Question

If I create an object that implements Runnable, and I start a thread with it…

ArrayList<Thread> threadlist = new ArrayList<Thread>();
{
  MergeThread mmt = new MergeThread();
  Thread t = new Thread(mmt);
  threadlist.add(mmt);
  t.start();
}

t.join();
Thread t = threadlist.get(0);

At this point is mmt guaranteed to exist or could it have gone away if garbage collection got it.

What I am asking is if the Thread object holds on to the Runnable class after the thread has ended.

edit: there’s a mistake in the above it should say
threadlist.add(t);

Answer 1

From the source of the Thread class:

/**
 * This method is called by the system to give a Thread
 * a chance to clean up before it actually exits.
 */
private void exit() {
if (group != null) {
    group.remove(this);
    group = null;
}
/* Aggressively null out all reference fields: see bug 4006245 */
target = null;
/* Speed the release of some of these resources */
    threadLocals = null;
    inheritableThreadLocals = null;
    inheritedAccessControlContext = null;
    blocker = null;
    uncaughtExceptionHandler = null;
}

So the system calls Thread#exit() and the reference to target (which is a runnable the thread is running) is released.

There is also a bug report #4006245 on bugs.sun.com that refers to clearing the target reference.