Why Expect Notify Together With Notifyall Called From Synchronized Block Or Method Inward Java

Most of the Java developers knows that wait(), notify(), in addition to notifyAll() methods of object aeroplane must accept to hold out called within synchronized method or synchronized block inwards Java but how many times nosotros idea why? Recently this questions was asked to inwards Java interview to i of my friend, he pondered for a minute in addition to replied that if nosotros don't telephone phone wait() or notify() method from synchronized context nosotros volition have IllegalMonitorStateException in Java. He was right inwards damage of the behaviour of linguistic communication but equally per him, interviewer was non completely satisfied amongst the response in addition to wanted to explicate him mor nearly it. After the interview, he discussed the same questions amongst me in addition to I idea he should accept to tell interviewr nearly race status betwixt wait() in addition to notify() inwards Java which could be if nosotros don't telephone phone them within synchronized method or block. Let’s come across how it could hap inwards a Java program.

It is too i of the pop thread interivew questions in addition to ofttimes asked inwards both telephonic in addition to face-to-face circular of  Java developer interviews. So, if y'all are preparing for Java interviews, y'all should laid upwards questions similar this in addition to i mass which tin genuinely handle y'all is the Java Programming Interivew exposed.

This is i of the rare mass which covers almost all of import theme of Java interviews e.g. pith Java, multi-threading, IO in addition to NIO in addition to frameworks similar Spring in addition to Hibernate etc. You tin banking concern jibe it out here.

Why wait(), notify() in addition to notifyAll() must hold out called from synchronized block or method inwards Java

We purpose wait(), notify(), or notifyAll() method to a greater extent than ofttimes than non for inter-thread communication inwards Java. One thread is waiting afterward checking a status e.g. In the classic Producer-Consumer problem, the Producer thread waits if the buffer is total in addition to Consumer thread notify Producer thread afterward it creates a infinite inwards the buffer past times consuming an element.

Calling notify() or notifyAll() methods issues a notification to a unmarried or multiple thread that a status has changed in addition to i time notification thread leaves synchronized block, all the threads which are waiting struggle for object lock on which they are waiting in addition to lucky thread returns from wait() method afterward reacquiring the lock in addition to maintain further.

Let’s dissever this whole performance into steps to come across a possibility of race status betwixt wait() in addition to notify() method inwards Java, nosotros volition purpose Produce Consumer thread example to empathise the scenario better:

   1. The Producer thread tests the status (buffer is total or not) in addition to confirms that it must hold back (after finding buffer is full).

   2. The Consumer thread sets the status afterward consuming an chemical ingredient from a buffer.

   3. The Consumer thread calls the notify () method; this goes unheard since the Producer thread is non silent waiting.

   4. The Producer thread calls the hold back () method in addition to goes into waiting state.

So due to race condition hither nosotros potential lost a notification in addition to if nosotros purpose buffer or simply i chemical ingredient Produce thread volition hold out waiting forever in addition to your computer program volition hang.

Now let's intend how does this potential race status acquire resolved? This race status is resolved past times using synchronized keyword in addition to locking provided past times Java. In guild to telephone phone the wait (), notify () or notifyAll () methods inwards Java, nosotros must accept obtained the lock for the object on which we're calling the method.

Since the wait() method inwards Java too releases the lock prior to waiting in addition to reacquires the lock prior to returning from the wait() method, nosotros must purpose this lock to ensure that checking the status (buffer is total or not) in addition to setting the status (taking chemical ingredient from buffer) is atomic which tin hold out achieved past times using synchronized method or block inwards Java.

I am non certain if this is what interviewer was genuinely expecting but this what I idea would at to the lowest degree brand sense, delight right me If I incorrect in addition to permit us know if at that topographic point is whatsoever other convincing argue of calling wait(), notify() or notifyAll method inwards Java.

Just to summarize nosotros telephone phone wait (), notify () or notifyAll method inwards Java from synchronized method or synchronized block inwards Java to avoid:

1) IllegalMonitorStateException inwards Java which volition occur if nosotros don't telephone phone hold back (), notify () or notifyAll () method from synchronized context.

2) Any potential race status betwixt hold back in addition to notify method inwards Java.

Some of my other favorite interview discussions are Why String is immutable or terminal inwards Java, how HashMap plant inwards Java in addition to what are differences betwixt HashMap in addition to hashtable inwards Java is.

Further Learning
Multithreading in addition to Parallel Computing inwards Java
Java Concurrency inwards Practice - The Book
Applying Concurrency in addition to Multi-threading to Common Java Patterns
Java Concurrency inwards Practice Course past times Heinz Kabutz

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