Setting Java System Properties with Ant

In this installment, we'll learn how to set a Java System Property with ant. Here is our Java class. It simply accesses the System Properties object and prints the value of the test.property key.

import java.util.Properties;

public class prop {
    public static void main( String[] args )
    {
        String     testProp = "test.property";
        Properties sysProps = System.getProperties();

        System.out.println( "Value of " + testProp + " is " +
                            sysProps.getProperty(testProp) );
    }
}

And here is the simple Ant build.xml that compiles and executes that class.

<project default="run">

  <target name="compile">
    <javac srcdir="." />
  </target>

  <target name="run" depends="compile">
    <java classname="prop"
          fork="true">
    </java>
  </target>

</project>

With the following result:

$ ant run
Buildfile: build.xml

compile:
    [javac] Compiling 1 source file

run:
     [java] Value of test.property is null

BUILD SUCCESSFUL
Total time: 6 seconds

To specify a System Property we use the sysproperty attribute of the java task. We'll change the run target to assign the value blue to the key test.property

  <target name="run" depends="compile">
    <java classname="prop"
          fork="true">
      <sysproperty key="test.property"
                   value="blue"
                   />
    </java>
  </target>

And we get the expected output on execution.

$ ant
Buildfile: build.xml

compile:

run:
     [java] Value of test.property is blue

BUILD SUCCESSFUL
Total time: 3 seconds

So far, so good. Of course, if test.property is going to have a fixed value, then we might as well set it from within our application. If test.property will typically have a wide range of simple values, then it is probably best managed as a parameter with a default value that can be overridden from the command line.

<project default="run">

  <target name="compile">
    <javac srcdir="." />
  </target>

  <property name="COLOR" value="red" />
  <target name="run" depends="compile">
    <java classname="prop"
          fork="true">
      <sysproperty key="test.property"
                   value="${COLOR}"
                   />
    </java>
  </target>

</project>

And if the property has a small number of complex values, then it can be set based upon another parameter that can be set from the command line.

<project default="run">

  <target name="compile">
    <javac srcdir="." />
  </target>

  <property name="COLOR" value="red" />
  <target name="setup-run">
    <condition property="COMPLEX-COLOR"
              value="complex.property.when.red.">
      <contains string="red" substring="${COLOR}" casesensitive="false" />
    </condition>
    <condition property="COMPLEX-COLOR"
              value="complex.property.when.blue.">
      <contains string="blue" substring="${COLOR}" casesensitive="false" />
    </condition>
    <condition property="COMPLEX-COLOR"
              value="complex.property.when.unknown.">
      <not>
        <isset property="COMPLEX-COLOR" />
      </not>
    </condition>
  </target>

  <target name="run" depends="compile,setup-run">
    <java classname="prop"
          fork="true">
      <sysproperty key="test.property"
                   value="${COMPLEX-COLOR}"
                   />
    </java>
  </target>

</project>

Which let's us control the property from the command line:

$ ant
Buildfile: build.xml

compile:

setup-run:

run:
     [java] Value of test.property is complex.property.when.red.

BUILD SUCCESSFUL
Total time: 3 seconds
$ ant -DCOLOR=BLue
Buildfile: build.xml

compile:

setup-run:

run:
     [java] Value of test.property is complex.property.when.blue.

BUILD SUCCESSFUL
Total time: 3 seconds
$ ant -DCOLOR=purple
Buildfile: build.xml

compile:

setup-run:

run:
     [java] Value of test.property is complex.property.when.unknown.

BUILD SUCCESSFUL
Total time: 3 seconds