When to use java.util.concurrent.Semaphore's acquire() and acquireUninterruptibly() method?

Question

I am new to Java concurrency/multi-threading. Can somebody please explain to me in what situations the acquire() and acquireUninterruptibly() must (or must not) be used, how they are different; and also how to use them (code sample?).

Thank you very much.

Answer 1

Even though previous answers are technically correct i'd like to point out the following:

• If you are using Semaphore for transaction coordination between threads you may want to use acquireUninterruptibly() to complete you transaction ad clean exit your process when the transaction (somehow) cleanly finish by testing thread interrupted state (you will know that the thread may have been interrupted after the call).

• If you can handle InterruptedException and immediately exit transaction without the need of any further operation you can use acquire().

Think you are waiting for a shared resource (a counter for example) to complete your transaction, if using acquire() and ignoring the exception you may get the wrong resource state (a non unique counter in this case), so you may still want to wait for the resource and check later what to do based on the thread state. If, for example, you are doing a batch of transactions you may want to complete the current one and skip the subsequents.

Answer 2

acquire() is interruptible. That means if a thread A is calling acquire() on a semaphore, and thread B interrupts threads A by calling interrupt(), then an InterruptedException will be thrown on thread A.

On the other hand acquireUninterruptibly() is not interruptible. That means if a thread A is calling acquireUninterruptibly() on a semaphore, and thread B interrupts threads A by calling interrupt(), then no InterruptedException will be thrown on thread A, just that thread A will have its interrupted status set after acquireUninterruptibly() returns.