Wednesday, January 8, 2014

Evolving Gradle Build from Ant Build: Importing Ant Build File

Changing the build system on a large project can be difficult and a lot of work. Fortunately for those migrating Ant builds to Gradle builds, Gradle provides particularly convenient mechanisms to facilitate this migration. Because Gradle is built on Groovy and Groovy includes built-in Ant support via AntBuilder, Gradle builds can use AntBuilder to call Ant tasks and run Ant targets. However, Gradle provides an even easier mechanism for referencing existing Ant targets from a Gradle build with Gradle's support for importing an Ant build via DefaultAntBuilder and that is the subject of this post.

Being able to call existing Ant targets from a new Gradle build is advantageous because it allows the migration to take place over time. One can start using Gradle almost immediately with all the real work delegated to the existing Ant build. Then, as time and priorities allow, different Ant tasks can be replaced with Gradle tasks.

To demonstrate how easy it is to import an Ant build in a Gradle build, I first provide the code listing for a simplified Ant build.

Ant Build File: build.xml
<?xml version="1.0" encoding="UTF-8"?>
<project name="JavaArrays" default="all" basedir=".">
   <description>Java Array Utility Functions</description>

   <property name="javac.debug" value="true" />
   <property name="src.dir" value="src" />
   <property name="dist.dir" value="dist" />
   <property name="classes.dir" value="classes" />
   <property name="javadoc.dir" value="${dist.dir}/javadoc" />

   <property name="jar.name" value="javaArrays.jar" />
   <property name="jar.filesonly" value="true" />

   <path id="classpath">
   </path>

   <target name="-init">
      <mkdir dir="${classes.dir}" />
      <mkdir dir="${dist.dir}" />
   </target>

   <target name="compile"
           description="Compile the Java code."
           depends="-init">
      <javac srcdir="${src.dir}"
             destdir="${classes.dir}"
             classpathref="classpath"
             debug="${javac.debug}"
             includeantruntime="false" />
   </target>

   <target name="jar"
           description="Package compiled classes into JAR file"
           depends="compile">
      <jar destfile="${dist.dir}/${jar.name}"
           basedir="${classes.dir}"
           filesonly="${jar.filesonly}">
      </jar>
   </target>

   <target name="all"
           description="Compile Java source, assemble JAR, and generate documentation"
           depends="jar, javadoc" />

   <target name="javadoc" description="Generate Javadoc-based documentation">
      <mkdir dir="${javadoc.dir}" />
      <javadoc doctitle="Examples of Java Array Utility Functions"
               destdir="${javadoc.dir}"
               sourcepath="${src.dir}"
               classpathref="classpath"
               private="true"
               author="Dustin" />
   </target>

   <target name="clean" description="Remove generated artifacts.">
      <delete dir="${classes.dir}" />
      <delete dir="${dist.dir}" />
   </target>

</project>

The above Ant build file has some fairly typical targets with names like "compile", "jar", "javadoc", and "clean". All of this functionality can be imported into a Gradle build file. The next code listing is the complete Gradle build file that does this.

Gradle build.gradle that imports Ant build.xml
ant.importBuild 'build.xml'

The one-line Gradle build file shown above imports the Ant build file shown earlier. The effects of this can be easily seen in the following screen snapshots. The initial screen snapshot shows that the single line Gradle build file makes the "arrays" project available to the Gradle build as well as "other tasks" of "all" and "clean" with the descriptions associated with those Ant targets.

One can use gradle tasks --all to see all Ant targets, including the dependent targets such as "compile", "jar", and "javadoc". This is demonstrated in the next screen snapshot.

The next screen snapshot demonstrates running the default "all" target in the Ant build from the Gradle build.

As the build listings and images have demonstrated, importing an existing Ant build in a Gradle build is a straightforward process.

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