Question

I have a SwingWorker as follows:

public class MainWorker extends SwingWorker(Void, MyObject) {
    :
    :
}

I invoked the above Swing Worker from EDT:

MainWorker mainWorker = new MainWorker();
mainWorker.execute();

Now, the mainWorker creates 10 instances of a MyTask class so that each instance will run on its own thread so as to complete the work faster.

But the problem is I want to update the gui from time to time while the tasks are running. I know that if the task was executed by the mainWorker itself, I could have used publish() and process() methods to update the gui.

But as the tasks are executed by threads different from the Swingworker thread, how can I update the gui from intermediate results generated by threads executing tasks.

Answer 1

The SwingWorker’s API documentation offers this hint:

The doInBackground() method is called
on this thread. This is where all
background activities should happen.
To notify PropertyChangeListeners
about bound properties changes use the
firePropertyChange and
getPropertyChangeSupport() methods. By
default there are two bound properties
available: state and progress.

MainWorker can implement PropertyChangeListener. It can then register itself with its PropertyChangeSupport:

getPropertyChangeSupport().addPropertyChangeListener( this );

MainWorker can supply its PropertyChangeSupport object to every MyTask object it creates.

new MyTask( ..., this.getPropertyChangeSupport() );

A MyTask object can then notify its MainWorker of progress or property updates by using PropertyChangeSupport.firePropertyChange methods.

MainWorker, so notified, can then use SwingUtilities.invokeLater or SwingUtilities.invokeAndWait to update the Swing components via the EDT.

protected Void doInBackground() {
    final int TASK_COUNT = 10;
    getPropertyChangeSupport().addPropertyChangeListener(this);
    CountDownLatch latch = new CountDownLatch( TASK_COUNT ); // java.util.concurrent
    Collection<Thread> threads = new HashSet<Thread>();
    for (int i = 0; i < TASK_COUNT; i++) {
        MyTask task = new MyTask( ..., latch, this.getPropertyChangeSupport() ) );
        threads.add( new Thread( task ) );
    }
    for (Thread thread: threads) {
        thread.start();
    }
    latch.await();
    return null;
}