Difference between wait() and sleep()

java multithreading sleep java-threads
Geek picture Geek · Jun 24, 2009 · Viewed 909.8k times · Source

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?

Answer

oxbow_lakes picture oxbow_lakes · Jun 24, 2009

A wait can be "woken up" by another thread 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(); }
}

Related questions

When does Java's Thread.sleep throw InterruptedException?

When does Java's Thread.sleep throw InterruptedException? Is it safe to ignore it? I am not doing any multithreading. I just want to wait for a few seconds before retrying some operation.

Read More
Sleep() in Android Java

I am following this tutorial to have a loading screen in my program. The tutorial says my activity should Sleep() using the Sleep() command, however it does not recognize Sleep() as a function and provides me with an error, asking …

Read More
Java: Thread.currentThread().sleep(x) vs. Thread.sleep(x)

I have this in my code Thread.currentThread().sleep(x); Eclipse tells me to use the static Thread.sleep(x); instead, why? What's the difference, is there some difference in functionality at all between these 2 methods?

Read More