Semaphore

Semaphore is an advanced lock that allows at most N threads to proceed after which it begins blocking. N is specified in the constructor.

Each call to acquire() in the semaphore object blocks if a permit is not available, else it takes a permit and proceeds. Each call to release() adds a permit, potentially making room for a waiting thread to proceed. However, no actual permit objects are used; the Semaphore just keeps a count of the number available and acts accordingly. Semaphores are often used to restrict the number of threads than can access some (physical or logical) resource.

The constructor for this class optionally accepts a fairness parameter which if set to true, guarantees that threads calling acquire() obtain the lock in a FIFO fashion thus guaranteeing fairness.