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Derek Honeybun
Derek Honeybun

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Posted on    July-03-2015 1:22 AM

 Java Java  Threading  Multiple Threading 
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What is the difference between a wait() and sleep() in Threads?

Is my understanding that a wait()-ing Thread is still in running mode and uses CPU cycles but a sleep()-ing does not consume any CPU cycles correct?

Why do we have both wait() and sleep(): how does their implementation vary at a lower level?


Mayank Tripathi
Mayank Tripathi

Total Post:397

Points:3117
Posted on    July-03-2015 8:37 AM

A wait can be "woken up" by another process calling notify on the monitor which is being waited on whereas a sleep cannot. Also a wait (and notify) must happen in a block synchronized on the monitor object whereas sleep does not:
Object mon = ...;
synchronized (mon) {
    mon.wait();
At this point the currently executing thread waits and releases the monitor. Another thread may do

synchronized (mon) { mon.notify(); }
(On the same mon object) and the first thread (assuming it is the only thread waiting on the monitor) will wake up.

You can also call notifyAll if more than one thread is waiting on the monitor - this will wake all of them up. However, only one of the threads will be able to grab the monitor (remember that the wait is in a synchronized block) and carry on - the others will then be blocked until they can acquire the monitor's lock.

Another point is that you call wait on Object itself (i.e. you wait on an object's monitor) whereas you call sleep on Thread.

Yet another point is that you can get spurious wakeups from wait (i.e. the thread which is waiting resumes for no apparent reason). You should always wait whilst spinning on some condition as follows:
synchronized {
    while (!condition) { mon.wait(); }
}

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