Execute Ant task just if a condition is met

An Ant target can have an optional if or unless clause. This means to execute the task only if the property is set, with an if clause, or is unset with the unless clause¹. Interestingly, that if or unless clause is checked after any dependent task is first executed.

This means, you can do this in standard Ant as a way of executing an Ant task only if a particular condition is met:

 <target name="test.if.tomcat.is.running">
      <condition property="tomcat.running">
          <socket server="${tomcat.host}" port="${tomcat.port}"/> 
      </condition>
</target>

<target name="my.target"
    if="tomcat.running"
    depends="test.if.tomcat.is.running">
    <yaddah/>
    <yaddah/>
    <yaddah/>
</target>

You specify that you want Ant to execute Target my.target. Ant notices that my.target depends upon the test.if.tomcat.is.running target, and will execute that first. The test.if.tomcat.is.running task will set the tomcat.running property if Tomcat is actually running. Otherwise, that property is not set.

Finally, Ant will go back to the my.target target and see if the property tomcat.running is set, and will only execute the target my.target if it is set.

Or, you can use the Ant-contrib tasks which may make your entire build process easier to understand.

If you want to go the Ant-Contrib route, there’s an easy way to setup Ant-Contrib, so the Ant-contrib jar is actually part of your project. If someone checks out your project from the version control system, they’ll also get the Ant-contrib jar, and thus won’t have to install Ant-Contrib themselves.

Download the Ant-Contrib jar, and put it into a directory in the root of your project called antlib/ac. The antlib can be used for all sorts of optional task jars such as Findbugs or PMD. Just put each optional Ant jar in their own directory under antlib (Like I put Ant-Contrib under the ac directory).

Then, in your build.xml, you specify the Ant-Contrib tasks this way:

<taskdef resource="net/sf/antcontrib/antlib.xml">
   <classpath>
       <fileset dir="${basedir}/antlib/ac"/>
   </classpath>
</taskdef>

Now, you can use the Ant-Contrib tasks without worrying whether or not they’re installed on a particular machine or not. You checkout your project, and you have access to those tasks automatically.

1. That’s right, the if/unless clause checks if a property is set and not true/false which can cause a lot of confusion. I’ve seen developers set a property to false or no, and then wonder why the target is actually executing since the if clause is set to false.