Difference between wait() and sleep()
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(); }}
One key difference not yet mentioned is that while sleeping a Thread does not release the locks it holds, while waiting releases the lock on the object that wait()
is called on.
synchronized(LOCK) { Thread.sleep(1000); // LOCK is held}synchronized(LOCK) { LOCK.wait(); // LOCK is not held}
I found this post helpful. It puts the difference between Thread.sleep()
, Thread.yield()
, and Object.wait()
in human terms. To quote:
It all eventually makes its way down to the OS’s scheduler, which hands out timeslices to processes and threads.
sleep(n)
says “I’m done with my timeslice, and please don’t give me another one for at least n milliseconds.” The OS doesn’t even try to schedule the sleeping thread until requested time has passed.
yield()
says “I’m done with my timeslice, but I still have work to do.” The OS is free to immediately give the thread another timeslice, or to give some other thread or process the CPU the yielding thread just gave up.
wait()
says “I’m done with my timeslice. Don’t give me another timeslice until someone calls notify().” As withsleep()
, the OS won’t even try to schedule your task unless someone callsnotify()
(or one of a few other wakeup scenarios occurs).Threads also lose the remainder of their timeslice when they perform blocking IO and under a few other circumstances. If a thread works through the entire timeslice, the OS forcibly takes control roughly as if
yield()
had been called, so that other processes can run.You rarely need
yield()
, but if you have a compute-heavy app with logical task boundaries, inserting ayield()
might improve system responsiveness (at the expense of time — context switches, even just to the OS and back, aren’t free). Measure and test against goals you care about, as always.