Using wait() without notify() when in synchronized code piece (java)

Question

I wanted to know if it's possible to use wait() on a synchronized piece of code without using notify(), something like this:

wait_on(B):
    synchronized(B.monitor) {
    B.count--
        while (B.count > 0) { /* wait */ }
    }

Thanks in advance

How to you reckon to come out of the wait() state without a notify()? What you are doing is some what equivalent to killing the thread without actually doing it. — noMAD
– noMAD, Commented Apr 19, 2012 at 18:41

Nathan Hughes · Accepted Answer · 2018-03-28 11:43:53Z

5

You need notify or notifyAll to awaken the thread from its wait state. In your sample the code would enter the wait and stay there (unless interrupted).

Know the difference between wait, yield, and sleep. Wait needs to be called in a synchronized block, once the wait is entered the lock is released, and the thread stays in that state until notify is called. Yield returns the thread to the ready pool and lets the scheduler decide when to run it again. Sleep means the thread goes dormant for a fixed period of time (and from there it goes to the ready pool).

Make sure you call wait on the same object that you’re synchronizing on (here it’s B.monitor).

edited Mar 28, 2018 at 11:43

answered Apr 19, 2012 at 18:38

Nathan Hughes

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6 Comments

MichBoy Over a year ago

But that happens until all threads will enter this method, and the last one will cause the count to be 0, then it'll get out, which will make an automatic notify(), or am I wrong?

Nathan Hughes Over a year ago

@MichBoy: there's no 'automatic' notify. once the counter goes to 0 then no more threads will enter the body of the while loop, that is all that will happen.

MichBoy Over a year ago

and if I get the synchronized off?

Nathan Hughes Over a year ago

@MichBoy: are you trying to implement a counting semaphore? because java.util.concurrent has those already: docs.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/…

MichBoy Over a year ago

I'm trying to do something similar but different, will it work without the synchronized?

|

Kalpak Gadre · Accepted Answer · 2012-04-19 18:40:22Z

0

No! Only option is to wait with a timeout, which surely will not help you.

answered Apr 19, 2012 at 18:40

Kalpak Gadre

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Comments

Eran Zimmerman Gonen · Accepted Answer · 2012-04-19 18:40:55Z

0

If you change the /* wait */ into a call to wait(), and no one will call notify() or notifyAll(), then this thread will never wake up...

answered Apr 19, 2012 at 18:40

Eran Zimmerman Gonen

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Comments

Pierre-Luc Bertrand · Accepted Answer · 2012-05-04 00:06:07Z

0

If it is a barrier that you want, you will need to notifyAll your other threads:

wait_on(B) {
    synchronized(B.monitor) {
        B.count--
        while (B.count > 0) {
            B.monitor.wait()
        }
        B.monitor.notifyAll();
    }
}

Regards,

Pierre-Luc

answered May 4, 2012 at 0:06

Pierre-Luc Bertrand

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