How does thread.join functions

[Anupam Sinha]

Pasted below in the code for join method in Thread class.

 public final synchronized void join(long millis) 
    throws InterruptedException {
	long base = System.currentTimeMillis();
	long now = 0;

	if (millis < 0) {
            throw new IllegalArgumentException("timeout value is negative");
	}

	if (millis == 0) {
	    while (isAlive()) {
		wait(0);
	    }
	} else {
	    while (isAlive()) {
		long delay = millis - now;
		if (delay <= 0) {
		    break;
		}
		wait(delay);
		now = System.currentTimeMillis() - base;
	    }
	}
    }

It checks it the current thread is alive. If the thread is alive it then calls wait on the thread. In this case it seems that if t.join() was called it will be t that is made to wait. Whereas it should be the thread from which t.join() was called, should wait. Secondly what happens to the lock when any of the thread goes waiting?

[Thakur Sachin Singh]

not know clearly...me also interested to know the answer of this question.

[Henry Wong]

Anupam Sinha wrote:
It checks it the current thread is alive. If the thread is alive it then calls wait on the thread. In this case it seems that if t.join() was called it will be t that is made to wait. Whereas it should be the thread from which t.join() was called, should wait. Secondly what happens to the lock when any of the thread goes waiting?

Please take a look at the JavaDoc for the wait() method -- you'll notice that it is the calling threads that waits.

Henry

[Subhasis Gorai]

Henry Wong wrote:

Anupam Sinha wrote:
It checks it the current thread is alive. If the thread is alive it then calls wait on the thread. In this case it seems that if t.join() was called it will be t that is made to wait. Whereas it should be the thread from which t.join() was called, should wait. Secondly what happens to the lock when any of the thread goes waiting?

Please take a look at the JavaDoc for the wait() method -- you'll notice that it is the calling threads that waits.

Henry

But who notifies the waiting thread(the calling thread), I mean is there any hook which gets executed as soon as the joining thread finishes its execution(job execution) and calls the notify()/notifyAll() on the joining thread? Or is it the combination of "Spurious wake up" and while(isAlive()) that works? Please clarify.
Thanks,

[Henry Wong]

Subhasis Gorai wrote:
But who notifies the waiting thread(the calling thread), I mean is there any hook which gets executed as soon as the joining thread finishes its execution(job execution) and calls the notify()/notifyAll() on the joining thread? Or is it the combination of "Spurious wake up" and while(isAlive()) that works? Please clarify.
Thanks,

When a thread completes (returns or exceptions from run() method), it goes into code that cleans up after the thread. One of the operations is a notifyAll() method call on the newly completed thread's Thread object.

This is also why you should not use a thread's Thread object for notification, as you will be sharing the notification object.

Henry

[Subhasis Gorai]

Thanks Henry. If I am not wrong Thread.exit() is probably the hook who does such clean up job, I didn't find out any code for notifying over there.
Can you please point out where exactly it's happening?

Thanks again!

[Henry Wong]

Subhasis Gorai wrote:Thanks Henry. If I am not wrong Thread.exit() is probably the hook who does such clean up job, I didn't find out any code for notifying over there.
Can you please point out where exactly it's happening?

Not sure what you are talking about. There is no such exit() method. And it is not a hook. It is part of the thread cleanup code. It has been a while since I looked at that part of the JVM, so don't have more detail than that.

Henry