Inter-thread communication in Java

Inter-thread communication or Co-operation is all about allowing synchronized threads to communicate with each other.

Cooperation (Inter-thread communication) is a mechanism in which a thread is paused running in its critical section and another thread is allowed to enter (or lock) in the same critical section to be executed.It is implemented by following methods of Object class:

  • wait()
  • notify()
  • notifyAll()

Causes current thread to release the lock and wait until either another thread invokes the notify() method or the notifyAll() method for this object, or a specified amount of time has elapsed.

The current thread must own this object's monitor, so it must be called from the synchronized method only otherwise it will throw exception.

public final void wait()throws InterruptedExceptionwaits until object is notified.
public final void wait(long timeout)throws InterruptedExceptionwaits for the specified amount of time.

Wakes up a single thread that is waiting on this object's monitor. If any threads are waiting on this object, one of them is chosen to be awakened. The choice is arbitrary and occurs at the discretion of the implementation. Syntax:

public final void notify()


Wakes up all threads that are waiting on this object's monitor. Syntax:

public final void notifyAll()

Understanding the process of inter-thread communication

inter thread communication in java

The point to point explanation of the above diagram is as follows:

  1. Threads enter to acquire lock.
  2. Lock is acquired by on thread.
  3. Now thread goes to waiting state if you call wait() method on the object. Otherwise it releases the lock and exits.
  4. If you call notify() or notifyAll() method, thread moves to the notified state (runnable state).
  5. Now thread is available to acquire lock.
  6. After completion of the task, thread releases the lock and exits the monitor state of the object.

Why wait(), notify() and notifyAll() methods are defined in Object class not Thread class?

It is because they are related to lock and object has a lock.

Difference between wait and sleep

Let's see the important differences between wait and sleep methods.

wait() method releases the locksleep() method doesn't release the lock.
is the method of Object classis the method of Thread class
is the non-static methodis the static method
is the non-static methodis the static method
should be notified by notify() or notifyAll() methodsafter the specified amount of time, sleep is completed.

Let's see the simple example of inter thread communication.

class Customer{
int amount=10000;

synchronized void withdraw(int amount){
System.out.println("going to withdraw...");

System.out.println("Less balance; waiting for deposit...");
try{wait();}catch(Exception e){}
System.out.println("withdraw completed...");

synchronized void deposit(int amount){
System.out.println("going to deposit...");
System.out.println("deposit completed... ");

class Test{
public static void main(String args[]){
final Customer c=new Customer();
new Thread(){
public void run(){c.withdraw(15000);}
new Thread(){
public void run(){c.deposit(10000);}

Output: going to withdraw... Less balance; waiting for deposit... going to deposit... deposit completed... withdraw completed
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