Why does the acquire() method in Semaphores not have to be synchronized?

Question

I am getting into Semaphores in Java and was reading this article http://docs.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/Semaphore.html . The only thing I don't get is why the acquire() method is not used in a synchronized context. Looking at the example from the above webiste:

They create a semaphore:

private Semaphore semaphore = new Semaphore(100);

and get a permit just like this:

semaphore.acquire();

Now, wouldn't it be possible that two or more threads try to acquire() at the same time? If so, there would be a little problem with the count.

Or, does the semaphore itself handle the synchronization?

Answer 1

Or, does the semaphore itself handle the synchronization?

Yes that's basically it. Semaphores are thread safe as explained in the javadoc:

Memory consistency effects: Actions in a thread prior to calling a "release" method such as release() happen-before actions following a successful "acquire" method such as acquire() in another thread.

Most operations on the objects in the java.util.concurrent package are thread safe. More details are provided at the very bottom of the package javadoc.

Answer 2

Semaphores ought to be fast and therefore use the atomic concurrency primitives from the Unsafe class, like CAS (compare and swap).

With these primitives synchronization happens on a much lower level and monitors are not needed. (Lock-free synchronization).

In fact the synchronization is performed by a loop continously using CAS until the expected value equals the written/read value.