A good small example to demonstrate wait() and notify() method in java

Question

Can anybody please provide me a good small example demonstrate wait() and notify() functionality in java. I've tried with the below piece of code but it's not showing what i expected.

public class WaitDemo {
    int i = 10;

    int display() {
        System.out.println("Lexmark");
        i++;
        return i;
    }
}

public class ClassDemo1 extends Thread {

    private WaitDemo wd = new WaitDemo();

    public static void main(String[] args) {
        ClassDemo1 cd1 = new ClassDemo1();
        ClassDemo1 cd2 = new ClassDemo1();
        cd1.setName("Europe");
        cd2.setName("America");
        cd1.start();
        cd2.start();

    }

    synchronized void display() {
        System.out.println("Hello");
        notifyAll();
    }

    public void run() {

        synchronized (this) {
            try {
                {
                    notify();
                    System.out.println("The thread is " + currentThread().getName());
                    wait();
                    System.out.println("The value is " + wd.display());
                }
            } catch (InterruptedException e) {

            }

        }
    }
}

The issue is that the method in the class WaitDemo is not getting executed and as per my idea the SOP after wait() should execute. Please help me out on this.

@Крысa: Remember that one of SO's goals is to be the top hit on Google (and other) searches. This is a perfectly reasonable question. It should be answered with an example -- here, on SO, not elsewhere -- and discussion (ideally) of where Sourav went wrong above. — T.J. Crowder, Dec 02 '11 at 15:33
Sourav, in order to help people help you, *please* get a good code formatter to deal with that ... *unusual* ... code indentation style. I'm not starting a style war, but you'd have to accept that the above is not the usual sort of thing, and keeping to the usual sort of thing will help people help you. With all due respect, it looks really quite sloppy and hard to follow, not like an actual style at all... **Update**: Ah, @Alex K fixed it for you. — T.J. Crowder, Dec 02 '11 at 15:35

score 3 · Answer 1 · edited Jan 15 '19 at 05:11

Below is an example of wait & notify in the Object class. The customer is trying to withdraw money of value 2000 but the account is having only 1000 so it has to wait for the deposit. Once the deposit is made, then the customer will be able to withdraw the amount. Until the deposit is made, the customer will be waiting.

class Cust {

    private int totalAmount = 1000;

    public synchronized void withdrawal(int amount) {
        System.out.println("Total amount " + totalAmount + " withdrawing amount " + amount);
        while (this.totalAmount < amount) {
            System.out.println("not enough amount..waiting for deposit..");
            try { wait(); } catch (Exception e) {}
        }
        this.totalAmount -= amount;     
        System.out.println("Withdrawal successful.. Remaining balance is "+totalAmount);    
    }

    public synchronized void deposit(int amount){
        System.out.println("Depositing amount "+amount);
        this.totalAmount += amount;
        System.out.println("deposit completed...and Now totalAmount is " + this.totalAmount);
        notify();
    }
}

class Depo implements Runnable {
    Cust c; int depo;

    Depo(Cust c, int depo){
        this.c = c;
        this.depo = depo;
    }

    @Override
    public void run() {
        c.deposit(depo);    
    }
}

class Withdrawal implements Runnable {
    Cust c; int with;

    Withdrawal(Cust c, int with){
        this.c = c;
        this.with = with; 
    }

    @Override
    public void run() {
        c.withdrawal(with);
    }
}

public class ObjectWaitExample {

    public static void main(String[] args) {
        Cust c = new Cust();
        Thread w = new Thread(new Withdrawal(c, 2000));
        Thread d1 = new Thread(new Depo(c, 50));
        Thread d2 = new Thread(new Depo(c, 150));
        Thread d3 = new Thread(new Depo(c, 900));
        w.start();
        d1.start();
        d2.start();
        d3.start();
    }

}

I have changed the `if` to `while` to prevent negative balance. — Raj, Jan 15 '19 at 05:11

jefflunt · Answer 2 · 2011-12-02T15:36:43.437

You've got two levels of braces { in your try block. If you remove the inner set (which doesn't appear to do anything), does that fix the problem?

There are several examples around, all of which demonstrate the use. The last link is a set of results that can help you out. If you need more specific things, let me know what it is that your app is trying to do, and I can try to find examples that are more specific to your situation.

Fullstack Guy · Answer 3 · 2016-03-23T06:04:08.893

I created two threads one for printing odd numbers (OddThread) and another for even numbers (EvenThread). Inside the run method of each of the threads I used the shared object of class Print to call printOdd() and printEven() for the Odd and EvenThread respectively. I made the shared object of Print static so that only one copy is made. Now synchronizing on the Print object I used a Boolean flag such that when the odd thread printed an odd number it will be sent into the waiting state and the at the same time notifying the even thread to execute. The logic is written in such a way that the odd thread will always print the odd number first no matter what, as the flag is set to false initially preventing the even thread to execute and sending it to a waiting state.

        package com.amardeep.test;

        public class ThreadDemo {
            // Shared object
            static Print print = new Print();

            public static void main(String[] args) {

                new Thread(new OddThread()).start();
                new Thread(new EvenThread()).start();

            }
        }

        class EvenThread implements Runnable {

            @Override
            public void run() {
                ThreadDemo.print.printEven();

            }

        }

        class OddThread implements Runnable {

            @Override
            public void run() {

                ThreadDemo.print.printOdd();
            }

        }

        class Print {
            public volatile boolean flag = false;

            public synchronized void printEven() {

                for (int i = 1; i <= 10; i++) {
                    if (!flag) {
                        try {
                            wait();
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    } else {
                        if (i % 2 == 0) {
                            System.out.println("from even " + i);
                            flag = false;
                            notifyAll();
                        }

                    }
                }
            }

            public synchronized void printOdd() {
                for (int i = 1; i <= 10; i++) {
                    if (flag) {
                        try {
                            wait();
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    } else {
                        if (i % 2 != 0) {
                            System.out.println("from odd " + i);
                            flag = true;
                            notifyAll();
                        }

                    }
                }
            }
        }
    output:-
    from odd 1
    from even 2
    from odd 3
    from even 4
    from odd 5
    from even 6
    from odd 7
    from even 8
    from odd 9
    from even 10

score 0 · Answer 4 · answered Nov 05 '17 at 06:58

What wait method does is , when some thread executed a synchronized block by locking some object (we call that object is "a") , then inside that synchronized block when the thread executed the wait method of object "a" like this

A a = new A (); // some class object call "a"
  synchronized (a){
  a.wait ();//exceptions must be handled
}

Then the a object will release and the thread has to go to the wait state until it has been release from that state.

and anothet thread now can use the a object beacause its a release object. so if another thread locked that object and it executed the notify method from that object like

 a.notify ()

Then one of a thread of the threads that went to wait state by object "a" can be released from the wait state. Other wise when call the notifyAll then the all the thread objects will release from that state.

score 0 · Answer 5 · answered Dec 02 '11 at 15:58

Your problem is that you are creating two instances of the Thread class. Thus when the wait() is called, it is on two different instances, neither of which has another thread that is in contention for your monitor, nor is there another thread to call notifyAll() to wake the thread from its wait state.

Thus each thread you have started will wait forever (or until interrupted for some other reason).

You want to have multiple threads accessing the same monitor, so start by trying to code something in which the code in question is not actually a thread, but is simply being used by a thread.

@normalocity has already provided links to multiple examples.

score 0 · Answer 6 · edited May 23 '17 at 12:33

I just updated this answer to include an SCCE.

The workers call pauseIfNeeded on the WorkerPauseManager. If the manager is paused when the worker thread calls pauseIfNeeded(), we call wait(), which tells the calling thread to wait until a different thread calls notify() or notifyAll() on the object being waited on. This happens when the Swing Event Dispatch Thread calls play() on the manager, which in turn calls notifyAll().

Note that you must have a synchronized lock on the object you are calling wait() or notify() on. Since the methods in WorkerPauseManager are synchronized, all the synchronized methods are getting a synchronized lock on the WorkerPauseManager itself.

import javax.swing.*;
import java.awt.event.ActionEvent;

/**
 * @author sbarnum
 */
public class WorkerPauseManagerTest {
    public static void main(String[] args) {
        final WorkerPauseManager pauseManager = new WorkerPauseManager();
        new Worker("Worker 1", pauseManager).start();
        new Worker("Worker 2", pauseManager).start();
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                JToggleButton playPauseButton = new JToggleButton(new AbstractAction("Pause") {
                    public void actionPerformed(final ActionEvent e) {
                        JToggleButton source = (JToggleButton) e.getSource();
                        if (source.isSelected()) {
                            pauseManager.start();
                            source.setText("Pause");
                        } else {
                            pauseManager.pause();
                            source.setText("Play");
                        }
                    }
                });
                playPauseButton.setSelected(true); // already running
                JOptionPane.showMessageDialog(null, playPauseButton, "WorkerPauseManager Demo", JOptionPane.PLAIN_MESSAGE);
                System.exit(0);
            }
        });

    }

    private static class Worker extends Thread {
        final String name;
        final WorkerPauseManager pauseManager;

        public Worker(final String name, final WorkerPauseManager pauseManager) {
            this.name = name;
            this.pauseManager = pauseManager;
        }

        @Override
        public void run() {
            while (!Thread.interrupted()) {
                try {
                    pauseManager.pauseIfNeeded();
                    System.out.println(name + " is running");
                    Thread.sleep(1000L);
                } catch (InterruptedException e) {
                    throw new RuntimeException(e);
                }
            }
        }
    }

    public static final class WorkerPauseManager {

        private boolean paused;

        public synchronized void pauseIfNeeded() throws InterruptedException {
            if (paused) wait();
        }

        public synchronized void pause() {
            this.paused = true;
        }

        public synchronized void start() {
            this.paused = false;
            notifyAll();
        }
    }
}

score -2 · Answer 7 · edited Oct 01 '12 at 12:13

/*
 *  the below program is like 
 * tread t1 will first run , and it comes to "notify()" method
 * there are no threds waiting bcoz this is the first thread.
 * so it will not invoke any other threads. next step is "wait()" method
 *will be called and the thread t1 in waiting state. next stament 
 * "System.out.println("The value is ..."+wd.display());" will not be  executed
 * because thread t1 is in waiting state.
 * 
 * thread t2 will run ,and it comes to "notify()" method ,there is already 
 * thread t1 is in waiting state ,then it will be invoked.now thread t1 will
 * continue execution and it prints the statement "System.out.println("The value is ..."+wd.display())"
 * and thread t2 will be in waiting state now.
 * 
 * if you uncomment "notifyAll()" method then, after t1 thread completes its execution
 *then immediately "notifyAll()" method will be called,by that time thread t2 is 
 * already in waiting state , then thread t2 will be invoked and continues execution.
 *or
 * if any other threadds are in waiting state all those threads will be invoked.
 */
package threadsex;

/**
 *
 * @author MaheshM
 */
/**
 * @param args the command line arguments
 */
public class WaitNotifyNotifyAllDemo implements Runnable {
    WaitDemo wd = new WaitDemo();

    public static void main(String[] args) {
        WaitNotifyNotifyAllDemo cd1 = new WaitNotifyNotifyAllDemo();
        Thread t1 = new Thread(cd1);
        t1.setName("mahi1");
        Thread t2 = new Thread(cd1);
        t2.setName("mahi2");
        t1.start();         
        t2.start();
    }

    @Override
    public void run() {
        synchronized (this) {
            try {

                System.out.println("The thread is=" + 
                        Thread.currentThread().getName());
                notify();
                wait();
                System.out.println("The value is ..." + wd.display());
                //         notifyAll();

            } catch (Exception ex) {
                ex.printStackTrace();
            }
        }
    }
}

A good small example to demonstrate wait() and notify() method in java

7 Answers7

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