Getting a thread to wake up after a notify to called

[Knute Snortum]

Hello all,

I am writing a proof-of-concept class to determine how to pause and resume a thread doing a cyclical process in Swing. The idea is to have an instance field called paused that the thread polls.  When the field is true, thread process will call Object.wait() to pause.  Then when the main proccess wants to resume, the paused field is set to false and an Object.notify() is called.

The thread process pauses just fine but when it comes to resuming, it never sees the notify() from the main process. Any help would be welcome.  Below is my code.

package swing;

import java.awt.BorderLayout;
import java.awt.event.ItemEvent;
import java.awt.event.ItemListener;

import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JToggleButton;
import javax.swing.SwingUtilities;

public class ThreadCommunication extends JFrame {
	private static final long serialVersionUID = -3326890771329640907L;

	private int counter = 0;
	private boolean paused = false;
	private volatile Thread counterThread;

	public ThreadCommunication() {
		
		setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		setSize(100, 100);
		setLocationRelativeTo(null);

		setLayout(new BorderLayout());
		JLabel counterLabel = new JLabel("0", JLabel.CENTER);
		add(counterLabel, BorderLayout.CENTER);

		JToggleButton pauseButton = new JToggleButton("Pause");
		add(pauseButton, BorderLayout.SOUTH);

		setVisible(true);

		counterThread = new Thread(() -> {
			while (true) {
				counterLabel.setText(String.valueOf(counter));
				counter++;

				try {
					Thread.sleep(300);
					
					if (paused) {
						System.out.println("In counterThread, waiting"); // debug
						synchronized(this) {
		                    while (paused) {
		                        wait();
		                        System.out.println("  not waiting"); // debug
		                    }
		                }
					}
				} catch (InterruptedException e) {
				}
			}
		});
		counterThread.setDaemon(true);
		counterThread.start();

		pauseButton.addItemListener(new ItemListener() {
			
			@Override
			public synchronized void itemStateChanged(ItemEvent event) {
				System.out.println("In itemStateChange..."); // debug
				
				if (event.getStateChange() == ItemEvent.SELECTED) {
					// communicate pause to thread
					System.out.println("  paused"); // debug
					paused = true;
				} else {
					// communicate resume to thread
					System.out.println("  resume"); // debug
					paused = false;
					notify();
				}
			}
		});
	}

	public static void main(String[] args) {
		SwingUtilities.invokeLater(() -> {
			new ThreadCommunication();
		});
	}

}

[Piet Souris]

In this code:

synchronized(this) {
   while (paused) {
       wait();
      ...

it is uncertain to what this 'this' refers to. But it is not the object that issues the 'notify' later on.

A simple remedy is to have a field Object object. Synchonize twice on this object, do object.wait() and object.notify(), and your code runs fine.

[Knute Snortum]

Thanks, that did it!