We continue with the Java Concurrency theme with semaphores. Semaphores are also a way to synchronize threads.

Semaphores are a really simple concept, invented by the famous Dutch computer scientist Edsger Dijkstra. Basically a semaphore is a counter (integer) that allows a thread to get into a critical region if the value of the counter is greater than 0. If it’s the case, the counter is decremented by one. Otherwise, the thread waits until it can go. And when the thread leaves the critical region, the counter is incremented by one to allow one more thread to pass through the critical region. A semaphore is created with a certain value for its counter. So, you can execute two actions on a semaphore P and V.

For example, if you have a critical section that cannot be executed concurrently, you can use a semaphore :

sem mutex = new sem(1)
 
P(mutex)
//Critical region
V(mutex) 

So you must always call the P operation by yourself before the critical region and V after it. We call a mutex (mutual exclusion) a semaphore with a value of one. So only one thread can enter the region guarded by the semaphore. This is the most used semaphore. The other use of semaphore is to guard a set of resources like database connections or a data pool.

In Java, a semaphore is created using the java.util.concurrent.Semaphore class. You can create it easily :

Semaphore mutex = new Semaphore(1);
Semaphore available = new Semaphore(100);

The P and V operations are represented using the acquire and release methods. The method acquire can be interrupted if the thread is interrupted. There is an ininterruptible version with the method acquireUninterruptibly(). There is also a third version with the tryAcquire method. This method acquires a permit only if there is one permit available, otherwise, this method return false. All the waiting methods also have an overloaded version with a timeout. You can also acquire several permits at once using the permits argument to the different versions of acquire methods.

A little example with a mutex using the same example as the previous post on Java concurrency :

public class Example {
    private int value = 0;
 
    private final Semaphore mutex = new Semaphore(1)
 
    public int getNextValue() throws InterruptedException {
        try {
            mutex.acquire();
            return value++;
        } finally {
            mutex.release();
        }
    }
}

For more information about Semaphore in Java, the best is to consult the Javadoc of the Semaphore class.

To conclude, semaphores are a powerful way to solve concurrency problems, but this is not adapted to all problems. If you only need mutual exclusion, synchronized blocks are a better solution. The problems with semaphores is that you can forget to call the release method and this can cause deadlocks that are sometimes difficult to find.

From http://www.baptiste-wicht.com/2010/08/java-concurrency-part-4-semaphores/