Why Wait() Together With Notify() Method Should Live On Called Within A Loop Inwards Java?

If yous create got used wait() together with notify() method inward Java together with hence yous know that the criterion idiom of calling wait() method uses a loop, but create got yous ever thought why? This is fifty-fifty advised past times none other than Joshua Bloch, a Java guru together with writer of pop producer-consumer problem, the waiting status for producer thread could survive written equally :

if(queue.isFull()){     queue.wait()   }   queue.add(item);

This says if the queue is total together with hence wait, otherwise insert an chemical component division into the queue. This looks fine at kickoff sight but when yous holler upwardly through nigh this inward a multi-threaded scenario, yous volition location the employment together with that's what yous volition acquire inward this article.

 Btw, this is my minute article on exploring how to locomote the wait-notify method inward Java. Earlier I create got written nigh why hold off together with notify is called from the synchronized context together with today you'll acquire why a loop is necessary for the wait-notify block. If yous haven't read that article, yous may detect it interesting together with useful.

What is the employment using if block amongst wait-notify inward Java?

This industrial plant fine inward ideal condition, but the programming basis is non e'er ideal. In an ideal condition, your thread volition exclusively wake upwardly if consumer thread has consumed 1 chemical component division together with directly the queue is non full, but inward the existent world, the waiting thread tin survive woken upwardly fifty-fifty if the status has non changed similar an evil thread sends an wrong notification.

If yous locomote the if block together with hence yous are non going to banking concern fit the waiting status 1 time to a greater extent than earlier start processing or adding but about other detail inward the queue. In that case, yous cease upwardly putting 1 to a greater extent than chemical component division inward the queue which is full, that could create error or exception.

This is why yous should e'er banking concern fit the waiting status inward a loop instead of if block.

There is but about other scenario, where multiple producers are waiting for a location inward the queue. If a user called notifyAll() together with informed all waiting threads nigh the 1 location beingness available.

After receiving this notification, 1 of the thread volition go for it together with fill upwardly that location but when other thread gets wake-up the location is already filled. If yous don't banking concern fit the status inward the loop together with hence yous cease upwardly adding to a greater extent than elements inward the already total queue.

When yous banking concern fit the waiting status inward the loop yous ensure that thread volition exam the status afterwards it wakes upwardly to encounter if the status however holds or not. If yous are non familiar amongst notify together with notifyAll method, yous tin farther see The Complete Java Masterclass to acquire more.

Reasons due to which a thread tin wake upwardly inward Java

Misdelivered notification: 
The gild inward which Java threads execute afterwards receipt of a notifyAll() holler for is unspecified. Which agency it's possible that an unrelated thread could start executing together with discovery that its status predicate is satisfied. Consequently, it could resume execution despite beingness required to stay dormant.

Spurious wakeups: 
Certain Java Virtual Machine (JVM) implementations are vulnerable to spurious wakeups that upshot inward waiting threads waking upwardly fifty-fifty without a notification

This is equally good truthful for await() method which is a novel method for Condition class but similar to wait().


The wrong way to locomote the wait() method

synchronized (object) {     if (condition does non hold) {        object.wait();     }     // Proceed when status holds   }

The right way to locomote the wait() method:

synchronized (object) {      while (condition does non hold) {        object.wait();      }      // Proceed when status holds   }

That's all nigh why wait() method should survive called within the loop inward Java instead of if block. As I create got said before, fifty-fifty the Java guru, Joshua Bloch, who has written many commutation classes of the java.lang package has advised this inward his classic Effective Java book, a must read for whatsoever serious Java programmer. It volition assist yous to avoid whatsoever such mistakes inward your day-to-day programming task.

Further Learning
The Complete Java Masterclass
50+ Java Multithreading Interview Questions inward Java
How to avoid deadlock inward Java programs
Top v Courses to Learn Multithreading and Concurrency inward Java
Multithreading together with Parallel Computing inward Java

Thanks for reading this article hence far. If yous similar this article together with hence delight portion amongst your friends together with colleagues. If yous create got whatsoever questions or feedback together with hence delight portion amongst your friends together with colleagues.