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 = ...;
// 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());


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:

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.