java - Why should wait() always be called inside a loop

Question

I have read that we should always call a wait() from within a loop:

while (!condition) { obj.wait(); }

It works fine without a loop so why is that?

Answer 1

You need not only to loop it but check your condition in the loop. Java does not guarantee that your thread will be woken up only by a notify()/notifyAll() call or the right notify()/notifyAll() call at all. Because of this property the loop-less version might work on your development environment and fail on the production environment unexpectedly.

For example, you are waiting for something:

synchronized (theObjectYouAreWaitingOn) {
   while (!carryOn) {
      theObjectYouAreWaitingOn.wait();
   }
}

An evil thread comes along and:

theObjectYouAreWaitingOn.notifyAll();

If the evil thread does not/can not mess with the carryOn you just continue to wait for the proper client.

Edit: Added some more samples. The wait can be interrupted. It throws InterruptedException and you might need to wrap the wait in a try-catch. Depending on your business needs, you can exit or suppress the exception and continue waiting.