13. Deadlock

deadlock

• When a synchronized code block holds "locks on more than two objects" at the same time, a "deadlock" problem may occur
• Deadlock: Multiple threads hold each other's resources and form a deadlock

Four conditions for deadlock

1. Mutual exclusion condition: a resource can only be used by one process at a time
2. Request and hold conditions: When a process is blocked by requesting resources, it will hold on to the acquired resources
3. No deprivation condition: The resource that the process has obtained cannot be forcibly deprived before it is used up
4. Circular waiting condition: A cyclic waiting resource relationship is formed between several processes.

The four necessary conditions for deadlock are listed above. We can avoid deadlock as long as we find a way to destroy any one or more of them.

package com.thread;

public class DeadLock {
    public static void main(String[] args) {
        Makeup g1 = new Makeup(0,"灰姑娘");
        Makeup g2 = new Makeup(1,"白雪公主");

        new Thread(g1).start();
        new Thread(g2).start();
    }
}

// 口红
class Lipstick{

}

// 镜子
class Mirror{

}

class Makeup implements Runnable{
    // 需要的资源只要一份，用static来保持只有一份
    static Lipstick lipstick = new Lipstick();
    static Mirror mirror = new Mirror();

    int choice; // 选择
    String girlName; //使用化妆品的人

    public Makeup(int choice, String girlName){
        this.choice = choice;
        this.girlName = girlName;
    }

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

    private void makeup() throws InterruptedException {
        if (choice==0){
            synchronized (lipstick){ // 获得口红的锁
                System.out.println(this.girlName + "获得口红的锁");
                Thread.sleep(1000);

                // synchronized (mirror){ // 一秒钟后想获得镜子的锁
                //     System.out.println(this.girlName + "获得镜子的锁");
                // }
            }
            // 解决方法，将锁放入外面,通过释放上一个锁来获取下一个锁
            synchronized (mirror){ // 一秒钟后想获得镜子的锁
                System.out.println(this.girlName + "获得镜子的锁");
            }
        }else {
            synchronized (mirror){ // 获得镜子的锁
                System.out.println(this.girlName + "获得镜子的锁");
                Thread.sleep(2000);

                // synchronized (lipstick){ // 两秒钟后想获得口红的锁
                //      System.out.println(this.girlName + "获得口红的锁");
                //  }
            }
            synchronized (lipstick){ // 两秒钟后想获得口红的锁
                System.out.println(this.girlName + "获得口红的锁");
            }
        }
    }
}