In this recipe, you will learn how to use the semaphore mechanism provided by the Java language. A semaphore is a counter that protects access to one or more shared resources.
The concept of a semaphore was introduced by Edsger Dijkstra in 1965 and was used for the first time in the THEOS operating system.
When a thread wants to access one of the shared resources, it must first acquire the semaphore. If the internal counter of the semaphore is greater than 0, the semaphore decrements the counter and allows access to the shared resource. A counter bigger than 0 implies that there are free resources that can be used, so the thread can access and use one of them.
Otherwise, if the counter is 0, the semaphore puts the thread to sleep until the counter is greater than 0. A value of 0 in the counter means all the shared resources are used by other threads...
