java.util.concurrent.Semaphore class is a counting semaphore.
That means that it has two main methods:
The counting semaphore is initialized with a given number of "permits". For each call to
acquire() a permit
is taken by the calling thread. For each call to
release() a permit is returned to the semaphore.
Thus, at most N threads can pass the
acquire() method without any
where N is the number of permits the semaphore was initialized with. The permits are just a simple counter.
Nothing fancy here.
As semaphore typically has two uses:
- To guard a critical section against entry by more than N threads at a time.
- To send signals between two threads.
Guarding Critical Sections
If you use a semaphore to guard a critical section, the thread trying to enter the critical section will typically first try to acquire a permit, enter the critical section, and then release the permit again after. Like this:
Semaphore semaphore = new Semaphore(1); //critical section semaphore.acquire(); ... semaphore.release();
Sending Signals Between Threads
If you use a semaphore to send signals between threads, then you would typically have one thread
acquire() method, and the other thread to call the
If no permits are available, the
acquire() call will block until a permit is released by
another thread. Similarly, a
release() calls is blocked if no more permits can be released
into this semaphore.
Thus it is possible to coordinate threads. For instance, if acquire was called after Thread 1 had inserted an
object in a shared list, and Thread 2 had called
release() just before taking an object from that
list, you had essentially created a blocking queue. The number of permits available in the semaphore would
correspond to the maximum number of elements the blocking queue could hold.
No guarantees are made about fairness of the
threads acquiring permits from the
Semaphore. That is, there is no guarantee that the first
thread to call
acquire() is also the first thread to obtain a permit. If the first thread
is blocked waiting for a permit, then a second thread checking for a permit just as a permit is released,
may actually obtain the permit ahead of thread 1.
If you want to enforce fairness, the
Semaphore class has a constructor that takes a boolean
telling if the semaphore should enforce fairness. Enforcing fairness comes at a performance / concurrency
penalty, so don't enable it unless you need it.
Here is how to create a
Semaphore in fair mode:
Semaphore semaphore = new Semaphore(1, true);
java.util.concurrent.Semaphore class has lots more methods. For instance:
Check out the JavaDoc for more details on these methods.