Monday, October 20, 2014

Conditional logic In Ant

Every so often I find myself needing some conditional logic in my ant build files based on either an automated build property or some property set by current state.

There is an open source library, ant-contrib, which gives you if/else statements in your Ant build files but I tend to not use ant-contrib for three reasons. First, it adds bloat to my project because of the requirement to include it's jar in my projects classpath. Second, you have to mess around with defining tasks in your build files which I just don't feel are very intuitive. Lastly, Ant already includes the ability to perform conditional logic by taking advantage of the Ant target's if attribute.

Performing conditional logic in Ant without an additional library is pretty easy. You simply need to define three targets. The first is the target you want to run in the TRUE scenario. The second is the target you want to run in the FALSE scenario. The third is the target that sets a property (or properties) based on some condition and calls the other targets.

Let's take a look at a very simple build file. This will print This build IS *nix if the isUnix property is set to true otherwise it will print This build is NOT *nix.

<?xml version="1.0" encoding="UTF-8"?>
<project name="example">
    <condition property="isUnix"><os family="unix"/></condition>

    <target name="-unix-build" if="performUnixBuild">
        <echo>This build IS *nix</echo>

    <target name="-non-unix-build" if="performNonUnixBuild">
        <echo>This build is NOT *nix</echo>

    <target name="build">
        <condition property="performUnixBuild"><istrue value="${isUnix}" /></condition>
        <condition property="performNonUnixBuild"><isfalse value="${isUnix}" /></condition>

        <antcall target="-unix-build" />
        <antcall target="-non-unix-build" />


You can see this in action by copying that file to a machine with Ant on it and running:
$ ant build.
If you're on a Unix like machine it will print This build IS *nix otherwise it will print This build is NOT *nix.

You can test the else logic by overriding the isUnix property at the command line using:
$ ant -DisUnix=false build.

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