CODEWITHSUNDEEP
javamultithreadingconcurrencysemaphore
Abhishek
Abhishek

Reputation: 647

Java Semaphor doesnt honour argument provided?

When I usesemaphor with 1 argument. And then Release it twice, it increases available permits to 2 but ideally i have already mentioned that i want 1 lock. Now since i have 2 available permits two thread can acquire the lock which defeats the purpose of argument.

Semaphor lock = new Semaphor(1);
lock.release();
lock.release();

lock.acquire(); // thread1
lock.acquire(); //thread2

Why does it behave like that?

Upvotes: 1

Views: 43

Answers (3)

Stephen C
Stephen C

Reputation: 719679

Now since i have 2 available permits two thread can acquire the lock which defeats the purpose of argument.

It doesn't defeat the purpose of a Semaphore. The purpose of Semaphore is to implement the counting semaphore abstraction as described here.

Why Semaphore doesn't honor the argument provided?

It does honor it. But the argument isn't a bound on the number of permits. Rather it represents the number of permits that initially may be acquire without blocking.

Why does it behave like that?

Because (in the opinion of the Java designers) that is the way that a counting semaphore should behave.

The standard Semaphore class does not have a way to restrict the number of permits. If you want a lock (or a binary semaphore) you should use a Lock class ...

But in multi threading app it is possible that release and acquire goes out of sync and it ends up calling release twice.

Yes. That would be a bug in the application.

Now ... if you wanted to detect that, you could extend the Semaphore class and override the acquire and release methods to check that the number of permits stays between prescribed bounds. (You could throw an unchecked exception if the check failed. Or you could silently ignore the request ... though that sounds like a bad idea.)

Upvotes: 0

xingbin
xingbin

Reputation: 28289

Why does it behave like that?

The doc mentions it:

When used in this way, the binary semaphore has the property (unlike many Lock implementations), that the "lock" can be released by a thread other than the owner (as semaphores have no notion of ownership). This can be useful in some specialized contexts, such as deadlock recovery.

You can use a wrapper class to track the initial permit. But usually, it is not supposed to call release until the thread aquire the lock, so it should not become a problem.

Upvotes: 1

phpnewbie
phpnewbie

Reputation: 9

release()

From Oracle doc : Releases a permit, increasing the number of available permits by one. If any threads are trying to acquire a permit, then one is selected and given the permit that was just released. That thread is (re)enabled for thread scheduling purposes.

There is no requirement that a thread that releases a permit must have acquired that permit by calling acquire(). Correct usage of a semaphore is established by programming convention in the application.

Upvotes: 0

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