Stop/cancel SwingWorker thread?

Question

I'm trying to find out how to stop a SwingWorker thread from running when I press a button. I have been looking around and I'm having some trouble working out how to do this. At the moment this is what I have:

new MySwingWorkerClass(args).execute();

I'm then creating a button which I want to use in order to stop the thread:

button = new JButton("Stop");
button.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent e) 
        {
        // Stop the swing worker thread
        }
});

I have already looked around in search of an answer, so far I have managed to find the cancel method. I don't understand how to use this to stop my swing worker though. I tried the following but it didn't work:

SwingWorker.cancel(true);

Answer 1

You need to keep a reference to your SwingWorker, then you use that reference to cancel the worker thread.

MySwingWorker myWorker = new MySwingWorkerClass(args).execute();

button.addActionListener(new ActionListener() {
    public void actionPerformed(ActionEvent e) 
    {
        // Stop the swing worker thread
        myWorker.cancel(true);
    }
});

Here is a full example:

public class WorkerDemo extends JFrame {
    private boolean isStarted = false;
    private JLabel counterLabel = new JLabel("Not started");
    private Worker worker = new Worker();
    private JButton startButton = new JButton(new AbstractAction("Start") {

        @Override
        public void actionPerformed(ActionEvent arg0) {
            if(!isStarted) { 
                worker.execute();
                isStarted = false;
            }
        }

    });
    private JButton stopButton = new JButton(new AbstractAction("Stop") {

        @Override
        public void actionPerformed(ActionEvent arg0) {
            worker.cancel(true);
        }

    });

    public WorkerDemo() {

        add(startButton, BorderLayout.WEST);
        add(counterLabel, BorderLayout.CENTER);
        add(stopButton, BorderLayout.EAST);
        pack();
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        setVisible(true);
    }

    class Worker extends SwingWorker<Void, Integer> {

        int counter = 0;

        @Override
        protected Void doInBackground() throws Exception {
            while(true) {
                counter++;
                publish(counter);
                Thread.sleep(60);
            }
        }

        @Override
        protected void process(List<Integer> chunk) {

            // get last result
            Integer counterChunk = chunk.get(chunk.size()-1);

            counterLabel.setText(counterChunk.toString());
        }

    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {

            @Override
            public void run() {
                new WorkerDemo();
            }

        });
    }

}

Answer 2

You need to periodically check its cancelled flag (i.e. isCancelled()). SwingWorker leaves how to handle the interrupt up to you.

For more information, see Cancelling Background Tasks.

Answer 3

I think you have to restart the counter after every stop.

For startButton, you have to have worker restarted as in

private JButton startButton = new JButton(new AbstractAction("Start") {

        @Override
        public void actionPerformed(ActionEvent arg0) {
            if(!isStarted) { 
                worker = new Worker();     // New line
                worker.execute();
                isStarted = false;
            }
        }
    });

To stop you can use

worker.setStopFlag(); // in stopButton actionPerformed block.

in Worker class private boolean stopFlag = false;

and add

if( stopFlag)
break;

after Thread.sleep(60); and finally put setter for stopFlag at the end of Worker class as

void setStopFlag(){
    stopFlag = true;
}

By the way, you can use cancel(true) if you want to have exception exit.

Answer 4

you can try overidding the done() method and put it in try catch and catch the java.util.concurrent.CancellationException. Something like

. . .

@Overide
done() {
   try {
     get();
   } catch (CancellationException e) {
       // Do your task after cancellation
   }
}

. . .