How does wait and notify work

Because your thread waits because some condition was not met it should be after returning to the method that it was suspended in, it needs to recheck that condition to see is it met yet. If condition is met it will not wait anymore, if it's not met it will call wait again as it is in while loop. The important thing to know here is. Stack Overflow for Teams — Collaborate and share knowledge with a private group.

Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. How does wait and notify work? Ask Question. Asked 8 years, 4 months ago. Active 1 year, 4 months ago. Viewed 7k times. Improve this question. Manish Doshi 1, 1 1 gold badge 8 8 silver badges 17 17 bronze badges. Could you share what you have been trying so far and what you would like to achieve? My program works like this, the wait doesn't execute at all.

To be able to help you we really need to see some code! Concurrency can be quite subtle. Add a comment. We now wait until the busyflag is free—no more and no less—and we no longer waste CPU cycles by waking up every milliseconds to test if the busyflag is free.

As noted, the wait and notify mechanism has a race condition that needs to be solved with the synchronization lock. Unfortunately, it is not possible to solve the race condition without integrating the lock into the wait and notify mechanism. This is why it is mandatory for the wait and notify methods to hold the locks for the objects for which they are waiting or notifying. The wait method releases the lock prior to waiting, and reacquires the lock prior to returning from the wait method. This is done so that no race condition exists.

If you recall, there is no concept of releasing and reacquiring a lock in the Java API. The wait method is actually tightly integrated with the synchronization lock, using a feature not available directly from the synchronization mechanism. In other words, it is not possible for us to implement the wait method purely in Java: it is a native method. This integration of the wait and notify and the synchronized method is actually standard.

In other systems, such as Solaris or POSIX threads, condition variables also require that a mutex lock be held for the mechanism to work.

There is another change: the getBusyFlag method is now synchronized. The getBusyFlag method was not synchronized in our earlier examples because the lock scope would have been too large.

It would not have been possible for the freeBusyFlag method to be called while the getBusyFlag method held the lock. However, because of the way in which the wait method works, there is no longer a danger of deadlock.

The wait method will release the lock, which will allow other threads to execute the freeBusyFlag method. Before the wait method returns, it will reacquire the lock, so that to the developer, it appears as if the lock has been held the entire time. What happens when notify is called and there is no thread waiting? This is a valid situation. Even with our BusyFlag class, it is perfectly valid to free the busyflag when there is no other thread waiting to get the busyflag.

Since the wait and notify mechanism does not know the condition about which it is sending notification, it assumes that a notification for which there is no thread waiting is a notification that goes unheard. In other words, if notify is called without another thread waiting, then notify simply returns. What are the details of the race condition that exists in wait and notify? In general, a thread that uses the wait method confirms that a condition does not exist typically by checking a variable and then calls the wait method.

When another thread sets the condition typically by setting that same variable , it then calls the notify method. A race condition occurs when:. The second thread calls the notify method; this goes unheard, since the first thread is not yet waiting. How does this potential race condition get resolved?

This race condition is resolved by the synchronization lock discussed earlier. This is mandatory: the methods will not work properly and will generate an exception condition if the lock is not held. Is there some mechanism possible which can notify to a particular thread only or a particular thread should be invoked. You may try this solution. Is wait directly got link with q object. Thanks a lot…. HI, Thanks for such an wonderful post. But recently, a question asked to my friend.

So, can you help me with that. Not with using wait notify both. So, it will be great if you can provide exaple with both. How can the threads decide among themselves?

An example could be group of threads one thread processing only a specific type of message watching a message queue. Upon placing a message in queue, producer can notify all consumer threads. Then each consumer thread may check if message is for it, and if it is then process it otherwise wait again. This design may look inefficient as correct thread may get chance to check in last; BUT it will start making sense when you start adding more message types and their handler threads into system without modifying code of other handlers.

I appreciate the article very much! For your question, a consumer indeed consumes only element location zero each time, and this is also the meaning of First Out for a queue. A blog about Java and its related technologies, the best practices, algorithms, interview questions, scripting languages, and Python. About Me. Contact Us. Privacy policy. Guest Posts. Secure Hash Algorithms. Best Way to Learn Java. How to Start New Blog. Skip to content. How to work with wait , notify and notifyAll in Java?

Last Updated: December 26, Java Concurrency. Read more : Difference between wait and sleep in Java 1. What are wait , notify and notifyAll methods? When another thread establishes the condition typically by setting the same variable , it calls the notify method. Was this post helpful?

Let us know if you liked the post. Binary Semaphore Tutorial and Example. Control concurrent access to multiple copies of a resource using Semaphore. Wonderful content and beautifully explained. Thank you for the blog! Consumed: 4 Queue is empty Consumer is waiting , size: 0 Exception in thread "Consumer" java. IllegalMonitorStateException at java.

Hello Lokesh, Firstly thanks for the clearing my doubt. And secondly, I just wrote like this in Producer and Consumer this. Could you please tell me which one is correct. Thanks Lokesh, Article is really helpful to understand concepts of java multi threading thread communication.

Nice article. It cleared many of my doubts regarding multithreading. Very Nice, Keep the Good Work. I would like to have hyperlink on this from my website. Thanks for kind words. And a hyperlink is much appreciated. If possible please answer. Thanks for the 2 answer. Just for an example. Make sense?