Java: Synchronizing on primitives?

You can try something with a ReentrantLock, such that you have a Map<Long,Lock>. Now after lock.release() You can test lock.hasQueuedThreads(). If that returns false you can remove it from the Map.

I invented a thing like that for myself some time ago. I call it an equivalence-class lock, meaning, it locks on all of the things that are equal to the given thing. You can get it from my github, and use it subject to the Apache 2 license, if you like, or just read it and forget it!

You can try the following little 'hack'

String str = UNIQUE_METHOD_PREFIX + Long.toString(id);
synchornized(str.intern()) { .. }

which is 100% guaranteed to return the same instance.

The UNIQUE_METHOD_PREFIX, may be a hardcoded constant, or may be obtained using:

StackTraceElement ste = Thread.currentThread().getStackTrace()[0];
String uniquePrefix = ste.getDeclaringClass() + ":" +ste.getMethodName();

which will guarantee that the lock happens only on this precise method. That's in order to avoid deadlocks.