Conditionals in Ant — Using if/unless and condition

Ant has no if/else like a programming language, but it does provide the if/unless attributes and the <condition> task for conditional logic.

if/unless on a target

The simplest form of conditional control — decide whether to run a target based on whether a property is set:

<target name="deploy" depends="build" if="do.deploy">
    <echo message="Starting deployment..." />
</target>

<target name="clean" unless="keep.temp">
    <delete dir="temp/" />
</target>

if="do.deploy": runs only when the property do.deploy is set
unless="keep.temp": runs only when the property keep.temp is not set

Usage:

# Run deployment
ant -Ddo.deploy=true

# Skip cleaning
ant -Dkeep.temp=true

The condition task

Used for a conditional evaluation that sets a property value:

<!-- Check whether a file exists -->
<condition property="config.exists">
    <available file="config.xml" />
</condition>

<!-- Check whether a directory exists -->
<condition property="build.dir.exists">
    <available file="build/" type="dir" />
</condition>

<!-- Check whether a class exists -->
<condition property="junit.available">
    <available classname="org.junit.Test" />
</condition>

Combining conditions

Combine multiple conditions with <and>, <or> and <not>:

<condition property="can.deploy">
    <and>
        <available file="release/app.jar" />
        <isset property="deploy.server" />
        <not>
            <equals arg1="${env}" arg2="dev" />
        </not>
    </and>
</condition>

String comparison

<condition property="is.production" value="true" else="false">
    <equals arg1="${env}" arg2="production" />
</condition>

<condition property="version.ok">
    <matches string="${app.version}" pattern="^\d+\.\d+\.\d+$" />
</condition>

Failing early with fail

Make the build fail when a condition is not met:

<fail message="JAVA_HOME is not set!">
    <condition>
        <not>
            <isset property="env.JAVA_HOME" />
        </not>
    </condition>
</fail>

Comparing file size

<condition property="jar.too.large">
    <length file="release/app.jar" when="greater" length="50000000" />
</condition>
<fail if="jar.too.large" message="The jar is over 50MB!" />

Practical example: choosing a different configuration per environment

<condition property="config.file" value="config.prod.properties" else="config.dev.properties">
    <equals arg1="${env}" arg2="production" />
</condition>
<property file="${config.file}" />

A small step forward every day is a new height!