SCJP/OCJP 6 (Threads)

[Tn Sharma]

class A extends Thread{
int count = 0;

public void run(){
System.out.println("run");
synchronized (this) {
for(int i =0; i < 50 ; i++){
count = count + i;

}
}

}
}


public class T{

public static void main(String argv[]) {
A a = new A();
a.start();

synchronized (a) {
System.out.println("waiting");
try{
a.wait();
}catch(InterruptedException e){
e.printStackTrace();
}
System.out.println(a.count);

}
}
}

notify() is not called anywhere BUT waiting Thread is waking up and printing value of a
why it is happening here.

[Henry Wong]

Tn Sharma wrote:
notify() is not called anywhere BUT waiting Thread is waking up and printing value of a
why it is happening here.

You are conflicting with the core library. Thread class instances are used internally to implement the join() mechanism. Threads that call the join() method will wait() on the Thread instance. And when a thread terminates, it will send a notification (all) on the Thread instance.

Henry

[Tn Sharma]

Henry Wong wrote:

Tn Sharma wrote:
notify() is not called anywhere BUT waiting Thread is waking up and printing value of a
why it is happening here.

You are conflicting with the core library. Thread class instances are used internally to implement the join() mechanism. Threads that call the join() method will wait() on the Thread instance. And when a thread terminates, it will send a notification (all) on the Thread instance.

Henry

And what if somehow notify() is called before wait() .then waiting will infinite?

[Henry Wong]

Tn Sharma wrote:
And what if somehow notify() is called before wait() .then waiting will infinite?

Who is calling wait()? Who is calling notify()? There are multiple use-cases in context with this topic, so, which one do you mean?

Anyway...

In the case of your original example (ie. your thread calling wait(), and the clean up code for the thread calling notifyAll()). ... It is not possible for your thread to call wait() after the clean up code for your thread to call notifyAll(). As long as your thread is running, the environment can't clean up after it.

In the case the core library implementation of the join() method (ie. the thread that called join() calling wait(), and the clean up code for the thread calling notifyAll()). ... It is not possible for wait() to be called by join() after the clean up call. The implementation of join() does not do a blind wait. It will check to confirm that the thread is still alive before calling wait().

In the general case (ie. general question -- and not related to this topic). ... Notifications that are sent, when there are no threads waiting for it, are simply discarded.

Henry

[Tn Sharma]

Henry Wong wrote:

Tn Sharma wrote:
And what if somehow notify() is called before wait() .then waiting will infinite?

Who is calling wait()? Who is calling notify()? There are multiple use-cases in context with this topic, so, which one do you mean?

Anyway...

In the case of your original example (ie. your thread calling wait(), and the clean up code for the thread calling notifyAll()). ... It is not possible for your thread to call wait() after the clean up code for your thread to call notifyAll(). As long as your thread is running, the environment can't clean up after it.

In the case the core library implementation of the join() method (ie. the thread that called join() calling wait(), and the clean up code for the thread calling notifyAll()). ... It is not possible for wait() to be called by join() after the clean up call. The implementation of join() does not do a blind wait. It will check to confirm that the thread is still alive before calling wait().

In the general case (ie. general question -- and not related to this topic). ... Notifications that are sent, when there are no threads waiting for it, are simply discarded.

Henry

class A extends Thread {
	int count = 0;

	public void run() {
		System.out.println("run");
		synchronized (this) {
			for (int i = 0; i < 50; i++) {
				count = count + i;
			}
			this.notify();
		}
	}
}

public class T {
	public static void main(String argv[]) {
		A a = new A();
		a.start();
		try {
			Thread.sleep(5000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		synchronized (a) {
			System.out.println("waiting");
			try {
				a.wait();
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
			System.out.println(a.count);
		}
	}
}

Please observe this code
As main thread is calling wait();
and run() method on thread a is calling notify();

But as per code
main is sleeping for 5 seconds while notify() from thread a is called before 5 seconds completed .
After 5 seconds main() will call wait() and there will no notify() or notifyAll() is there.

So this main() will infinitely waiting ( tested by me)

Please differentiate the behavior of this code with original code

[Henry Wong]

Tn Sharma wrote:
Please differentiate the behavior of this code with original code

These are two different examples/cases. The first (original) case is explained already. And the second case, is explained by the "general case" in my previous post.

Henry