1.死锁代码

//死锁：多个线程互相抱着对方需要的资源，然后形成僵局 public class DeadLock {public static void main(String[] args) {Makeup girl1 = new Makeup(0,"灰姑娘");Makeup girl2 = new Makeup(1,"白雪公主");girl1.start();girl2.start();}}//口红 class Lipstick{}//镜子 class Mirror{}class Makeup extends Thread{//需要的资源只能有一份，用static来保证只有一份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;}@Overridepublic void run(){try {makeup();//化妆} 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System.out.println(this.girlName+"获取镜子的锁");}}}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System.out.println(this.girlName+"获取口红的锁");}}}} }

关键代码

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System.out.println(this.girlName+"获取镜子的锁");}}}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System.out.println(this.girlName+"获取口红的锁");}}}}

上述代码解析，当一个线程A锁住口红不放，同时想拿镜子
但是镜子被线程B锁住，线程B要拿到了口红才会释放镜子
那么线程A就会一直等待线程B释放镜子锁
但是线程B要拿不到口红锁就不会释放镜子锁
所以两个线程互相等待，形成死锁，程序崩溃
运行结果

2.解决后的代码

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System.out.println(this.girlName+"获取镜子的锁");}}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System.out.println(this.girlName+"获取口红的锁");}}}

线程A拿到了口红锁，然后释放了口红锁，想拿镜子锁
线程B拿到了镜子锁，然后释放了镜子锁，再拿口红锁
双方达成共识，不会永久的等待

运行结果

总结

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