This article shows you how to use a semaphore in your code. Then, using examples, I will explain what a Semaphore is and how you can use them.
The quick answer
Don't have time to read an entire post? The following example is the quick and easy answer on creating and using a semaphore.
It is important to note that every thread uses the same semaphore instance. For example, with the previous example, you want
to pass an instance of
UsingSemaphores to every thread. So don't create an
UsingSemaphores for each thread.
What is a Semaphore
A semaphore is a class you can use to control the number of threads accessing a given resource. For example, you can use a semaphore to shield a part of your code so only five threads can access it sanctimoniously.
A semaphore in Java holds a number of permits. When a thread obtains a permit, it can continue to run the code shielded by the semaphore. With a semaphore, you can be sure that there aren't more threads running the shielded-off code than there are permits. When there are no more permits lefts for the semaphore to give away, a thread must wait until they are available again. When is thread is done with its permit, it can choose to release the permit back to the semaphore; then, another thread can use it.
How to create a Semaphore
The first step is to create a semaphore which is very easy. In the following example, we create two semaphores. The first semaphore we create has five permits it can give to threads. In the second semaphore, we create acts fair with its permits. When a permit becomes available, the thread waiting for the longest for a permit will get it.
Acquire a permit
When you create a semaphore, the next thing you want to do is to acquire a permit. To do so, you can use the
The following example shows you four common ways to acquire a permit. It's important to note that
tryAcquire() will ignore
the fairness setting of a semaphore.
Acquire all available permits
It is also possible to claim all the available permits. To do so, you need to use the
this method ignores the fairness setting and immediately claims all permits.
Release a permit
When you have a permit, it is important to release it; else, the semaphore can never give the permit to another thread.
Releasing is very straightforward. All you have to do is call the
release() method. You must never release
more permits than you hold.
This article showed you how to create a semaphore, claim a permit, and release it. We also looked at different ways, like how to claim multiple permits, drain a semaphore, and release multiple permits at once.
More about multithreading in Java: