Question

I need a ReadWriteLock that is NOT reentrant, because the lock may be released by a different thread than the one that acquired it. (I realized this when I started to get IllegalMonitorStateException intermittently.)

I’m not sure if non-reentrant is the right term. A ReentrantLock allows the thread that currently holds to lock to acquire it again. I do NOT want this behaviour, therefore I’m calling it “non-reentrant”.

The context is that I have a socket server using a thread pool. There is NOT a thread per connection. Requests may get handled by different threads. A client connection may need to lock in one request and unlock in another request. Since the requests may be handled by different threads, I need to be able to lock and unlock in different threads.

Assume for the sake of this question that I need to stay with this configuration and that I do really need to lock and unlock in different requests and therefore possibly different threads.

It’s a ReadWriteLock because I need to allow multiple “readers” OR an exclusive “writer”.

It looks like this could be written using AbstractQueuedSynchronizer but I’m afraid if I write it myself I’ll make some subtle mistake. I can find various examples of using AbstractQueuedSynchronizer but not a ReadWriteLock.

I could take the OpenJDK ReentrantReadWriteLock source and try to remove the reentrant part but again I’m afraid I wouldn’t get it quite right.

I’ve looked in Guava and Apache Commons but didn’t find anything suitable. Apache Commons has RWLockManager which might do what I need but I’m not sure and it seems more complex than I need.

Answer 1

A Semaphore allows different threads to perform the acquire and release of permits. An exclusive write is equivalent to having all of the permits, as the thread waits until all have been released and no additional permits can be acquired by other threads.

final int PERMITS = Integer.MAX_VALUE;
Semaphore semaphore = new Semaphore(PERMITS);

// read
semaphore.acquire(1);
try { ... }
finally {
  semaphore.release(1);
}

// write
semaphore.acquire(PERMITS);
try { ... }
finally {
  semaphore.release(PERMITS);
}