The Object class in java contains three final methods that allows threads to communicate about the lock status of a resource. These methods arewait(),notify()andnotifyAll(). So today we will look into wait, notify and notifyAll in java program.

The current thread which invokes these methods on any object should have the objectmonitorelse it throwsjava.lang.IllegalMonitorStateExceptionexception.

wait

Object wait methods has three variance, one which waits indefinitely for any other thread to call notify or notifyAll method on the object to wake up the current thread. Other two variances puts the current thread in wait for specific amount of time before they wake up.

notify

notify method wakes up only one thread waiting on the object and that thread starts execution. So if there are multiple threads waiting for an object, this method will wake up only one of them. The choice of the thread to wake depends on the OS implementation of thread management.

notifyAll

notifyAll method wakes up all the threads waiting on the object, although which one will process first depends on the OS implementation.

Exampe:

Message

public class Message {
    private String msg;
    
    public Message(String str){
        this.msg=str;
    }

    public String getMsg() {
        return msg;
    }

    public void setMsg(String str) {
        this.msg=str;
    }

}

Waiter

public class Waiter implements Runnable{
    
    private Message msg;
    
    public Waiter(Message m){
        this.msg=m;
    }

    @Override
    public void run() {
        String name = Thread.currentThread().getName();
        synchronized (msg) {
            try{
                System.out.println(name+" waiting to get notified at time:"+System.currentTimeMillis());
                msg.wait();
            }catch(InterruptedException e){
                e.printStackTrace();
            }
            System.out.println(name+" waiter thread got notified at time:"+System.currentTimeMillis());
            //process the message now
            System.out.println(name+" processed: "+msg.getMsg());
        }
    }

}

Notifier

public class Notifier implements Runnable {

    private Message msg;
    
    public Notifier(Message msg) {
        this.msg = msg;
    }

    @Override
    public void run() {
        String name = Thread.currentThread().getName();
        System.out.println(name+" started");
        try {
            Thread.sleep(1000);
            synchronized (msg) {
                msg.setMsg(name+" Notifier work done");
                msg.notify();
                // msg.notifyAll();
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        
    }

}

WaitNotifyTest

public class WaitNotifyTest {

    public static void main(String[] args) {
        Message msg = new Message("process it");
        Waiter waiter = new Waiter(msg);
        new Thread(waiter,"waiter").start();
        
        Waiter waiter1 = new Waiter(msg);
        new Thread(waiter1, "waiter1").start();
        
        Notifier notifier = new Notifier(msg);
        new Thread(notifier, "notifier").start();
        System.out.println("All the threads are started");
    }

}

Output

If we comment the notify() call and uncomment the notifyAll() call in Notifier class, below will be the output produced.