Java Locks and Conditions

Question

Say I have three threads, thread 1, thread 2, and thread 3 all sharing the same lock. Thread 2 acquires the lock, does some work and then blocks via a call to the await method. Thread 1 then acquires the lock, does some work, and during the middle of it, thread 3 tries to acquire the lock but is blocked since thread 1 is holding it. Thread 1 finishes working and, before terminating, signals thread 2 that it can reacquire the lock. So what happens then? Will thread 2 or thread 3 acquire the lock next?

Thank you so much for your time and help in advance.

Answer 1

If no priority is given, whoever comes first will acquire the lock.

While mutual exclusion may provide safety property, it does not ensure liveness property. There can be cases where a thread keeps coming first to acquire the lock, resulting in starvation (other threads wait forever because someone keeps occupying).

Google with the keywords highlighted will help you understand more. I found these slides really comprehensive http://www.cs.cornell.edu/Courses/cs414/2004su/slides/05_schedule.pdf

Answer 2

If you're using a ReentrantLock (or any of its subclasses), you can pass a "fairness" flag to the constructor. If set to true, this will ensure that control of the lock passes to the longest-waiting thread, in this case your Thread 1.

Lock lock = new ReentrantLock(true);