Importance of wait(), notify() and notifyAll() methods in Java?

The threads can communicate with each other through wait(), notify() and notifyAll() methods in Java. These are final methods defined in the Object class and can be called only from within a synchronized context. The wait() method causes the current thread to wait until another thread invokes the notify() or notifyAll() methods for that object.

The notify() method wakes up a single thread that is waiting on that object’s monitor. The notifyAll() method wakes up all threads that are waiting on that object’s monitor. A thread waits on an object’s monitor by calling one of the wait() method. These methods can throw IllegalMonitorStateException if the current thread is not the owner of the object’s monitor.

wait() method Syntax

public final void wait() throws InterruptedException

notify() Method Syntax

public final void notify()

notifyAll() method Syntax

public final void notifyAll()

Example

public class WaitNotifyTest {
   private static final long SLEEP_INTERVAL = 3000;
   private boolean running = true;
   private Thread thread;
   public void start() {
      print("Inside start() method");
      thread = new Thread(new Runnable() {
         @Override
         public void run() {
            print("Inside run() method");
            try {
               Thread.sleep(SLEEP_INTERVAL);
            } catch(InterruptedException e) {
               Thread.currentThread().interrupt();
            }
            synchronized(WaitNotifyTest.this) {
               running = false;
               WaitNotifyTest.this.notify();
            }
         }
      });
      thread.start();
   }
   public void join() throws InterruptedException {
      print("Inside join() method");
      synchronized(this) {
         while(running) {
            print("Waiting for the peer thread to finish.");
            wait(); //waiting, not running
         }
         print("Peer thread finished.");
      }
   }
   private void print(String s) {
      System.out.println(s);
   }
   public static void main(String[] args) throws InterruptedException {
      WaitNotifyTest test = new WaitNotifyTest();
      test.start();
      test.join();
   }
}

Output

Inside start() method
Inside join() method
Waiting for the peer thread to finish.
Inside run() method
Peer thread finished.

raja

Updated on: 27-Nov-2023

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