Condition Property Override

Question

I want to use an condition property to set the property value to X if another property is defined and Y otherwise. However, I don't want the user to be able to override the condition property from the command line.

How can this be achieved?

Answer 1

Starting from ant 1.8 for some use cases local task may be applicable. Since a property is made local it starts with an empty value. It's scope is limited to current target, but you may pass it to subsequent targets using param argument in antcall.

Answer 2

Nope, you can't override a property set on the command line. At least, it's not easy to do. The whole purpose of overriding properties on the command line is to allow users to override defaults in order to make modification in the way your project builds. For example:

 <property file="${basedir}/build.properties"/>
 <property name="javac.debug"  value="no"/>

 <target name="compile">
    <javac destdir="${main.destdir}"
         debug="${javac.debug}">

By default, the Java code is compiled without debugging information. Maybe this is done to make jar files smaller, or faster interpretation, or maybe to make the code harder to decompile and read. Whatever reason, this build won't put debug information into the classfiles.

However, developers do want this debugging information, so they want to be able to override this setting:

$ ant -Djavac.debug=true compile

Or, they can create a build.properties file and put the value in there.

This type of issue comes up when you're not using Ant for builds. I know several sites that use Ant scripts to do deployments. I usually discourage this because Ant isn't really made for this type of thing. For example, Ant doesn't have any built in logic or loops. Once a property is set, it can't be changed. These are good ideas for a build language, but a terrible idea for a general purpose programming language.

Also, developers shouldn't be doing builds for QA or production. Those should be done by a build server that won't override defaults.

---

Now how to destroy this whole well thought out system and cause absolute havoc:

You can use the ant-contrib tasks in your project. Doing this will allow you to access the Ant Contrib var task to unset properties.

Download the ant-contrib.jar file (whatever the latest version is), and put it in a lib directory under your project. Then you can do this:

<project name="danger-will-robinson" default="package" basedir="."
   xmlns:ac="http://ant-contrib.sourceforge.net">

  <!-- Define the Ant-Contrib tasks -->
  <taskdef=resource="net/sf/antcontrib/antlib.xml"
       uri="http://ant-contrib.sourceforge.net">
       <classpath>
            <fileset dir="${basedir}/lib">
                <include name="ant-contrib*.jar"/>
            </fileset>
       </classpath>
  </taskdef>

  <!-- Unset Property "foo", so you can use it -->
  <ac:var name="foo" unset="true"/>

Note that the <classpath> points to the ant-contrib jar in the ${basedir}/lib directory. If you check that into your source repository, it will allow everyone who checks out your project to be able to do the build without installing the ant-contrib jar on their system.

Note that I've defined a "ac" XML namespace, so Ant-Contrib tasks won't overlap other possible third party tasks.

Answer 3

Properties in ant once set are immutable by design. You may overwrite an existing property with any scripting language that provides access to ant api, i.e. javascript.
JDK >= 1.6 already ships with a javascript engine, so you may use something like :

<project>

 <property name="x" value="whatever"/>

 <script language="javascript">
  project.getProperty('x') ?
   project.setProperty('foo', 'true') :
    project.setProperty('foo', 'false');
 </script>

 <echo>$$[foo} => ${foo}</echo>

</project>

out of the box.
But that won't help if someone uses ant -f yourbuild.xml -Dfoo=bla !! as userproperties (those properties defined via -Dkey=value) have a special protection.
So your requirement "..However, I don't want the user to be able to override the condition property from the command line".
is not fullfilled.

But the let task from Ant addon Flaka provides the possibillity to overwrite even userproperties :

<project xmlns:fl="antlib:it.haefelinger.flaka">

<property name="x" value="whatever"/>

 <!--
  := defines a new property whereas
  ::= overwrites any existing property
  even userproperties
 -->
 <fl:let> foo ::= has.property['x'] ? 'true' : 'false'</fl:let>

 <echo>$$[foo} => ${foo}</echo>

</project>

Run both scripts with ant -f yourbuild.xml -Dfoo=bla to see the difference.

Ant api has also method project.setUserProperty(String,String) so you may use also:

...
 <script language="javascript">

 project.getProperty('x') ?
    project.setProperty('foo', 'true') :
    project.setProperty('foo', 'false');

 project.getUserProperty('x') ?
  project.setUserProperty('foo', 'true') :
  project.setUserProperty('foo', 'false');

 </script>
...

to prevent the foo property to be set via .. -D .. and it will work even if property x is defined on commandline -Dx=whatever
You have to make your choice, script task with javascript out of the box or Flaka let task oneline solution but Flaka jar needed.