Simple Background Tasks

Let's start with a task that is very simple, but potentially time-consuming. The TumbleItem applet loads a set of graphic files used in an animation. If the graphic files are loaded from an initial thread, there may be a delay before the GUI appears. If the graphic files are loaded from the event dispatch thread, the GUI may be temporarily unresponsive.

To avoid these problems, TumbleItem creates and executes an instance of SwingWorker from its initial threads. The object's doInBackground method, executing in a worker thread, loads the images into an ImageIcon array, and returns a reference to it. Then the done method, executing in the event dispatch thread, invokes get to retrieve this reference, which it assigns to an applet class field named imgs. This allows TumbleItem to construct the GUI immediately, without waiting for the images to finish loading.

Here is the code that defines and executes the SwingWorker object.

SwingWorker worker = new SwingWorker<ImageIcon[], Void>() {
    @Override
    public ImageIcon[] doInBackground() {
        final ImageIcon[] innerImgs = new ImageIcon[nimgs];
        for (int i = 0; i < nimgs; i++) {
            innerImgs[i] = loadImage(i+1);
        }
        return innerImgs;
    }

    @Override
    public void done() {
        //Remove the "Loading images" label.
        animator.removeAll();
        loopslot = -1;
        try {
            imgs = get();
        } catch (InterruptedException ignore) {}
        catch (java.util.concurrent.ExecutionException e) {
            String why = null;
            Throwable cause = e.getCause();
            if (cause != null) {
                why = cause.getMessage();
            } else {
                why = e.getMessage();
            }
            System.err.println("Error retrieving file: " + why);
        }
    }
};

All concrete subclasses of SwingWorker implement doInBackground; implementation of done is optional.

Notice that SwingWorker is a generic class, with two type parameters. The first type parameter specifies a return type for doInBackground, and also for the get method, which is invoked by other threads to retrieve the object returned by doInBackground. SwingWorker's second type parameter specifies a type for interim results returned while the background task is still active. Since this example doesn't return interim results, Void is used as a placeholder.

You may wonder if the code that sets imgs is unnecessarily complicated. Why make doInBackground return an object and use done to retrieve it? Why not just have doInBackground set imgs directly? The problem is that the object imgs refers to is created in the worker thread and used in the event dispatch thread. When objects are shared between threads in this way, you must make sure that changes made in one thread are visible to the other. Using get guarantees this, because using get creates a happens before relationship between the code that creates imgs and the code that uses it. For more on the happens before relationship, refer to Memory Consistency Errors in the Concurrency lesson.

There are actually two ways to retrieve the object returned by doInBackground.

• Invoke SwingWorker.get with no arguments. If the background task is not finished, get blocks until it is.
• Invoke SwingWorker.get with arguments indicating a timeout. If the background task is not finished, get blocks until it is — unless the timeout expires first, in which case get throws java.util.concurrent.TimeoutException.

Be careful when invoking either overload of get from the event dispatch thread; until get returns, no GUI events are being processed, and the GUI is "frozen". Don't invoke get without arguments unless you are confident that the background task is complete or close to completion.

For more on the TumbleItem example, refer to How to Use Swing Timers in the lesson Using Other Swing Features.