# Semaphore A counting semaphore. Conceptually, a semaphore maintains a set of permits. Each [`acquire()`](https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/Semaphore.html#acquire--) blocks if necessary until a permit is available, and then takes it. Each [`release()`](https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/Semaphore.html#release--) adds a permit, potentially releasing a blocking acquirer. 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.

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