Setting Properties with Ant Tasks

In this Ant tutorial installment, we'll take a closer look at setting properties. The combination of properties and conditional execution allows more robust build files that adapt to the computing environment. Start with this basic build.xml file.

<project default="set">
  <target name="set">
    <property name="status" value="false"/>
    <echo message="the status is ${status}" />
  </target>

  <property name="status" value="true"/>
</project>

And then execute ant.

$ ant
Buildfile: build.xml

set:
     [echo] the status is true

BUILD SUCCESSFUL
Total time: 2 seconds

This demonstrates two characteristics of the property task:

1. Properties are immutable. Once they are set, they stay set (with some inevitable exceptions). If properties were mutable, then the property assignment task <property name="status" value="false"/> would result in the message the status is false.
2. Property tasks that are direct children of the project are executed before any target task is executed. In this case, the property assignment task <property name="status" value="true"/> is a direct child of the project and is executed before the target set.

Now let's see how we can use the available task to set a property value. The available task can be used to test for the existence of a file or directory, test for the existence of a class in a classpath, and test for the existence of a system resource. We'll just work with the first option, please see the fine manual for the other choices. This build file tests for the presence of foo.

<project default="all">
  <target name="init">
    <available property="present"
               file="foo"
               />
  </target>

  <target name="exists" depends="init" if="present">
    <echo message="foo exists" />
  </target>

  <target name="all" depends="exists" />
</project>

Execute this build.xml file twice. Once with an empty directory and once after creating file foo with the touch command.

$ ls
build.xml
$ ant
Buildfile: build.xml

init:

exists:

all:

BUILD SUCCESSFUL
Total time: 3 seconds
$ touch foo
$ ant
Buildfile: build.xml

init:

exists:
     [echo] foo exists

all:

BUILD SUCCESSFUL
Total time: 3 seconds

In the previous example, we only checked for the existence of foo. In this example, we will also identify whether it is a directory or a file. The available task's type="dir" attribute specifically tests for a directory and is used to set property isaDir. We could use the type="file" attribute to set property isaFile but we'll use the condition task instead. The condition task is the generalized method for setting parameters. Please see the manual for a complete list of the supported conditions.

<project default="all">
  <target name="init">
    <available property="present"
               file="foo"
               />
    <available property="isaDir"
               file="foo"
               type="dir"
               />
    <condition property="isaFile">
      <and>
        <isset property="present" />
        <not>
          <isset property="isaDir" />
        </not>
      </and>
    </condition>
  </target>

  <target name="exists" depends="init" if="present">
    <echo message="foo exists" />
  </target>

  <target name="isFile" depends="init" if="isaFile">
    <echo message="foo is is a file" />
  </target>

  <target name="isDir" depends="init" if="isaDir">
    <echo message="foo is is a directory" />
  </target>

  <target name="all" depends="exists,isFile,isDir" />
</project>

Execute this build file three times. Once with an empty directory, once after creating directory foo, and once after removing directory and creating file foo.

$ ls
build.xml
$ ant
Buildfile: build.xml

init:

exists:

isFile:

isDir:

all:

BUILD SUCCESSFUL
Total time: 3 seconds
Ant/Setting 41 $ mkdir foo
mkdir foo
Ant/Setting 42 $ ant
ant
Buildfile: build.xml

init:

exists:
     [echo] foo exists

isFile:

isDir:
     [echo] foo is is a directory

all:

BUILD SUCCESSFUL
Total time: 3 seconds
$ rmdir foo
$ touch foo
$ ant
Buildfile: build.xml

init:

exists:
     [echo] foo exists

isFile:
     [echo] foo is is a file

isDir:

all:

BUILD SUCCESSFUL
Total time: 3 seconds