inter thread communication notify is not getting called

[amithk kumar]

i have created a class called hotel  in which  i have set a boolean variable called conditionVar to false. inside the run method i have used two synchonized blocks, one is for wait and other is for notify .But the application is showing output as

thread A in the waiterThread
thread A in the while  waiterThread
thread B in the waiterThread
thread B in the while  waiterThread

it is not entering in the synchronized block of notify . Is this an concept of busy waiting? and how can the thread call notify?

public class Hotel implements Runnable {

	public boolean conditionVar = false;

	public String str = new String();

	public void run() {


		// synchronized block of waiter thread
		synchronized (str) {
			System.out.println(Thread.currentThread().getName() + " in the waiterThread");
			while (!conditionVar) {
				System.out.println(Thread.currentThread().getName() + " in the while  waiterThread");

				try {
					str.wait();
				} catch (InterruptedException e) {
					// TODO Auto-generated catch block
					e.printStackTrace();
				}

			}

		}

		// synchronized block of notifier thread
		synchronized (str) {
			System.out.println(Thread.currentThread().getName() + " in the notifierThread");

			conditionVar = true;

			str.notify();

		}

	

	}
 
}

public class Check {

	public static void main(String[] args) {
  
		Hotel hotel=new Hotel();
		
		Thread t1=new Thread(hotel,"thread A");
		Thread t2=new Thread(hotel,"thread B");
		
		
		t1.start();
		t2.start();
		
		
		
	}

}

[Paul Clapham]

The block starting at line 28 can never be entered by either thread.

Why? Because the block starting at line 11 is an infinite loop. Control cannot exit from that block until the conditionVar variable is set to true, which cannot happen.

Still not persuaded? Consider thread A. It can't get out of the line-11 block until some other thread sets conditionVar to true. Now consider thread B. It can't get out of the line-11 block until some other thread sets conditionVar to true. So B would have to set it to allow A to proceed, and A would have to set it for B to proceed. I suppose you could call that self-inflicted deadlock.

[amithk kumar]

so how to make the thread call the notify?

[Norman Radder]

amithk kumar wrote:so how to make the thread call the notify?

Can you explain which thread you want to call notify and what state it needs detect to know that it  should call notify?

How are the two threads supposed to interact?  Why the use of wait and notify?

[Paul Clapham]

If you remove lines 13 and 23 from your code then it will call wait and then go on directly to call notify. That satisfies your stated goal of calling notify, but like Norman I have no idea of why you would want it to do that. I don't understand what the code is supposed to be doing.

[Paul Clapham]

Paul Clapham wrote:If you remove lines 13 and 23 from your code then it will call wait and then go on directly to call notify.

No, actually it won't. You still have the deadlock where both threads start out by waiting. How you "fix" that I have no idea, because that would require knowing what the code was really meant to do.

[Piet Souris]

In the current situation, only an external factor can break the deadlock. For instance:

class Check {
 
    public static void main(String[] args) {
   
        Hotel hotel=new Hotel();
         
        Thread t1 = new Thread(hotel,"thread A");
        Thread t2 = new Thread(hotel,"thread B");
        t1.start();
        t2.start();
        
        try {
            System.out.println("Going to sleep for 6 seconds...");
            Thread.sleep(6_000);
            System.out.println("out of the 6 second sleep... activating the waiting threads...");
        }
        catch (InterruptedException e) {
        }

        synchronized (hotel.str) {
            hotel.conditionVar = true;  // if you comment this out, then still a deadlock
            hotel.str.notifyAll();
        }