multithreading - Java: notify() vs. notifyAll() all over again -

if 1 googles "difference between notify() , notifyall()" lot of explanations pop (leaving apart javadoc paragraphs). boils down number of waiting threads being waken up: 1 in notify() , in notifyall().

however (if understand difference between these methods right), 1 thread selected further monitor acquisition; in first case 1 selected vm, in second case 1 selected system thread scheduler. exact selection procedures both of them (in general case) not known programmer.

what's useful difference between notify() , notifyall() then? missing something?

simply put, depends on why threads waiting notified. want tell 1 of waiting threads happened, or want tell of them @ same time?

in cases, waiting threads can take useful action once wait finishes. example set of threads waiting task finish; once task has finished, waiting threads can continue business. in such case use notifyall() wake waiting threads @ same time.

another case, example mutually exclusive locking, 1 of waiting threads can useful after being notified (in case acquire lock). in such case, rather use notify(). implemented, could use notifyall() in situation well, unnecessarily wake threads can't anyway.