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()

1) wait() method
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 InterruptedException :  waits until object is notified.
public final void wait(long timeout)throws InterruptedException :  waits for the specified amount of time.

2) notify() method
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()

3) notifyAll() method

Wakes up all threads that are waiting on this object's monitor. Syntax:
Syntax:
public final void notifyAll()

Difference between wait and sleep?

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

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

Example of inter thread communication in java

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...");
          if(this.amount<amount){
                 System.out.println("Less balance; waiting for deposit...");
                 try{wait();}catch(Exception e){}
         }
         this.amount-=amount;
         System.out.println("withdraw completed...");
     }
   synchronized void deposit(int amount){
         System.out.println("going to deposit...");
         this.amount+=amount;
         System.out.println("deposit completed... ");
         notify();
  }
}

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



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