ant-contrib - if/then/else task - ant

Question

I am using ant, and I have a problem with if/then/else task, (ant-contrib-1.0b3.jar).
I am running something that can be simplified with build.xml below.
I am expecting to obtain from 'ant -Dgiv=Luke' the message
input name: Luke
should be overwritten with John except for Mark: John
but it seems property "giv" is not overwritten inside if/then/else..
input name: Luke
should be overwritten with John except for Mark: Luke
Is it depending from the fact I am using equals task with ${giv} ?
Otherwise what is wrong in my code?
build.xml CODE:
<project name="Friend" default="ifthen" basedir=".">
<property name="runningLocation" location="" />
<taskdef resource="net/sf/antcontrib/antcontrib.properties">
<classpath>
<pathelement location="${runningLocation}/antlib/ant-contrib-1.0b3.jar" />
</classpath>
</taskdef>
<target name="ifthen">
<echo message="input name: ${giv}" />
<if>
<equals arg1="${giv}" arg2="Mark" />
<then>
</then>
<else>
<property name="giv" value="John" />
</else>
</if>
<echo message="should be overwritten with John except for Mark: ${giv}" />
</target>
</project>

Answer 1

In Ant a property is always set once, after that variable is not alterable anymore.
Here follows a solution using standard Ant (without ant-contrib) which could be useful for the people who does not want an extra dependency.
<target name="test" >
<echo message="input name: ${param}" />
<condition property="cond" >
<equals arg1="${param}" arg2="Mark" />
</condition>
</target>
<target name="init" depends="test" if="cond">
<property name="param2" value="Mark" />
</target>
<target name="finalize" depends="init">
<property name="param2" value="John" />
<echo message="should be overwritten with John except for Mark: ${param2}" />
</target>

Answer 2

Ant Properties are very hard to overwrite (if not impossible). What you need is a Variable. These are also defined in the Ant Contrib JAR.
Editing your example:
<target name="ifthen">
<var name="Evangelist" value="${giv}" />
<echo message="input name: ${Evangelist}" />
<if>
<equals arg1="${Evangelist}" arg2="Mark" />
<then>
</then>
<else>
<var name="Evangelist" value="John" />
</else>
</if>
<echo message="should be overwritten with John except for Mark: ${Evangelist}" />
</target>

Answer 3

<project name="Friend" default="ifthen" basedir=".">
<property name="runningLocation" location="" />
<taskdef resource="net/sf/antcontrib/antcontrib.properties">
<classpath>
<pathelement location="${runningLocation}/antlib/ant-contrib-1.0b3.jar" />
</classpath>
</taskdef>
<target name="ifthen">
<echo message="input name: ${giv}" />
<if>
<equals arg1="${giv}" arg2="Mark" />
<then>
</then>
<else>
<var name="giv" unset="true"/>
<property name="giv" value="John" />
</else>
</if>
<echo message="should be overwritten with John except for Mark: ${giv}" />
</target>
</project>
We can use var task to unset the property also.

Answer 4

I know this is old, but should prove handy to others searching for a solution.
to re-assign a property without using ant-contrib, use macrodef with a script.
<macrodef name="property-change">
<attribute name="name"/>
<attribute name="value"/>
<sequential>
<script language="javascript"><![CDATA[
project.setProperty("#{name}", "#{value}");
]]></script>
</sequential>
</macrodef>
then in anywhere in ant, just call this like the property tag
<property-change name="giv" value="John"/>
to Implement this in your original version of xml, it would look like this:
<project name="Friend" default="ifthen" basedir=".">
<property name="runningLocation" location="" />
<taskdef resource="net/sf/antcontrib/antcontrib.properties">
<classpath>
<pathelement location="${runningLocation}/antlib/ant-contrib-1.0b3.jar" />
</classpath>
</taskdef>
<target name="ifthen">
<echo message="input name: ${giv}" />
<if>
<equals arg1="${giv}" arg2="Mark" />
<then>
</then>
<else>
<property-change name="giv" value="John" />
</else>
</if>
<echo message="should be overwritten with John except for Mark: ${giv}" />
</target>
</project>
This sample is given purely as an example on writing a macro to replace the <var> command in ant-contrib. In a situation like this one, where the <if> command is being used, it makes more sense to use <var> sinnce ant-contrib is already loaded, and <var> might be faster in processing.
Hope this helps.

Answer 5

It is possible to re-assign the value of a property using the ant-contrib 'propertycopy'. This is an alternative to using ant-contrib Variables.
This way the property "giv" can be overwritten.
<target name="ifthen">
<echo message="input name: ${giv}" />
<if>
<equals arg1="${giv}" arg2="Mark" />
<then>
</then>
<else>
<property name="tempName" value="John" />
<propertycopy name="giv" from="tempName" override="true" />
</else>
</if>
<echo message="should be overwritten with John except for Mark: ${giv}" />
</target>
Be aware this assumes the property tempName is not already set to a value other than 'John'.