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 ? Recently this questions was asked to in Java interview to one of my friend, he pondered for a moment and replied that if we don't call wait () or notify () method from synchronized context we will receive IllegalMonitorStateException in java. He was right in terms of behavior of language but as per him interviewer was not completely satisfied with the answer and wanted to explain more about it. After the interview he discussed the same questions with me and I thought he might have told about race condition between wait () and notify () in Java that could exists if we don't call them inside synchronized method or block. Let’s see how it could happen: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 use Produce 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.
Some of my other favorite interview discussions are Why String is immutable or final in Java, how HashMap works in Java and what are differences between HashMap and hashtable in Java is.