Can a waiting thread be notified without an explicit notify invocation on the synchronized object ?

[Rajanand Pandaraparambil Kuttappan]

public class Fizz {  
    static Thread laurel, hardy;  
    public static void main(String[] args) {  
  
        laurel = new Thread() {  
            public void run() {  
                System.out.println("laurel: starts running");  
                try {  
                	System.out.println("laurel: going to sleep");
                    Thread.sleep(1000);  
                    System.out.println("laurel: out from  sleep");
                } catch (Exception e) {  
                    System.out.println("laurel: in exception from sleep");  
                }  
                System.out.println("laurel: is exiting run");  
            }  
        };  
        hardy = new Thread() {  
            public void run() {  
                System.out.println("hardy: starts running");  
                try {  
                    synchronized (laurel){ // Synchronize on the laurel instance  
                    	System.out.println("hardy: going to wait for laurel");
                        laurel.wait();  
                        System.out.println("hardy: IS OUT FROM WAIT ON LAUREL.");
                    }  
                } catch (Exception e) {  
                    System.out.println("hardy: in exception from wait");  
                }  
                System.out.println("hardy: is exiting run");  
            }  
        };  
        laurel.start();  
        hardy.start();  
    }  
}  

The output is :-

laurel: starts running
laurel: going to sleep
hardy: starts running
hardy: going to wait for laurel
laurel: out from sleep
laurel: is exiting run
hardy: IS OUT FROM WAIT ON LAUREL.
hardy: is exiting run

Can any one please explain how the thread 'hardy' came-out from the waiting and who notified on the synchronized object 'laurel'? Please note that there is no laurel.notify() statement anywhere in the code-snippet.

Also if I comment-out the try-catch including the Thread.sleep, hardy waits forever, though laurel completes its execution.

Thanks
Raj

[Ankit Garg]

When you wait on a Thread object then you get a notification when the target thread completes. So if Thread A is waiting on Thread B object, then Thread A will be notified when Thread B completes...

[Rajanand Pandaraparambil Kuttappan]

if Thread A is waiting on Thread B object, then Thread A will be notified when Thread B completes...

So, JLS treats this an exceptional case for the thread being used as a monitor, right? Because, theoretically speaking, if there is an object1.wait() from ThreadA, ThreadA would wait until there comes a object1.notify() from another thread which again synchronized on object1. And, in our case, we do not have an explicit notify call at all.

-Raj

[Ankit Garg]

You cannot rely on this behavior. This happens because Thread.join has been implemented this way but JLS doesn't specify this behavior. So this behavior might change in the future thus it will not be asked in SCJP exam ...

[Harsha Smith]

And also I think wait(), notify(). notifyAll() are not part of OCP-JP syllabus anymore. Please check.