation you could think of. There is a large library of available Ant tasks for everything from running JUnit tests to transforming XML to copying files to a remote server using SCP. An overview of available Ant tasks can be found online in the Apache Ant Manual. You can use these tasks as a low-level build customization language, and you can also write a Maven plugin where, instead of a Mojo written in Java, you can pass parameters to a Mojo which is an Ant build target.