Java Semaphore

Java introduced Semaphore since 1.5, let's see how we can use it to maintain an order for thread access.

Suppose we have the following code:
class Foo {
public:
A(.....);
B(.....);
}
Foo f;
f.A(.....);
f.B(.....);
f.C(.....);

Suppose we have the following code to use class Foo. We do not know how the threads
will be scheduled in the OS.
Foo f;
f.A(.....);
f.B(.....);
f.C(.....);
f.A(.....);
f.B(.....);
f.C(.....);
i) Can you design a mechanism to make sure that all the methods will be executed in
sequence?

--------------------------------------
we can use Semaphore sem = new Semaphore(1) to init a mutex, at this time, available permit for this semaphore is 1.
we can use sem.acquire() for P operation, so the permit will be reduced to 0, any other places uses sem.acquire() will be blocked until sem.release() is called and permit is increased to 1 again. So to maintain an access order, we wanna acquire semaphore first and use predecessor to release this semaphore.

public class SemaphoreTest {

	/**
	 * @param args
	 * @throws InterruptedException 
	 */
	public static void main(String[] args) throws InterruptedException {
		// TODO Auto-generated method stub
		final Semaphore sem_c = new Semaphore(1);
		final Semaphore sem_b = new Semaphore(1);	//permit is 1
		final Semaphore sem_all = new Semaphore(1);
		System.out.println("first, sem_b permit: " + sem_b.availablePermits());
		System.out.flush();
		sem_b.acquire();	//permit from 1 to 0
		System.out.println("after acquire, sem_b permit: " + sem_b.availablePermits());
		System.out.flush();
		sem_c.acquire();
		sem_all.acquire();
		
		A(sem_b);		
		B(sem_c, sem_b);		
		C(sem_c, sem_all, sem_b);

//		System.out.println("second round");
		System.out.flush();
		
		System.out.println("sem_all permit: " + sem_all.availablePermits());
		sem_all.acquire();
		System.out.println("sem_b permit: " + sem_b.availablePermits());
		sem_b.acquire();
		sem_c.acquire();
//
		A(sem_b);		
		B(sem_c, sem_b);		
		C(sem_c, sem_all, sem_b);
	}

	private static void C(final Semaphore sem_c, final Semaphore sem_all, final Semaphore sem_b) {
		new Thread(new Runnable() {

			@Override
			public void run() {
				// TODO Auto-generated method stub
				try {
					sem_c.acquire();
				} catch (InterruptedException e) {
					// TODO Auto-generated catch block
					e.printStackTrace();
				}
				System.out.println("c");
				System.out.flush();
				sem_c.release();
//				System.out.println("sem_all permit: " + sem_all.availablePermits());

				sem_all.release();
			}}).start();
	}

	private static void B(final Semaphore sem_c, final Semaphore sem_b) {
		new Thread(new Runnable() {

			@Override
			public void run() {
				// TODO Auto-generated method stub
				try {
					Thread.sleep(1000);
					sem_b.acquire();
				} catch (InterruptedException e) {
					// TODO Auto-generated catch block
					e.printStackTrace();
				}
				sem_b.release();
				System.out.println("b");
				System.out.flush();
				sem_c.release();
			}}).start();
	}

	private static void A(final Semaphore sem_b) {
		new Thread(new Runnable() {

			@Override
			public void run() {
				try {
					Thread.sleep(2000);
				} catch (InterruptedException e) {
					// TODO Auto-generated catch block
					e.printStackTrace();
				}
				System.out.println("a");
				System.out.flush();
//				System.out.println("sem_b permit: " + sem_b.availablePermits());
				sem_b.release();
//				System.out.println("sem_b permit after: " + sem_b.availablePermits());
				System.out.flush();
			}}).start();
	}

}