Interthread Communication

Java includes an interprocess communication mechanism via the wait( ), notify( ), and notifyAll( ) methods. These methods are implemented as final methods in Object, so all classes have them. All three methods can be called only from within a synchronized context.

wait( ): tells the calling thread to give up the monitor and go to sleep until some other thread enters the same monitor and calls notify( ).
notify( ): wakes up a thread that called wait( ) on the same object.
notifyAll( ): wakes up all the threads that called wait( ) on the same object. One of the threads will be granted access.

These methods are declared within Object, as shown here:


final void wait( ) throws InterruptedException 
final void notify( ) 
final void notifyAll( )

The proper way to write this program in Java is to use wait( ) and notify( ) to signal in both directions:


class SharedBox {
  int n;
  boolean valueSet = false;

  synchronized int get() {
    while (!valueSet)
      try {
        wait();
      } catch (Exception e) {
      }
    System.out.println("Got: " + n);
    valueSet = false;
    notify();
    return n;
  }
  synchronized void put(int n) {
    while (valueSet)
      try {
        wait();
      } catch (Exception e) {
      }
    this.n = n;
    valueSet = true;
    System.out.println("Put: " + n);
    notify();
  }
}

class Producer implements Runnable {
  SharedBox box;

  Producer(SharedBox q) {
    this.box = q;
    new Thread(this, "Producer").start();
  }

  public void run() {
    int i = 0;
    while (true) {
      box.put(i++);
    }
  }
}

class Consumer implements Runnable {
  SharedBox box;

  Consumer(SharedBox q) {
    this.box = q;
    new Thread(this, "Consumer").start();
  }

  public void run() {
    while (true) {
      box.get();
    }
  }
}

public class Main {
  public static void main(String args[]) throws Exception {
    SharedBox q = new SharedBox();
    new Producer(q);
    new Consumer(q);
    
    Thread.sleep(2000);
    System.exit(0);
  }
}

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