《Cracking the Coding Interview》——第16章：线程与锁——题目3

2014-04-27 19:26

题目：哲学家吃饭问题，死锁问题经典模型（专门用来黑哲学家的？）。

解法：死锁四条件：1. 资源互斥。2. 请求保持。3. 非抢占。4. 循环等待。所以，某砖家拿起一只筷子后如果发现没有另一只了，就必须把手里这只筷子放下，这应该是通过破坏“请求保持”原则来防止死锁产生，请求资源失败时，连自己的资源也进一步释放，然后在下一轮里继续请求，直到成功执行。

代码：

// This is the class for chopsticks.
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class Chopstick {
    private Lock lock;
    
    public Chopstick() {
        lock = new ReentrantLock();
    }
    
    public boolean pickUp() {
        return lock.tryLock();
    }
    
    public void putDown() {
        lock.unlock();
    }
}

//------------------------------------I'm a delimiter------------------------------------
// This is the class for philosophers.
import java.util.Vector;

public class Philosopher extends Thread {
    private Chopstick left;
    private Chopstick right;
    private int id;
    int appetite;

    final int FULL_APPETITE = 10;

    public Philosopher(Chopstick left, Chopstick right, int id) {
        // TODO Auto-generated constructor stub
        appetite = 0;
        this.left = left;
        this.right = right;
        this.id = id;
    }

    private boolean pickUp() {
        if (!left.pickUp()) {
            return false;
        }
        if (!right.pickUp()) {
            left.putDown();
            return false;
        }
        return true;
    }

    private void putDown() {
        left.putDown();
        right.putDown();
    }

    public boolean eat() {
        while (appetite < FULL_APPETITE) {
            if (!pickUp()) {
                return false;
            }
            System.out.println(id + ":chew~");
            ++appetite;
            putDown();
        }
        return appetite == FULL_APPETITE;
    }

    @Override
    public void run() {
        // TODO Auto-generated method stub
        super.run();
        while (!eat()) {
            // Not full yet.
        }
    }

    public static void main(String[] args) {
        final int n = 6;
        Vector<Chopstick> chopsticks = new Vector<Chopstick>();
        Vector<Philosopher> philosophers = new Vector<Philosopher>();

        for (int i = 0; i < n; ++i) {
            chopsticks.add(new Chopstick());
        }
        for (int i = 0; i < n; ++i) {
            philosophers.add(new Philosopher(chopsticks.elementAt(i),
                    chopsticks.elementAt((i + 1) % n), i + 1));
        }
        
        for (int i = 0; i < n; ++i) {
            philosophers.elementAt(i).start();
        }
    }
}