96 Java - Inter Thread Communication

If you are aware of interprocess communication then it will be easy for you to understand interthread communication.

Inter-thread Communication in Java

Inter-thread communication is important when you develop an application where two or more threads exchange some information. Inter-thread communication is achieved by using the wait(), notify(), and notifyAll() methods of the Object class.

Methods used for Inter-thread Communication

There are three simple methods and a little trick which makes thread communication possible. All the three methods are listed below −

Sr.No.	Method & Description
1	public void wait() Causes the current thread to wait until another thread invokes the notify().
2	public void notify() Wakes up a single thread that is waiting on this object's monitor.
3	public void notifyAll() Wakes up all the threads that called wait( ) on the same object.

These methods have been implemented as final methods in Object, so they are available in all the classes. All three methods can be called only from within a synchronized context.

Example of Inter-thread Communication in Java

This examples shows how two threads can communicate using wait() and notify() method. You can create a complex system using the same concept.

class Chat {
   boolean flag = false;

   public synchronized void Question(String msg) {
      if (flag) {
         try {
            wait();
         } catch (InterruptedException e) {
            e.printStackTrace();
         }
      }
      System.out.println(msg);
      flag = true;
      notify();
   }

   public synchronized void Answer(String msg) {
      if (!flag) {
         try {
            wait();
         } catch (InterruptedException e) {
            e.printStackTrace();
         }
      }

      System.out.println(msg);
      flag = false;
      notify();
   }
}

class T1 implements Runnable {
   Chat m;
   String[] s1 = { "Hi", "How are you ?", "I am also doing fine!" };

   public T1(Chat m1) {
      this.m = m1;
      new Thread(this, "Question").start();
   }

   public void run() {
      for (int i = 0; i < s1.length; i++) {
         m.Question(s1[i]);
      }
   }
}

class T2 implements Runnable {
   Chat m;
   String[] s2 = { "Hi", "I am good, what about you?", "Great!" };

   public T2(Chat m2) {
      this.m = m2;
      new Thread(this, "Answer").start();
   }

   public void run() {
      for (int i = 0; i < s2.length; i++) {
         m.Answer(s2[i]);
      }
   }
}
public class TestThread {
   public static void main(String[] args) {
      Chat m = new Chat();
      new T1(m);
      new T2(m);
   }
}

When the above program is complied and executed, it produces the following result −

Output

Hi
Hi
How are you ?
I am good, what about you?
I am also doing fine!
Great!