Why does my thread stop when I try to access a synchronized list?

Question

For some reason, the output of this:

 public void msgNeedParts() {
    // Blabla...
    System.out.println(name + ": Try to print 'tasks'...");
    synchronized(tasks) {
        System.out.println(name + ": Tasks--" + tasks);
        System.out.println(name + ": Did I manage to print it?");
        tasks.add(new BinToDump(feeder, binNum));
    }
    stateChanged();
 }

Just prints out "GantryAgent: Try to print 'tasks'..." but not any of the following messages. I'm guessing the thread somehow 'gets stuck' when trying to access the synchronized list 'tasks', but I don't know why this is happening.

'tasks' was declared and initialized like this:

private List<BinToDump> tasks = 
    Collections.synchronizedList(new ArrayList<BinToDump>());

Can anybody point out what I'm missing?

Ah! I suspect I may have a culprit:

    /* If nothing left to do, return to original position. */

    synchronized (tasks) {

        if (tasks.isEmpty()) {

            doReturnToOriginalPos();

        }

    }

In my scheduler (this is an agent design), I check to see if 'tasks' is empty, then I call doReturnToOriginalPos(). Maybe this is just happening over and over so fast that other methods don't get a chance to modify it?

That was indeed the problem! It kept getting called so fast in my scheduler that nothing else could access 'tasks'. Thanks all for the help!

Answer 1

Something has a lock on tasks. Depending on what sort of application this is, you should be able to get a full stack dump of the system, but the method varies. For example, I think CTRL-Break on most windows-based appservers will do this, and I think sending a SIGQUIT on linux will do the same.

Once you get a stack dump, you can look through it to try and find out which other thread has a lock on that object.

You can also use VisualVM to get a stack dump, for the same end goal:

You can use Java VisualVM to take a thread dump (stack trace) while a local application is running. Taking a thread dump does not stop the application. When you print the thread dump you get a printout of the thread stack that includes thread states for the Java threads.

When you print a thread dump in Java VisualVM, the tool prints a stack trace of the active threads of the application. Using Java VisualVM to take a thread dump can be very convenient in cases where you do not have a command-line console for the application. You can use a stack trace to help diagnose a number of issues such as deadlocks or when an application hangs.

Answer 2

Is it possible that another thread is holding a synchronization lock against tasks?