Why wait (), notify () and notifyAll () must be called from synchronized block or method in Java ?

Most of Java developer knows that wait() ,notify() and notifyAll() method of object class must have to be called inside synchronized method or synchronized block in Java but how many times we thought why ?

We use wait () and notify () or notifyAll () method mostly for inter-thread communication. One thread is waiting after checking a condition e.g. In Producer Consumer example Producer Thread is waiting if buffer is full and Consumer thread notify Producer thread after he creates a space in buffer by consuming an element. calling notify() or notifyAll() issues a notification to a single or multiple thread that a condition has changed and once notification thread leaves synchronized block , all the threads which are waiting fight for object lock on which they are waiting and lucky thread returns from wait() method after reacquiring the lock and proceed further. Let’s divide this whole operation in steps to see a possibility of race condition between wait () and notify () method in Java, we will useProduce Consumer thread example to understand the scenario better:

   1. The Producer thread tests the condition (buffer is full or not) and confirms that it must wait (after finding buffer is full).

   2. The Consumer thread sets the condition after consuming an element from buffer.

   3. The Consumer thread calls the notify () method; this goes unheard since the Producer thread is not yet waiting.

   4. The Producer thread calls the wait () method and goes into waiting state.

So due to race condition here we potential lost a notification and if we use buffer or just one element Produce thread will be waiting forever and your program will hang.

Now let's think how does this potential race condition get resolved? This race condition is resolved by using synchronized keyword and locking provided by java. In order to call the wait (), notify () or notifyAll () methods in Java, we must have obtained the lock for the object on which we're calling the method. Since the wait () method in Java also releases the lock prior to waiting and reacquires the lock prior to returning from the wait () method, we must use this lock to ensure that checking the condition (buffer is full or not) and setting the condition (taking element from buffer) is atomic which can be achieved by using synchronized method or block in Java.

Just to summarize we call wait (), notify () or notifyAll method in Java from synchronized method or synchronized block in Java to avoid:

1) IllegalMonitorStateException in Java which will occur if we don't call wait (), notify () or notifyAll () method from synchronized context.

2) Any potential race condition between wait and notify method in Java.