A Counting semaphore with one permit is known as binary semaphore because it has only two state permit available or permit unavailable. Binary semaphore can be used to implement mutual exclusion or critical section where only one thread is allowed to execute. Thread will wait on acquire() until Thread inside critical section release permit by calling release() on semaphore.
here is a simple example of counting semaphore in Java where we are using binary semaphore to provide mutual exclusive access on critical section of code in java:
import java.util.concurrent.Semaphore;
public class SemaphoreTest {
Semaphore binary = new Semaphore(1);
public static void main(String args[]) {
final SemaphoreTest test = new SemaphoreTest();
new Thread(){
@Override
public void run(){
test.mutualExclusion();
}
}.start();
new Thread(){
@Override
public void run(){
test.mutualExclusion();
}
}.start();
}
private void mutualExclusion() {
try {
binary.acquire();
//mutual exclusive region
System.out.println(Thread.currentThread().getName() + " inside mutual exclusive region");
Thread.sleep(1000);
} catch (InterruptedException i.e.) {
ie.printStackTrace();
} finally {
binary.release();
System.out.println(Thread.currentThread().getName() + " outside of mutual exclusive region");
}
}
}
Output:
Thread-0 inside mutual exclusive region
Thread-0 outside of mutual exclusive region
Thread-1 inside mutual exclusive region
Thread-1 outside of mutual exclusive region
Some Scenario where Semaphore can be used:
1) To implement better Database connection pool which will block if no more connection is available instead of failing and handover Connection as soon as its available.
2) To put a bound on collection classes. by using semaphore you can implement bounded collection whose bound is specified by counting semaphore.
That's all on Counting semaphore example in Java. Semaphore is real nice concurrent utility which can greatly simply design and implementation of bounded resource pool. Java 5 has added several useful concurrent utility and deserve a better attention than casual look.
