How do I read a SwingWorker's result *without* busy wait?

Question

I'm writing an application that executes its file menu actions using SwingWorker. Every called method returns a boolean value that tells, whether the operation was successfully executed or not.

At the moment I'm using busy waiting for the result, like this:

public boolean executeOperation() {
    final SwingWorker<Boolean, Void> worker = new SwingWorker<Boolean, Void>() {
        @Override
        protected Boolean doInBackground() throws Exception {
            // ..

            if (aborted) {
                return false;
            }

            // ..

            return true;
        }
    };

    worker.execute();

    // busy wait
    while (!worker.isDone())
        ;

    try {
        return worker.get().booleanValue();
    } catch (Exception e) {
        // handle exceptions ..
        return false;
    }
}

Is there a less polling-intense way of solving this?

Using worker.get() directly wouldn't work, as it blocks the EDT, waiting for the task to finish - meaning even the dialogs I open from within the SwingWorker wouldn't get painted.

EDIT: If possible, I would like to avoid that the method (or the worker) to communicate their result asynchronously. I'm implementing several short methods (file -> open, new, close, save, save as, exit) that rely on each other (i. e. when the trying to exit, exit calls close, close might call save, save might call save as). Solving this asynchronously seems to make the code much more complicated.

Answer 1

The point of the SwingWorker is precisely to launch some task in the background and don't block the EDT. Either you want something synchronous, and the EDT will be blocked whatever you try, or you want something asynchronous, and the background task should update its status using the publish method of the SwingWorker.

You could display a blocking modal dialog with a progress bar while the task is running, and hide it once the task completes.

The alternative is to block for some time, hoping the task will be quick to finish, and then backup to an asynchronous way of doing. This can be done using the get method taking a timeout as argument.

Answer 2

You could use an asynchronous paradigm. Look at Observer / Observable and use the job to transfer the result back to the object which is currently doing the polling.

Answer 3

Using worker.get() directly wouldn't work, as it blocks the EDT, waiting for the task to finish - meaning even the dialogs I open from within the SwingWorker wouldn't get painted.

They don't with the current code either. Your busy wait blocks the EDT as much as calling worker.get() does - there is only one event dispatch thread, and the dialogs in the SwingWorker are just as blocked if that thread is spinning in a loop or awaiting a lock. The problem here is that if a method runs on the EDT, it simply can't return a value from an asynchronous operation (without hogging the EDT) to its caller.

The correct way to react to completed async processing is overriding the done() method in SwingWorker.

Also check out http://java.sun.com/products/jfc/tsc/articles/threads/threads2.html for more info.

Answer 4

One way as mentioned by several folks above is to override the SwingWorker's done method. However if for some reason you want the post SwingWorker code outside of the SwingWorker and in the calling code, you can take advantage of SwingWorker's property change support. Simply add a PropertyChangeListener to the SwingWorker and listen for the state property which has a property name of "state". You can then extract the SwingWorker's state with its getState() method. When it is done it will return the DONE value of the SwingWorker.StateValue enum. For example (from an answer I've given in another thread here on SO):

  if (turn == white) {
     try {
        final SwingWorker<Move, Void> mySwingWorker = new SwingWorker<Move, Void>() {
           @Override
           protected Move doInBackground() throws Exception {
              Engine e = new Engine(); // Engine is implemented by runnable
              e.start();
              Move m = e.getBestMove(board);                  
              return m;
           }
        };

        mySwingWorker.addPropertyChangeListener(new PropertyChangeListener() {
           public void propertyChange(PropertyChangeEvent evt) {
              if (StateValue.DONE == mySwingWorker.getState()) {
                 try {
                    Move m = mySwingWorker.get();

                    // TODO: insert code to run on the EDT after move determined

                 } catch (InterruptedException e) {
                    e.printStackTrace();
                 } catch (ExecutionException e) {
                    e.printStackTrace();
                 }
              }
           }
        });
        mySwingWorker.execute();

     } catch (Exception e) {
        e.printStackTrace();
     }
  }

Answer 5

I ran into a similar problem when I wanted a function to return a value that would be calculated in a swing worker. I didn't want to simply get that thread to block the EDT. I also didn't want it to block. So I used a semaphore like this:

public boolean executeOperation() {
    final Semaphore semaphore = new Semaphore(1);
    semaphore.acquire(1); // surround by try catch...
    final SwingWorker<Boolean, Void> worker = new SwingWorker<Boolean, Void>() {
        @Override
        protected Boolean doInBackground() throws Exception {
            // ..

            if (aborted) {
                semaphore.release();
                return false;
            }

            // ..
            semaphore.release();
            return true;
        }
    };

    worker.execute();



    try {
        semaphore.tryAcquire(1, 600, TimeUnit.SECONDS); // awakes when released or when 10 minutes are up.
        return worker.get().booleanValue(); // blocks here if the task doesn't finish in 10 minutes.
    } catch (Exception e) {
        // handle exceptions ..
        return false;
    }
}

I guess this is not ideal for all situations. But I thought it was an alternative approach that was very useful for me.