122 Java Concurrency In Practice

Listing 9.5. LongǦrunning Task with User Feedback.

button.addActionListener(new ActionListener() { public void actionPerformed(ActionEvent e) {

button.setEnabled(false);

label.setText("busy"); backgroundExec.execute(new Runnable() {

public void run() { try {

doBigComputation(); } finally {

GuiExecutor.instance().execute(new Runnable() { public void run() {

button.setEnabled(true);

label.setText("idle");

}

});

}

}

});

}

});

The task triggered when the button is pressed is composed of three sequential subtasks whose execution alternates between the event thread and the background thread. The first subtask updates the user interface to show that a long running operation has begun and starts the second subtask in a background thread. Upon completion, the second subtask queues the third subtask to run again in the event thread, which updates the user interface to reflect that the operation has completed. This sort of "thread hopping" is typical of handling long running tasks in GUI applications.

9.3.1. Cancellation

Any task that takes long enough to run in another thread probably also takes long enough that the user might want to cancel it. You could implement cancellation directly using thread interruption, but it is much easier to use Future, which was designed to manage cancellable tasks.

When you call cancel on a Future with mayInterruptIfRunning set to true, the Future implementation interrupts the thread that is executing the task if it is currently running. If your task is written to be responsive to interruption, it can return early if it is cancelled. Listing 9.6 illustrates a task that polls the thread's interrupted status and returns early on interruption.

Listing 9.6. Cancelling a LongǦrunning Task.

Future<?> runningTask = null; // thread-confined

...

startButton.addActionListener(new ActionListener() { public void actionPerformed(ActionEvent e) {

if (runningTask != null) {

runningTask = backgroundExec.submit(new Runnable() { public void run() {

while (moreWork()) {

if (Thread.currentThread().isInterrupted()) { cleanUpPartialWork();

break;

}

doSomeWork();

}

}

});

};

}});

cancelButton.addActionListener(new ActionListener() { public void actionPerformed(ActionEvent event) {

if (runningTask != null) runningTask.cancel(true);

}});

Because runningTask is confined to the event thread, no synchronization is required when setting or checking it, and the start button listener ensures that only one background task is running at a time. However, it would be better to be notified when the task completes so that, for example, the cancel button could be disabled. We address this in the next section.

9.3.2. Progress and Completion Indication

Using a Future to represent a long running task greatly simplified implementing cancellation. FutureTask also has a done hook that similarly facilitates completion notification. After the background Callable completes, done is called. By