Java 9 Concurrency Cookbook - Second Edition

Monitoring a Semaphore class

A semaphore is a counter that protects the access to one or more shared resources.

The concept of semaphore was introduced by Edsgar Dijkstra in 1965 and was used for the first time in the THEOS operating system.

When a thread wants to use shared resources, it must acquire a semaphore. If the internal counter of the semaphore is greater than 0, the semaphore decrements the counter and allows the access to the shared resource. If the counter of the semaphore is 0, the semaphore blocks the thread until the counter is greater than 0. When the thread has finished using the shared resource, it must release the semaphore. That operation increases the internal counter of the semaphore.

In Java, semaphores are implemented ...