CODEWITHSUNDEEP
javasemaphoreinterrupted-exception
Ernie
Ernie

Reputation: 1219

Need to semaphore.relase() if semaphore.acquire() gets InterruptedException?

From the Java java.util.concurrent.Semaphore docs it wasn't quite clear to me what happens if semaphore.acquire() blocks the thread and later gets interrupted by an InterruptedException. Has the semaphore value been decreased and so is there a need to release the semaphore?

Currently I am using code like this:

try {
  // use semaphore to limit number of parallel threads
  semaphore.acquire();
  doMyWork();
}
finally {
  semaphore.release();
}

Or should I rather not call release() when an InterruptedException occurs during acquire() ?

Upvotes: 14

Views: 13989

Answers (3)

jblack
jblack

Reputation: 820

nos's accepted answer is partially correct, except semaphore.acquire() also throws InterruptedException. So, to be 100% correct, the code would look like:

try {
    semaphore.acquire();
    try {
        doMyWork();
    } catch (InterruptedException e) { 
        // do something, if you wish
    } finally {
        semaphore.release();
    }
} catch (InterruptedException e) {
    // do something, if you wish
}

Upvotes: 8

dan
dan

Reputation: 13272

If the thread is interrupted before acquire method call, or while waiting to acquire a permit the InterruptedException will be thrown and no permit will be hold, so no need to release. Only when you are certain that a permit was acquired (after calling the acquire method call) you will need to release the permit. So you better acquire before your try block starts, something like:

sem.acquire();
try{
   doMyWork();
}finally{
   sem.release();
}

Upvotes: 0

nos
nos

Reputation: 229224

call release() when an InterruptedException occurs during acquire() ?

You should not. If .acquire() is interrupted, the semaphore is not acquired, so likely should not release it.

Your code should be 

// use semaphore to limit number of parallel threads
semaphore.acquire();
try {
  doMyWork();
}
finally {
  semaphore.release();
}

Upvotes: 13

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