java - Binary Semaphore vs a ReentrantLock

Question

I've been trying to understand Reentrant locks and Semaphores ( the nesting of Reentrant locks vs release/unlock mechanism ).

It seems that having a Semaphore requires you to write a more thoroughly tested application because the release() method does not check if the thread releasing the permit is actually holding it. When I tested my test code, I found out that this may subsequently increase the number of permits beyond the initial limit. On the other hand, if a thread is not holding a reentrant lock when it invokes the unlock method, we get an IllegalMonitorException.

So would it be right to say that there is no real reason ever to have a binary semaphore as everything that a binary semaphore can do can also be done by a ReentrantLock. If we use binary semaphores we would have to check the entire method call stack to see if a permit was acquired before ( also was it released too if there is a possibility of a subsequent acquire - which might block if a release does not proceed it and so on ). Also since reentrant locks also provide one lock per object, isn't it always a better idea to prefer a reentrant lock to a binary semaphore?

I have checked a post here that talks about difference between a binary semaphore and a mutex but is there a thing like a mutex in Java?

Thanks, Chan.

P.S - I had posted this question in another forum ( http://www.coderanch.com/t/615796/threads/java/reason-prefer-binary-Semaphore-Reentrant ) and I haven't received a response yet. I thought I'd post it here as well to see what I can get.

Answer 1

there is no real reason ever to have a binary semaphore as everything that a binary semaphore can do can also be done by a ReentrantLock

If all you need is reentrant mutual exclusion, then yes, there is no reason to use a binary semaphore over a ReentrantLock. If for any reason you need non-ownership-release semantics then obviously semaphore is your only choice.

Also since reentrant locks also provide one lock per object, isn't it always a better idea to prefer a reentrant lock to a binary semaphore?

It depends on the need. Like previously explained, if you need a simple mutex, then don't choose a semaphore. If more than one thread (but a limited number) can enter a critical section you can do this through either thread-confinement or a semaphore.

I have checked a post here that talks about difference between a binary semaphore and a mutex but is there a thing like a mutex in Java?

ReentrantLock and synchronized are examples of mutexes in Java.