Java - Why wait() must be called in a synchronized block?

- 日理万妓 2021-10-18 14:38 215阅读 0赞

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A `wait()` only makes sense when there is also a `notify()`, so it's always about communication between threads, and that needs synchronization to work correctly. One could argue that this should be implicit, but that would not really help, for the following reason:

Semantically, you never just `wait()`. You need some condition to be satsified, and if it is not, you wait until it is. So what you really do is:

if(!condition){
        wait();
    }

But the condition is being set by a separate thread, so in order to have this work correctly you need synchronization.

A couple more things wrong with it, where just because your thread quit waiting doesn't mean the condition you are looking for is true:

*  You can get spurious wakeups (meaning that a thread can wake up from waiting without ever having received a notification), or
 *  The condition can get set, but a third thread makes the condition false again by the time the waiting thread wakes up (and reacquires the monitor).

To deal with these cases what you really need is *always* some variation of this:

synchronized(lock){
        while(!condition){
            lock.wait();
        }
    }

Better yet, don't mess with the synchronization primitives at all and work with the abstractions offered in the `java.util.concurrent`packages.

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