Calls to the notify method rather than notifyAll may fail to wake up the correct thread if an object's monitor (intrinsic lock) is used for multiple conditions. notify only wakes up a single arbitrary thread that is waiting on the object's monitor, whereas notifyAll wakes up all such threads.

Ensure that the call to notify instead of notifyAll is a correct and desirable optimization. If not, call notifyAll instead.

In the following example, the methods produce and consume both use notify to tell any waiting threads that an object has been added or removed from the buffer. However, this means that only one thread is notified. The woken-up thread might not be able to proceed due to its condition being false, immediately going back to the waiting state. As a result no progress is made.

When using notifyAll instead of notify, all threads are notified, and if there are any threads that could proceed, we can be sure that at least one of them will do so.

J. Bloch. Effective Java (second edition), p. 277. Addison-Wesley, 2008.

Java API Specification: Object.notify(), Object.notifyAll().