It does not solve the following case:. The second thread calls tryGetBusyFlag , which returns false. The second thread executes the wait method, which frees the synchronization lock. The first thread enters the freeBusyFlag method, obtaining the synchronization lock.
The third thread attempts to call getBusyFlag and blocks waiting for the synchronization lock. The first thread exits the freeBusyFlag method, releasing the synchronization lock. The third thread acquires the synchronization lock and enters the getBusyFlag method. Because the busyflag is free, it obtains the busyflag and exits the getBusyFlag method, releasing the synchronization lock. The second thread, having received notification, returns from the wait method, reacquiring the synchronization lock along the way.
The second thread calls the tryGetBusyFlag method again, confirms that the flag is busy, and calls the wait method. All we know when the wait method returns is that at some point in the past, the condition had been satisfied and another thread called the notify method; we cannot assume that the condition is still satisfied without testing the condition again.
Hence, we always need to put the call to the wait method in a loop. Skip to main content. Java Threads, Second Edition by. Start your free trial.
Wait and Notify. Wait and Notify and Synchronization As noted, the wait and notify mechanism has a race condition that needs to be solved with the synchronization lock. The first thread tests the condition and confirms that it must wait. The second thread sets the condition.
The first thread calls the wait method. Wait and Notify and the Synchronized Block In all the wait and notify examples so far, we have used synchronized methods. The first thread acquires the busyflag. The first thread calls the notify method.
Concurrency can be quite subtle. Add a comment. Active Oldest Votes. Improve this answer. Cole Smith 1 1 silver badge 7 7 bronze badges. Manish Doshi Manish Doshi 1, 1 1 gold badge 8 8 silver badges 17 17 bronze badges. Can you add sample output for clarity?
If you don't do this the wait can miss the notify or the wait can wake spuriously. As written in " Java Concurrency in Practice " Every call to wait is implicitly associated with a specific condition predicate. When calling wait regarding a particular condition predicate, the caller must already hold the lock associated with the condition queue, and that lock must also guard the state variables from which the condition predicate is composed.
JovoH JovoH 6 6 bronze badges. Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password. I think this is a simple question,you should only answer : wait notify notifyAll is designed by thread communication. We can not release a lock using wait or notifying other threads to acquire lock with owning lock for any object. So before calling this function we need to own a lock using synchronized block or synchronized method.
I super like your bloggings. Whenever i feel doubt regarding any topic i always go for your articles. Here in this article i have one doubt that when "The Consumer thread calls the notify method; this goes unheard since the Producer thread is not yet waiting. Javin your argument is incorrect. Lets assume that inside Object class wait and notify method where synchronized.
No because 'is buffer empty condition check and wait' must be atomic. Similarly, 'buffer is not empty check and notify' must be atomic operation. Every object, in addition to having an associated monitor, has an associated wait set. A wait set is a set of threads. When an object is first created, its wait set is empty. Elementary actions that add threads to and remove threads from wait sets are atomic.
Wait sets are manipulated solely through the methods Object. I am not sure if your producer-consumer example is correct from the standpoint of race condition. What I mean is, producer uses push Object x function to put something in queue -- so this will be synchronized for all producer threads. Similarly a consumer will use the pop method to empty the queue -- so this will be synchronized only for all consumer threads.
If either of these methods are not synchronized a race condition should occur between the different producers OR different consumers. Not sure if the race condition will occur between producer AND consumer cos both are working on a different set of locks. Even if I synchronize push or pop methods Let me know if my thinking is right. I think Nitin's and Satpan's explanation make more sense to me.
But anyways, you are a great guy. Your tutorials are really helpful Race condition is prevented by conditional check on shared state of the object in loop around call of wait.
Friends, Simple answer is to let Java know which object lock should be released when wait method is called, as thread may have multiple lock. Calling wait outside sync context well leave Java clueless on which lock should release. Thanks, Gk. Hi, When thread calls wait on an object, it releases the lock it holds on that object and goes to wait, this is the reason we need to call wait inside synchronized block. Hi Anonymous, the problem in your code is on which you are calling the notify.
I see that you are having a static Thread on which you are calling the notify, but the wait is happening on the main thread.
I modified your code and changed it to notify instead of b. In java Inter thread communication means acquiring or releasing the lock on an object. So no question of releasing the lock at all. This is the same scenario for notify method too. There are a few reasons. For example, there might be more than one condition to wait for.
Since we cannot control which thread gets the notification, it is entirely possible that a notification wakes up a thread that is waiting for an entirely different condition.
By waking up all the threads, we can design the program so that the threads decide among themselves which thread should execute next. Another option could be when producers generate data that can satisfy more than one consumer. Since it may be difficult to determine how many consumers can be satisfied with the notification, an option is to notify them all, allowing the consumers to sort it out among themselves.
Subscribe to get new post notifications, industry updates, best practices, and much more. Directly into your inbox, for free. Thank you. Threads are objects so they have lock like other objects.
When we use wait and notify in synchronized methods, which lock do we release? This is an excellent article. However, with the above code I get a very orderly production, until the queue is full, and a very orderly consumption, until the queue is empty, and then this repeats.
The example would be much better if the Thread. The critical section synchronized block is simply too large, and by the time the block ends, it repeats immediately and therefore the same thread is likely to reacquire the lock immediately, thus the long strings of production and consumption, without much interleaving.
Move the sleep outside and randomize it, and you will have a much better example. Hi, Thanks for such a wonderful post. To produce or consume they require lock. Hello Lokesh,my problem is when i run the program post in your blog ,the output is regular,the consumer do not wake up until the queue is full,can you help me?
Above output is also a valid output. To have the process interleave at all I needed to add a Thread. Hi Lokesh, Can u tell me that what r the conditions to get the object lock. Means can we lock the object through synchronised method and what is the role of wait method for getting the objects lock. See i was really devasted and could not understand how to explain this concept in practical scenarios you showed how easily it can be explained.
Hello Lokesh, I implemented the code mentioned in the blog and mentioned below is the output I was getting. Now, the question : Q. So what happens when tProducer. It gets the lock of taskQueue object and starts producing and then releases the lock when wait is called. At this time tConsumer which was was in waiting for the lock of taskQueue object in consume method to get into synchronized block and goes into consuming the item from list and when size is zero it goes into wait and releases the lock and tProducer acquires it and this keeps on going.
Am I correct with this understanding? So rather than creating a new monitor object for this purpose, I used this existing object. Regarding error, not sure how you have modified the wait and notify calls. Please share the modified code. Firstly thanks for the clearing my doubt. And secondly, I just wrote like this in Producer and Consumer.
Thanks for such a quick response and clearing my doubt. To own the monitor of that object, you must synchronize on it. So if you want to use this. Thanks for the post.
0コメント