Java Thread Start-Stop-Start on same button click in Eclipse

Question

I am creating a simple java program with a GUI built with the help of window builder in eclipse. The GUI consists of just a button.

My Aim :- On button click,start a thread that will print to the console numbers infinitely until it is stopped by clicking the same button again.

Here is my implementation:-

Runner.java (The thread Class)

public class Runner extends Thread{

    private volatile boolean running = true;
    private int i = 1;

    @Override
    public void run() {

        while(running)
        {
            System.out.println(i++ +"\n");
            try {
                sleep(1000);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
    }



    public void shutdown()
    {
        running = false;
    }

}

MainGUI.java (The ui class)

Runner runIT = new Runner();
final JButton btnNewButton = new JButton("Start Thread");
        btnNewButton.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent arg0) {

                String buttonText = btnNewButton.getText();
                if(buttonText.equals("Start Thread"))
                {
                    btnNewButton.setText("Stop Thread");
                    runIT.start();
                }

                else if(buttonText.equals("Stop Thread"))
                {
                    btnNewButton.setText("Start Thread");
                    runIT.shutdown();

                }
            }
        });

My Problem :- The thread starts and stops perfectly but just once. That is, I am able to start and stop the thread only once. When I try to restart the stopped thread it fails. I want the thread to continue printing number from the same value at which it stopped.

Error :- Exception in thread "AWT-EventQueue-0" java.lang.IllegalThreadStateException
    at java.lang.Thread.start(Thread.java:705)
    at com.zakoi.java.thread.GUI.MainGUI$2.actionPerformed(MainGUI.java:62)
    at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)
    at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341)
    at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
    at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
    at java.awt.Component.processMouseEvent(Component.java:6516)
    at javax.swing.JComponent.processMouseEvent(JComponent.java:3311)
    at java.awt.Component.processEvent(Component.java:6281)
    at java.awt.Container.processEvent(Container.java:2229)
    at java.awt.Component.dispatchEventImpl(Component.java:4872)
    at java.awt.Container.dispatchEventImpl(Container.java:2287)
    at java.awt.Component.dispatchEvent(Component.java:4698)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832)
    at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4492)
    at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4422)
    at java.awt.Container.dispatchEventImpl(Container.java:2273)
    at java.awt.Window.dispatchEventImpl(Window.java:2719)
    at java.awt.Component.dispatchEvent(Component.java:4698)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:735)
    at java.awt.EventQueue.access$200(EventQueue.java:103)
    at java.awt.EventQueue$3.run(EventQueue.java:694)
    at java.awt.EventQueue$3.run(EventQueue.java:692)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
    at java.awt.EventQueue$4.run(EventQueue.java:708)
    at java.awt.EventQueue$4.run(EventQueue.java:706)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:705)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)

Please help.

Answer 1

You should have the thread run but act as a service. It should then be controlled through some form of communication with it.

It is quite expensive to create threads - they should not be created/destroyed unnecessarily.

This code uses locks to implement a Thread that can be paused and resumed. There's a bit more here than you are looking for but it demonstrates the theory.

/**
 * PauseableThread is a Thread with pause/resume and cancel methods.
 *
 * The meat of the process must implement `step`.
 *
 * You can either extend this and implement `step` or use the factory.
 *
 * I cannot extend Thread because my resume will clash.
 *
 */
public abstract class PauseableThread implements Runnable {

    // The lock.

    private final ReadWriteLock pause = new ReentrantReadWriteLock();
    private final Lock readLock = pause.readLock();
    private final Lock writeLock = pause.writeLock();
    // Flag to cancel the wholeprocess.
    private volatile boolean cancelled = false;
    // The exception that cause it to finish.
    private Exception thrown = null;
    // The thread that is me.
    private Thread me = null;

    @Override
    // The core run mechanism.
    public void run() {
        // Track my current thread.
        me = Thread.currentThread();
        try {
            while (!finished()) {
                // Block here if we're paused.
                blockIfPaused();
                // Don't do any more work if we've been asked to stop.
                if (!finished()) {
                    // Do my work.
                    step();
                }
            }
        } catch (Exception ex) {
            // Just fall out when exception is thrown.
            thrown = ex;
        }
    }

    // Have we finished yet?
    private boolean finished() {
        return cancelled || !me.isInterrupted();
    }

    // Block if pause has been called without a matching resume.
    private void blockIfPaused() throws InterruptedException {
        try {
            // Grab a write lock. Will block if a read lock has been taken.
            writeLock.lockInterruptibly();
        } finally {
            // Release the lock immediately to avoid blocking when pause is called.
            writeLock.unlock();
        }
    }

    // Pause the work. NB: MUST be balanced by a resume.
    public void pause() {
        // We can wait for a lock here.
        readLock.lock();
    }

    // Resume the work. NB: MUST be balanced by a pause.
    public void resume() {
        // Release the lock.
        readLock.unlock();
    }

    // Stop.
    public void cancel() {
        // Stop everything.
        cancelled = true;
    }

    // Stop immediately (if param is true).
    public void cancel(boolean interrupt) {
        if (interrupt) {
            // Interrupt me.
            me.interrupt();
        } else {
            // Or cancel me.
            cancel();
        }
    }

    // Wait for completion.
    public void await() throws InterruptedException {
        // Wait 'till we've finished. NB: Will wait forever if you haven't instigated a cancel of some kind.
        while (me.isAlive()) {
            Thread.sleep(0);
        }
    }

    // Start - like a thread.
    public void start() {
        // Wrap me in a thread and fire the sucker up!
        new Thread(this).start();
    }

    // Get the exception that was thrown to stop the thread or null if the thread was cancelled.
    public Exception getThrown() {
        return thrown;
    }

    // Expose my Thread.
    public Thread getThread() {
        return me;
    }

      // Create this method to do stuff.
    // Calls to this method will stop when pause is called.
    // Any thrown exception stops the whole process.
    public abstract void step() throws Exception;

    // Factory to wrap a Stepper in a PauseableThread
    public static PauseableThread make(Stepper stepper) {
        StepperThread pauseableStepper = new StepperThread(stepper);
        // That's the thread they can pause/resume.
        return pauseableStepper;
    }

    // One of these must be used.
    public interface Stepper {

        // A Stepper has a step method.
        // Any exception thrown causes the enclosing thread to stop.

        public void step() throws Exception;
    }

    // Holder for a Stepper.
    private static class StepperThread extends PauseableThread {

        // The actual stepper I am proxying.

        private final Stepper stepper;

        StepperThread(Stepper stepper) {
            this.stepper = stepper;
        }

        @Override
        public void step() throws Exception {
            stepper.step();
        }
    }

      // !!!! Testing only below !!!!
    // My test counter.
    static int n = 0;

    // Test/demo.
    public static void main(String[] args) throws InterruptedException {

        try {
            // Simple stepper that just increments n.
            Stepper s = () -> {
                n += 1;
                Thread.sleep(1);
            };
            PauseableThread pt = PauseableThread.make(s);
            // Start it up.
            pt.start();
            Thread.sleep(1000);
            pt.pause();
            System.out.println("Paused: " + n);
            Thread.sleep(1000);
            System.out.println("Resuminng: " + n);
            pt.resume();
            Thread.sleep(1000);
            pt.cancel();
            System.out.println("Finished: " + n);

            // Start again to test agressive cancelling.
            pt.await();
            n = 0;
            pt = PauseableThread.make(s);
            // Start it up.
            pt.start();
            Thread.sleep(1000);
            pt.pause();
            System.out.println("Paused: " + n);
            Thread.sleep(1000);
            System.out.println("Resuminng: " + n);
            pt.resume();
            Thread.sleep(1000);
            // Cancel aggressively.
            pt.cancel(true);
            System.out.println("Finished: " + n);
            System.out.println("thrown: " + pt.getThrown());

        } catch (InterruptedException e) {
        }
    }
}

Answer 2

It is never legal to start a thread more than once. In particular, a thread may not be restarted once it has completed execution.

-- from the Thread API

Answer 3

First of all never handwrite GUI code, there are plenty of awesome tools for that generate perfectly organized modular and readable code. My personal favourites are if its Swing : Netbeans GUI builder, if its JavaFX (which is awesome in comparison to Swing) then its SceneBuilder which in its turn is oficially supported and provided by Oracle.

Second and most important Thread can be started only once... so make sure your code doesent try to start it again... thats why you'll get IllegalStateException because Thread instance execution is already finished...

Also threads are heavy objects, one should be carefull creating those, in your case its ok of course but generally Its good to have thread pool and reuse threads... Always avoid creating threads manually, as you must track them to be properly finished, if you leave any non deamon thread alive your application wont finish its execution...

Answer 4

Sketch of a simple solution:

• Your class does not need to extend Thread , simply implement Runnable
• When listening to the button events, first make sure you react only on those events you really want to
• In your ActionListener , create a new Thread each time you want to start one, initialize it with your Runnable object and call Thread.start()