How to set an ANT property only if it is unset

Question

Hi,

I can't figure out how to set an ANT property on the condition that it has not been set(i.e: it is not defined in the properties file and should automatically default).

So far, I only have the following code:

 <condition property="core.bin" value="../bin">
  <isset property="core.bin"/>
 </condition>

But this only seems to work if the value is defined in a tag.

Does anyone know how to conditionally set a property for the first time if it currently unset?

Answer 1

You simply can set the property with the property-task. If the property is already set, the value is unchanged, because properties are immutable.

But you can also include 'not' in your condition:

<condition property="core.bin" value="../bin">
   <not>  
      <isset property="core.bin"/>
   </not>
</condition>

Answer 2

Ant does this by default; if the property is already set; setting it again has no effect:

<project name="demo" default="demo">
    <target name="demo" >
        <property name="aProperty" value="foo" />
        <property name="aProperty" value="bar" /> <!-- already defined; no effect -->
        <echo message="Property value is '${aProperty}'" /> <!-- Displays 'foo' -->
    </target>
</project>

Gives

   /c/scratch> ant -f build.xml
Buildfile: build.xml

demo:
     [echo] Property value is '${aProperty}'

BUILD SUCCESSFUL
Total time: 0 seconds
/c/scratch> ant -f build.xml
Buildfile: build.xml

demo:
     [echo] Property value is 'foo'

BUILD SUCCESSFUL

Properties cannot be redefined; to do this you need to use something like the variable task from ant-contrib.