哲学家进餐问题

哲学家进餐问题是著名的死锁问题，5个哲学家，5根筷子，每个哲学家进餐需要获得左右两根筷子才可以；

信号量

使用信号量完美解决，因为只有5根筷子，所以永远最多只能有两个不相邻的哲学家同时进餐；
选定0号和2号哲学家先开始进餐，每人进餐完毕唤醒下一个人；

// 筷子类
public class Kuaizi {
    long id;

    public Kuaizi(long id) {
        this.id = id;
    }
}

// 哲学家类
public class Zhexuejia implements Runnable {
    long id;
    Kuaizi left, right;
    Semaphore semaphore;
    Semaphore next;

    public Zhexuejia(long id, Kuaizi left, Kuaizi right, Semaphore semaphore, Semaphore next) {
        this.id = id;
        this.left = left;
        this.right = right;
        this.semaphore = semaphore;
        this.next = next;
    }

    public void eat() throws InterruptedException {
        synchronized (left) {
            System.out.println("哲学家" + id + "号拿起左手筷子" + left.id);
            TimeUnit.SECONDS.sleep(1);
            synchronized (right) {
                System.out.println("哲学家" + id + "号拿起右手筷子" + right.id);
                TimeUnit.SECONDS.sleep(1);
                System.out.println("哲学家" + id + "号吃饭");
                TimeUnit.SECONDS.sleep(1);

            }
            System.out.println("哲学家" + id + "号放下右手筷子" + right.id);
        }
        System.out.println("哲学家" + id + "号放下左筷子" + left.id);
    }

    @Override
    public void run() {
        for (; ; ) {
            try {
                semaphore.acquire();
                eat();
                next.release(1);
            } catch (InterruptedException e) {
            }
        }
    }
}

// 运行类
public class Test {

    public static void main(String[] args) {
        Kuaizi k0 = new Kuaizi(0);
        Kuaizi k1 = new Kuaizi(1);
        Kuaizi k2 = new Kuaizi(2);
        Kuaizi k3 = new Kuaizi(3);
        Kuaizi k4 = new Kuaizi(4);

        Semaphore s0 = new Semaphore(1);
        Semaphore s1 = new Semaphore(0);
        Semaphore s2 = new Semaphore(1);
        Semaphore s3 = new Semaphore(0);
        Semaphore s4 = new Semaphore(0);

        Zhexuejia z0 = new Zhexuejia(0, k0, k4, s0, s1);
        Zhexuejia z1 = new Zhexuejia(1, k1, k0, s1, s2);
        Zhexuejia z2 = new Zhexuejia(2, k2, k1, s2, s3);
        Zhexuejia z3 = new Zhexuejia(3, k3, k2, s3, s4);
        Zhexuejia z4 = new Zhexuejia(4, k4, k3, s4, s0);

        ExecutorService executorService = Executors.newFixedThreadPool(5);
        executorService.execute(z0);
        executorService.execute(z1);
        executorService.execute(z2);
        executorService.execute(z3);
        executorService.execute(z4);

        LockSupport.park();

    }
}

运行结果

image.png

混进一半的左撇子