死锁

产生死锁的四个必要条件

• 互斥条件：一个资源每次只能被一个进程使用
• 请求与保持条件：一个进程因请求资源而阻塞时，对已获得的资源保持不放
• 不剥夺条件：进程已获得的资源在未使用完之前，不能强行剥夺
• 循环等待条件：若干进程之间形成一种头尾相接的循环等待资源关系

死锁示例

package com.example.multi_thread;

public class TestLock {
    public static void main(String[] args) {
        Makeup makeup = new Makeup("灰姑娘", 0);
        Makeup makeup1 = new Makeup("白雪公主", 1);
        new Thread(makeup).start();
        new Thread(makeup1).start();
    }
}

class Mirror {

}

class Lipstick {

}

class Makeup implements Runnable {
    static Mirror mirror = new Mirror();
    static Lipstick lipstick = new Lipstick();

    private String name;
    private int choose;

    public Makeup(String name, int choose) {
        this.name = name;
        this.choose = choose;
    }

    @Override
    public void run() {
        try {
            makeup();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    private void makeup() throws InterruptedException {

        if (choose == 0) {
            synchronized (mirror) {
                System.out.println(Thread.currentThread().getName() + "获取镜子");
                Thread.sleep(100);
                synchronized (lipstick) {
                    System.out.println(Thread.currentThread().getName() + "获取口红");
                }
            }
        } else {
            synchronized (lipstick) {
                System.out.println(Thread.currentThread().getName() + "获取口红");
                Thread.sleep(100);
                synchronized (mirror) {
                    System.out.println(Thread.currentThread().getName() + "获取镜子");
                }
            }
        }
    }
}

输出：

Thread-0获取镜子
Thread-1获取口红

修复后：

package com.example.multi_thread;

public class TestLock {
    public static void main(String[] args) {
        Makeup makeup = new Makeup("灰姑娘", 0);
        Makeup makeup1 = new Makeup("白雪公主", 1);
        new Thread(makeup).start();
        new Thread(makeup1).start();
    }
}

class Mirror {

}

class Lipstick {

}

class Makeup implements Runnable {
    static Mirror mirror = new Mirror();
    static Lipstick lipstick = new Lipstick();

    private String name;
    private int choose;

    public Makeup(String name, int choose) {
        this.name = name;
        this.choose = choose;
    }

    @Override
    public void run() {
        try {
            makeup();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    private void makeup() throws InterruptedException {

        if (choose == 0) {
            synchronized (mirror) {
                System.out.println(Thread.currentThread().getName() + "获取镜子");
                Thread.sleep(100);
            }
            synchronized (lipstick) {
                System.out.println(Thread.currentThread().getName() + "获取口红");
            }
        } else {
            synchronized (lipstick) {
                System.out.println(Thread.currentThread().getName() + "获取口红");
                Thread.sleep(100);
            }
            synchronized (mirror) {
                System.out.println(Thread.currentThread().getName() + "获取镜子");
            }
        }
    }
}

输出：

Thread-0获取镜子
Thread-1获取口红
Thread-1获取镜子
Thread-0获取口红