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

Most of the Java developers knows that wait(), notify(), together with notifyAll() methods of object course of written report must convey to last called within synchronized method or synchronized block inward Java but how many times nosotros idea why? Recently this questions was asked to inward Java interview to i of my friend, he pondered for a 2d together with replied that if nosotros don't telephone telephone wait() or notify() method from synchronized context nosotros volition have IllegalMonitorStateException in Java. He was right inward damage of the demeanour of linguistic communication but equally per him, interviewer was non completely satisfied amongst the response together with wanted to explicate him mor virtually it. After the interview, he discussed the same questions amongst me together with I idea he should convey to tell interviewr virtually race status betwixt wait() together with notify() inward Java which could be if nosotros don't telephone telephone them within synchronized method or block. Let’s run across how it could come about inward a Java program.

It is besides i of the pop thread interivew questions together with oftentimes asked inward both telephonic together with face-to-face circular of  Java developer interviews. So, if yous are preparing for Java interviews, yous should cook questions similar this together with i mass which tin genuinely assistance yous 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. heart Java, multi-threading, IO together with NIO together with frameworks similar Spring together with Hibernate etc. You tin banking concern tally it out here.

Why wait(), notify() together with notifyAll() must last called from synchronized block or method inward Java

We travel wait(), notify(), or notifyAll() method generally for inter-thread communication inward Java. One thread is waiting later checking a status e.g. In the classic Producer-Consumer problem, the Producer thread waits if the buffer is total together with Consumer thread notify Producer thread later it creates a infinite inward 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 together with i time notification thread leaves synchronized block, all the threads which are waiting struggle for object lock on which they are waiting together with lucky thread returns from wait() method later reacquiring the lock together with maintain further.

Let’s separate this whole functioning into steps to run across a possibility of race status betwixt wait() together with notify() method inward Java, nosotros volition travel Produce Consumer thread example to sympathise the scenario better:

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

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

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

   4. The Producer thread calls the hold back () method together with goes into waiting state.

So due to race condition hither nosotros potential lost a notification together with if nosotros travel buffer or only i chemical ingredient Produce thread volition last waiting forever together with your programme volition hang.

Now let's remember how does this potential race status acquire resolved? This race status is resolved past times using synchronized keyword together with locking provided past times Java. In fellowship to telephone telephone the wait (), notify () or notifyAll () methods inward Java, nosotros must convey obtained the lock for the object on which we're calling the method.

Since the wait() method inward Java besides releases the lock prior to waiting together with reacquires the lock prior to returning from the wait() method, nosotros must travel this lock to ensure that checking the status (buffer is total or not) together with setting the status (taking chemical ingredient from buffer) is atomic which tin last achieved past times using synchronized method or block inward 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 together with permit us know if at that spot is whatever other convincing argue of calling wait(), notify() or notifyAll method inward Java.

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

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

2) Any potential race status betwixt hold back together with notify method inward Java.

Some of my other favorite interview discussions are Why String is immutable or in conclusion inward Java, how HashMap industrial plant inward Java together with what are differences betwixt HashMap together with hashtable inward Java is.

Further Learning
Multithreading together with Parallel Computing inward Java
Java Concurrency inward Practice - The Book
Applying Concurrency together with Multi-threading to Common Java Patterns
Java Concurrency inward Practice Course past times Heinz Kabutz

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