Query on the behavior of Thread sleep

[Rajanand Pandaraparambil Kuttappan]

Hi,
I was going through the Thread Self Test Question #12 in Page 767 given in the "Kathy Sierra & Bert Bates" SCJP 6 Study Guide and I quite understood it. Then I modified the question as is given below. I made the laurel.wait() to synchronize on laurel. Then what I see is, hardy waits and continues, once the laurel came out from the sleep after one second.

Does it mean that laurel obtains a lock on itself and notifies hardy once it releases the lock, though there was no synchronized keyword used within the laurel run method? My understanding was that, a 'wait' can continue only upon a 'notification' which must be through an explicit 'notify' invocation and sleep would never notify.

The reason why I ask is, if I comment-out the sleep code within laurel(including its exception handling), then the synchronized wait in hardy waits forever.

A reply is much appreciated.

The modified code snippet is given below in the bold font.

public class TwoThreads {
	static Thread laurel, hardy;
	public static void main(String[] args) {

		laurel = new Thread() {
			public void run() {
				System.out.println("A");
				try {
					hardy.sleep(1000); // makes laurel to sleep
				} catch (Exception e) {
					System.out.println("B");
				}
				System.out.println("C");
			}
		};
		hardy = new Thread() {
			public void run() {
				System.out.println("D");
				try {
					synchronized (laurel){ // synchronized on the laurel instance
						laurel.wait();
					}
				} catch (Exception e) {
					System.out.println("E");
				}
				System.out.println("F");
			}
		};
		laurel.start();
		hardy.start();
	}
}

The output obtained was:-

A <- from laurel
D <- from hardy, then hardy waits
C <- from laurel after 1000 milli seconds
F <- from hardy, out from the waiting

If 'sleep' from laurel is removed including the try-catch, output is

A <- from laurel
C <- from laurel
D <- from hardy, then hardy waits indefinitely.

program waits indefinitely.

Thanks
Raj

[Rajanand Pandaraparambil Kuttappan]

Hi All,

Anyone to extend help on this ?

If its because of a too lengthy content in the original mail, let me sum-up my queries down here.

1) Thread.sleep within an InterruptedException try-catch block - does it obtain a lock on the thread object, though there is no explicit synchronized block specified around it ?

2) Once the thread is out from the sleep and completes the run method, does it notify the other threads waiting on the this thread object ?

If answers are NO, then I am not able reason out the behavior I observed in the code snippet I posted with the original content. Though, as far as I read and understood, sleep does not obtain any lock implicitly.

Any feedback is very much appreciated.

Thanks
Raj