DEFAULT_CLASSPATH will matter to you if you're trying to launch a task using ILaunchConfigurationWorkingCopy using a separate JRE.

To launch in a separate JVM, you'll have to first, get a JVM.. 




  ILaunchManager launchManager = DebugPlugin.getDefault().getLaunchManager();

  ILaunchConfigurationType type = launchManager.getLaunchConfigurationType(IAntLaunchConfigurationConstants.ID_ANT_LAUNCH_CONFIGURATION_TYPE);

  ILaunchConfigurationWorkingCopy workingCopy = type.newInstance(null, "Execute Ant Target");

  IProject project = ...;

  IVMInstall vm = JavaRuntime.getVMInstall((IJavaProject) project);

  workingCopy.setAttribute( IJavaLaunchConfigurationConstants.ATTR_VM_INSTALL_TYPE, vm.getVMInstallType().getId());

  workingCopy.setAttribute(IJavaLaunchConfigurationConstants.ATTR_VM_INSTALL_NAME, vm.getName());

  // ... more codes.. 

  workingCopy.launch(ILaunchManager.RUN_MODE, new NullProgressMonitor()); 

  // now this runs in separate JVM as ur Eclipse
Now, for my case I need to append quite a bit of classpath for my ant tasks to run, when NOT running as separate JVM, its relatively easy.. just modify the classpath set in the AntCorePlugin:


  IAntClasspathEntry [] newUrls = ...;

  AntCorePlugin.getPlugin().getPreferences().setAdditionalClasspathEntries(newUrls);

  // and when u're done launching/launch has terminated.. just remember to reset it back

  // its impolite to make permanent change to user's preference without asking you know..
However, the above method of modifying classpath doesn't work when you're intending to run as a separate JRE, we'll have to do with something called IMemento:


  List mementoList = new ArrayList();

  mementoList.add((new AntHomeClasspathEntry()).getMemento());

  mementoList.add((new ContributedClasspathEntriesEntry()).getMemento());

  mementoList.add(JavaRuntime.newArchiveRuntimeClasspathEntry(new Path(workingDir.getAbsolutePath())).getMemento());

  // and more custom classpaths.. 

  workingCopy.setAttribute(IJavaLaunchConfigurationConstants.ATTR_CLASSPATH, mementoList);
But what I find is, that there's this attribute ATTR_DEFAULT_CLASSPATH that is always set to true. And when its true, the classpath that I've painstakingly assembled above doesn't apply, its value is always true even if I explicitly set:


  workingCopy.setAttribute(IJavaLaunchConfigurationConstants.ATTR_DEFAULT_CLASSPATH, false);

  workingCopy.launch(ILaunchManager.RUN_MODE, new NullProgressMonitor());
Frustrating.

Its only after debugging into Eclipse's plugin's source code (yippie! open source!) that I realise that somewhere inside workingCopy.launch itself, there's a call to AntUtil.migrateToNewClasspathFormat() that effectively screws up that attribute. I haven't dwell into more details.. so my current fix is simple, I preempt:


  AntUtil.migrateToNewClasspathFormat(workingCopy);

  workingCopy.setAttribute(IJavaLaunchConfigurationConstants.ATTR_DEFAULT_CLASSPATH, false);

  workingCopy.launch(ILaunchManager.RUN_MODE, new NullProgressMonitor());
Now, the value remains as false and my custom classpath takes effect.