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?

Solution 1

Depending on how you want to use the modified property, you can use macrodefs.

For example, instead of writing the following:

<target name="foo">
   <echo message="${my_property}"/>
</target>

and not being able to call ant foo with another message, you could write:

<macrodef name="myecho">
    <attribute name="msg"/>
    <sequential>
        <echo message="@{msg}"/>
    </sequential>
</macrodef>

<target name="foo">
   <myecho msg="${my_property}"/>
   <property name="my_property2" value="..."/>
   <myecho msg="${my_property2}"/>
</target>

Solution 2

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.

Solution 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.

Solution 4

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>

Solution 5

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

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Comments

[Andreas Kraft]

The var task is especially nice for "local variables", e.g. in for loops (also a task from the excellent ant-contrib). One drawback, though, is that the var task doesn't support the "location" attribute.

[kailash gaur]

it's very helpful if you give one example and explain more.