哲学家问题死锁

image.png

package com.kang.multithread;

import lombok.SneakyThrows;

/**
 * @author klr
 * @create 2020-09-15-19:58
 */

//哲学家问题
public class TestDeadLock {

    public static void main(String[] args) {
        Chopstick c1 = new Chopstick("1");
        Chopstick c2 = new Chopstick("2");
        Chopstick c3 = new Chopstick("3");
        Chopstick c4 = new Chopstick("4");
        Chopstick c5 = new Chopstick("5");

        new Philosopher("苏格拉底",c1,c2).start();
        new Philosopher("柏拉图",c2,c3).start();
        new Philosopher("亚里士多德",c3,c4).start();
        new Philosopher("赫拉克利特",c4,c5).start();
        new Philosopher("阿基米德",c5,c1).start();


    }
}

class Chopstick{
    private String name;

    public Chopstick(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

class Philosopher extends Thread {

    String name;
    Chopstick left;
    Chopstick right;

    public Philosopher(String name, Chopstick left, Chopstick right) {
        super(name);
        this.name=name;
        this.left = left;
        this.right = right;
    }

    @SneakyThrows
    @Override
    public void run() {
        while (true) {
            synchronized (left) {
                synchronized (right) {
                    //获取了两个筷子后才能吃饭
                    eat();
                }
            }
        }
    }

    public void eat() throws InterruptedException {
        System.out.println("eating..."+name);
        Thread.sleep(1000);
    }
}

解决方法

1.改变拿筷子的次序
c1,c2/c1,c5
这样做后有的线程得到锁的机会就少了

2.使用reentrantlock
这样获取锁失败并不会进入阻塞队列等待，而是直接跳过

package com.kang.multithread;

import lombok.SneakyThrows;

import java.util.concurrent.locks.ReentrantLock;

/**
 * @author klr
 * @create 2020-09-15-19:58
 */

//哲学家问题
public class TestDeadLock {

    public static void main(String[] args) {
        Chopstick c1 = new Chopstick("1");
        Chopstick c2 = new Chopstick("2");
        Chopstick c3 = new Chopstick("3");
        Chopstick c4 = new Chopstick("4");
        Chopstick c5 = new Chopstick("5");

        new Philosopher("苏格拉底",c1,c2).start();
        new Philosopher("柏拉图",c2,c3).start();
        new Philosopher("亚里士多德",c3,c4).start();
        new Philosopher("赫拉克利特",c4,c5).start();
        new Philosopher("阿基米德",c5,c1).start();


    }
}

class Chopstick extends ReentrantLock {
    private String name;

    public Chopstick(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

class Philosopher extends Thread {

    String name;
    Chopstick left;
    Chopstick right;

    public Philosopher(String name, Chopstick left, Chopstick right) {
        super(name);
        this.name=name;
        this.left = left;
        this.right = right;
    }

    @SneakyThrows
    @Override
    public void run() {
        while (true) {
//            synchronized (left) {
//                synchronized (right) {
//                    //获取了两个筷子后才能吃饭
//                    eat();
//                }
//            }
            //尝试获取左筷子
            if (left.tryLock()) {
                try {
                    //尝试获取右筷子
                    if (right.tryLock()) {
                        try {
                            eat();
                        }
                        finally {
                            right.unlock();
                        }
                    }
                }
                finally {
                    left.unlock();//释放左筷子
                }
            }
        }
    }

    public void eat() throws InterruptedException {
        System.out.println("eating..."+name);
        Thread.sleep(1000);
    }
}