How to over-write the property in Ant?

Question

Is there a way to re-assign the value for the Ant property task? Or is there another task available for that purpose?

Answer 1

Since Ant 1.8, you can use the "local" task to change the value of a property within a target. Note that this does NOT change the value of the global property with the same name but it is a way to solve some problems.

See

http://ant.apache.org/manual/Tasks/local.html

Answer 2

You can't change the value of a property in Ant.

If you have some Ant tasks you want to run repeatedly passing in different values I recommend the macrodef task as you can run the same macro repeatedly passing in different attributes.

For example:

<macrodef name="copythings">
  <attribute name="todir"/>
  <sequential>
    <copy todir="@{todir}">
      <fileset dir="${src}">
        <exclude name='**/*svn' />
      </fileset>
    </copy>
  </sequential>
</macrodef>

<copythings todir="/path/to/target1"/>
<copythings todir="/path/to/target2"/>

Note that ${property} is used to reference properties and @{attribute} is used to reference the attributes passed to the <macrodef> task.

Answer 3

For the sake of justice, there is a hack that allows to alter ant's immutable properties without any additional libs (since java 6):

<scriptdef name="propertyreset" language="javascript"
    description="Allows to assign @{property} new value">
    <attribute name="name"/>
    <attribute name="value"/>
        project.setProperty(attributes.get("name"), attributes.get("value"));
</scriptdef>

Usage:

    <property name="x" value="10"/>
    <propertyreset name="x" value="11"/>
    <echo>${x}</echo>   <!-- will print 11 -->

As others mentioned, this should be used with care after all canonical approaches proved not to fit.

Answer 4

ant-contrib's Variable task can do this:

<property name="x" value="6"/>
<echo>${x}</echo>   <!-- will print 6 -->
<var name="x" unset="true"/>
<property name="x" value="12"/>
<echo>${x}</echo>   <!-- will print 12 -->

Not recommended, though, it can lead to weird side-effects if parts of your Ant scripts assume immutable property values, and other parts break this assumption.

Answer 5

Properties are immutable in ant.

You may be interested in ant-contrib's var task.

<var name="my_var" value="${my_property}" />

<echo>Addressed in the same way: ${my_var} and ${my_property}</echo>

Answer 6

Properties are immutable in ant. But that's not as terrible a limitation as it may seem. There's a whole class of programming languages where (most) variables are constant and yet they get stuff done   this is called "functional programming."

You can "change" values used by different tasks by deriving new, changed properties from old ones, or changing parameters when calling tasks with the subant or antcall tasks. If you're creative you can usually find a way to solve your problem.