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Is notify() mandatory?

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Hi All, I have a doubt related to the wait() and notify(). Please look at the following code:

1. class ThreadA {
2. public static void main(String [] args) {
3. ThreadB b = new ThreadB();
4. b.start();
6. synchronized(b) {
7. try {
8. System.out.println("Waiting for b to complete...");
9. b.wait();
10. System.out.println("After waiting");
11. } catch (InterruptedException e) {}
12. System.out.println("Total is: " +;
13. }
14. }
15. }
17. class ThreadB extends Thread {
18. int total;
20. public void run() {
21. synchronized(this) {
22. for(int i=0;i<100;i++) {
23. total += i;
24. }
25. notify();
26. }
27. }
28. }

In line number no 25, the notify() method is included. If I remove this notify(), will the thread that invokes the wait() method will wait for ever??.

I tried removing the notify() method. I get the same results irrespective of the presence of notify() method.

Output(Irrespective of whether I include notify() or not:

Wating for b to complete
After waiting
Please explain whether the notify() method is mandatory if we call the wait() method or once the called thread is completed, will the waiting method resume by itself..

[ November 07, 2008: Message edited by: Loganathan Karunakaran ]
Posts: 23879
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What you are seeing is an implementation detail of the join() method. The join() method checks the status of the thread, and if the thread is still alive, it will call wait on the thread object. For this to work, there is a notifyAll() call in the thread's cleanup code -- meaning once the run() method terminates, during the cleanup, all the joining threads will be woken up, as soon as the thread is marked as no longer alive.

Now... It is unfortunate that the designers of the thread class, chose to use the thread class for this notification. I guess they didn't think that developers would like to wait on the thread instance for other reasons too. But regardless, this is an implementation detail. It may change in future implementation -- do not depend on notifications being sent upon thread termination.

Loganathan Karunakaran
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Henry, Thanks for taking time to explain this.

Thanks much.

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I believed they defended this on the grounds it was an example of 'spurious thread wakeup' (I'm not commenting on the argument)
ie wait should always be used in a loop testing the reason for wakeup as wait said it could just wake up without a notify.

So I'd put the wait in a loop as other reasons for spurious thread wakeup exist and new ones do appear.
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