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?
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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 stubfinal Semaphore sem_c = new Semaphore(1);final Semaphore sem_b = new Semaphore(1);//permit is 1final 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 0System.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() {@Overridepublic void run() {// TODO Auto-generated method stubtry {sem_c.acquire();} catch (InterruptedException e) {// TODO Auto-generated catch blocke.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() {@Overridepublic void run() {// TODO Auto-generated method stubtry {Thread.sleep(1000);sem_b.acquire();} catch (InterruptedException e) {// TODO Auto-generated catch blocke.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() {@Overridepublic void run() {try {Thread.sleep(2000);} catch (InterruptedException e) {// TODO Auto-generated catch blocke.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();}}
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