大白话聊聊Java并发面试问题之谈谈你对AQS的理解?【石杉的架构笔记】

13,280 阅读9分钟

欢迎关注微信公众号:石杉的架构笔记(id:shishan100)

我的新课**《C2C 电商系统微服务架构120天实战训练营》在公众号儒猿技术窝**上线了,感兴趣的同学,可以点击下方链接了解详情:

《C2C 电商系统微服务架构120天实战训练营》

一、写在前面

上一篇文章聊了一下java并发中常用的原子类的原理和Java 8的优化,具体请参见文章:大白话聊聊Java并发面试问题之Java 8如何优化CAS性能?

这篇文章,我们来聊聊面试的时候比较有杀伤力的一个问题:聊聊你对AQS的理解?

之前有同学反馈,去互联网公司面试,面试官聊到并发时就问到了这个问题。当时那位同学内心估计受到了一万点伤害。。。

因为首先,很多人还真的连AQS是什么都不知道,可能听都没听说过。或者有的人听说过AQS这个名词,但是可能连具体全称怎么拼写都不知道。

更有甚者,可能会说:AQS?是不是一种思想?我们平时开发怎么来用AQS?

总体来说,很多同学估计都对AQS有一种云里雾里的感觉,如果用搜索引擎查一下AQS是什么?看几篇文章,估计就直接放弃了,因为密密麻麻的文字,实在是看不懂!

所以,基于上述痛点,咱们这篇文章,就用最简单的大白话配合N多张手绘图,给大家讲清楚AQS到底是什么?让各位同学面试被问到这个问题时,不至于不知所措。

二、ReentrantLock和AQS的关系

首先我们来看看,如果用java并发包下的ReentrantLock来加锁和释放锁,是个什么样的感觉?

这个基本学过java的同学应该都会吧,毕竟这个是java并发基本API的使用,应该每个人都是学过的,所以我们直接看一下代码就好了:

上面那段代码应该不难理解吧,无非就是搞一个Lock对象,然后加锁和释放锁。

你这时可能会问,这个跟AQS有啥关系?关系大了去了!因为java并发包下很多API都是基于AQS来实现的加锁和释放锁等功能的,AQS是java并发包的基础类。

举个例子,比如说ReentrantLock、ReentrantReadWriteLock底层都是基于AQS来实现的。

那么AQS的全称是什么呢?AbstractQueuedSynchronizer,抽象队列同步器。给大家画一个图先,看一下ReentrantLock和AQS之间的关系。

我们来看上面的图。说白了,ReentrantLock内部包含了一个AQS对象,也就是AbstractQueuedSynchronizer类型的对象。这个AQS对象就是ReentrantLock可以实现加锁和释放锁的关键性的核心组件。

三、ReentrantLock加锁和释放锁的底层原理

好了,那么现在如果有一个线程过来尝试用ReentrantLock的lock()方法进行加锁,会发生什么事情呢?

很简单,这个AQS对象内部有一个核心的变量叫做state,是int类型的,代表了加锁的状态。初始状态下,这个state的值是0。

另外,这个AQS内部还有一个关键变量,用来记录当前加锁的是哪个线程,初始化状态下,这个变量是null。

接着线程1跑过来调用ReentrantLock的lock()方法尝试进行加锁,这个加锁的过程,直接就是用CAS操作将state值从0变为1。

如果不知道CAS是啥的,请看上篇文章,大白话聊聊Java并发面试问题之Java 8如何优化CAS性能?

如果之前没人加过锁,那么state的值肯定是0,此时线程1就可以加锁成功。

一旦线程1加锁成功了之后,就可以设置当前加锁线程是自己。所以大家看下面的图,就是线程1跑过来加锁的一个过程。

其实看到这儿,大家应该对所谓的AQS有感觉了。说白了,就是并发包里的一个核心组件,里面有state变量、加锁线程变量等核心的东西,维护了加锁状态。

你会发现,ReentrantLock这种东西只是一个外层的API,内核中的锁机制实现都是依赖AQS组件的

这个ReentrantLock之所以用Reentrant打头,意思就是他是一个可重入锁。

可重入锁的意思,就是你可以对一个ReentrantLock对象多次执行lock()加锁和unlock()释放锁,也就是可以对一个锁加多次,叫做可重入加锁。

大家看明白了那个state变量之后,就知道了如何进行可重入加锁!

其实每次线程1可重入加锁一次,会判断一下当前加锁线程就是自己,那么他自己就可以可重入多次加锁,每次加锁就是把state的值给累加1,别的没啥变化。

接着,如果线程1加锁了之后,线程2跑过来加锁会怎么样呢?

**我们来看看锁的互斥是如何实现的?**线程2跑过来一下看到,哎呀!state的值不是0啊?所以CAS操作将state从0变为1的过程会失败,因为state的值当前为1,说明已经有人加锁了!

接着线程2会看一下,是不是自己之前加的锁啊?当然不是了,**“加锁线程”**这个变量明确记录了是线程1占用了这个锁,所以线程2此时就是加锁失败。

给大家来一张图,一起来感受一下这个过程:

接着,线程2会将自己放入AQS中的一个等待队列,因为自己尝试加锁失败了,此时就要将自己放入队列中来等待,等待线程1释放锁之后,自己就可以重新尝试加锁了

所以大家可以看到,AQS是如此的核心!AQS内部还有一个等待队列,专门放那些加锁失败的线程!

同样,给大家来一张图,一起感受一下:

接着,线程1在执行完自己的业务逻辑代码之后,就会释放锁!他释放锁的过程非常的简单,就是将AQS内的state变量的值递减1,如果state值为0,则彻底释放锁,会将“加锁线程”变量也设置为null!

整个过程,参见下图:

接下来,会从等待队列的队头唤醒线程2重新尝试加锁。

好!线程2现在就重新尝试加锁,这时还是用CAS操作将state从0变为1,此时就会成功,成功之后代表加锁成功,就会将state设置为1。

此外,还要把**“加锁线程”**设置为线程2自己,同时线程2自己就从等待队列中出队了。

最后再来一张图,大家来看看这个过程。

四、总结

OK,本文到这里为止,基本借着ReentrantLock的加锁和释放锁的过程,给大家讲清楚了其底层依赖的AQS的核心原理。

基本上大家把这篇文章看懂,以后再也不会担心面试的时候被问到:谈谈你对AQS的理解这种问题了。

其实一句话总结AQS就是一个并发包的基础组件,用来实现各种锁,各种同步组件的。它包含了state变量、加锁线程、等待队列等并发中的核心组件。

并发系列文章,正在更新中,欢迎关注:
大白话聊聊Java并发面试问题之volatile到底是什么? 大白话聊聊Java并发面试问题之Java 8如何优化CAS性能? 大白话聊聊Java并发面试问题之谈谈你对AQS的理解?
大白话聊聊Java并发面试问题之公平锁与非公平锁是啥?敬请期待
大白话聊聊Java并发面试问题之微服务注册中心的读写锁优化?敬请期待

END

如有收获,请帮忙转发,您的鼓励是作者最大的动力,谢谢!

一大波微服务、分布式、高并发、高可用的原创系列文章正在路上

欢迎扫描下方二维码,持续关注:

石杉的架构笔记(id:shishan100)

十余年BAT架构经验倾囊相授

**> **推荐阅读:** > > 1、[拜托!面试请不要再问我Spring Cloud底层原理](https://link.juejin.im?target=https%3A%2F%2Flink.juejin.im%3Ftarget%3Dhttps%253A%252F%252Flink.juejin.im%252F%253Ftarget%253Dhttps%25253A%25252F%25252Flink.juejin.im%25253Ftarget%25253Dhttps%2525253A%2525252F%2525252Flink.juejin.im%2525253Ftarget%2525253Dhttps%252525253A%252525252F%252525252Flink.juejin.im%252525252F%252525253Ftarget%252525253Dhttps%25252525253A%25252525252F%25252525252Flink.juejin.im%25252525253Ftarget%25252525253Dhttps%2525252525253A%2525252525252F%2525252525252Flink.juejin.im%2525252525252F%2525252525253Ftarget%2525252525253Dhttps%252525252525253A%252525252525252F%252525252525252Flink.juejin.im%252525252525252F%252525252525253Ftarget%252525252525253Dhttps%25252525252525253A%25252525252525252F%25252525252525252Fjuejin.im%25252525252525252Fpost%25252525252525252F5be13b83f265da6116393fc7) > > 2、[【双11狂欢的背后】微服务注册中心如何承载大型系统的千万级访问?](https://link.juejin.im?target=https%3A%2F%2Flink.juejin.im%3Ftarget%3Dhttps%253A%252F%252Flink.juejin.im%252F%253Ftarget%253Dhttps%25253A%25252F%25252Flink.juejin.im%25253Ftarget%25253Dhttps%2525253A%2525252F%2525252Flink.juejin.im%2525253Ftarget%2525253Dhttps%252525253A%252525252F%252525252Flink.juejin.im%252525252F%252525253Ftarget%252525253Dhttps%25252525253A%25252525252F%25252525252Flink.juejin.im%25252525253Ftarget%25252525253Dhttps%2525252525253A%2525252525252F%2525252525252Flink.juejin.im%2525252525252F%2525252525253Ftarget%2525252525253Dhttps%252525252525253A%252525252525252F%252525252525252Flink.juejin.im%252525252525252F%252525252525253Ftarget%252525252525253Dhttps%25252525252525253A%25252525252525252F%25252525252525252Fjuejin.im%25252525252525252Fpost%25252525252525252F5be3f8dcf265da613a5382ca) > > 3、[【性能优化之道】每秒上万并发下的Spring Cloud参数优化实战](https://link.juejin.im?target=https%3A%2F%2Flink.juejin.im%3Ftarget%3Dhttps%253A%252F%252Flink.juejin.im%252F%253Ftarget%253Dhttps%25253A%25252F%25252Flink.juejin.im%25253Ftarget%25253Dhttps%2525253A%2525252F%2525252Flink.juejin.im%2525253Ftarget%2525253Dhttps%252525253A%252525252F%252525252Flink.juejin.im%252525252F%252525253Ftarget%252525253Dhttps%25252525253A%25252525252F%25252525252Flink.juejin.im%25252525253Ftarget%25252525253Dhttps%2525252525253A%2525252525252F%2525252525252Flink.juejin.im%2525252525252F%2525252525253Ftarget%2525252525253Dhttps%252525252525253A%252525252525252F%252525252525252Flink.juejin.im%252525252525252F%252525252525253Ftarget%252525252525253Dhttps%25252525252525253A%25252525252525252F%25252525252525252Fjuejin.im%25252525252525252Fpost%25252525252525252F5be83e166fb9a049a7115580) > > 4、[微服务架构如何保障双11狂欢下的99.99%高可用](https://link.juejin.im?target=https%3A%2F%2Flink.juejin.im%3Ftarget%3Dhttps%253A%252F%252Flink.juejin.im%252F%253Ftarget%253Dhttps%25253A%25252F%25252Flink.juejin.im%25253Ftarget%25253Dhttps%2525253A%2525252F%2525252Flink.juejin.im%2525253Ftarget%2525253Dhttps%252525253A%252525252F%252525252Flink.juejin.im%252525252F%252525253Ftarget%252525253Dhttps%25252525253A%25252525252F%25252525252Flink.juejin.im%25252525253Ftarget%25252525253Dhttps%2525252525253A%2525252525252F%2525252525252Flink.juejin.im%2525252525252F%2525252525253Ftarget%2525252525253Dhttps%252525252525253A%252525252525252F%252525252525252Flink.juejin.im%252525252525252F%252525252525253Ftarget%252525252525253Dhttps%25252525252525253A%25252525252525252F%25252525252525252Fjuejin.im%25252525252525252Fpost%25252525252525252F5be99a68e51d4511a8090440) > > 5、[兄弟,用大白话告诉你小白都能听懂的Hadoop架构原理](https://link.juejin.im?target=https%3A%2F%2Flink.juejin.im%3Ftarget%3Dhttps%253A%252F%252Flink.juejin.im%252F%253Ftarget%253Dhttps%25253A%25252F%25252Flink.juejin.im%25253Ftarget%25253Dhttps%2525253A%2525252F%2525252Flink.juejin.im%2525253Ftarget%2525253Dhttps%252525253A%252525252F%252525252Flink.juejin.im%252525252F%252525253Ftarget%252525253Dhttps%25252525253A%25252525252F%25252525252Flink.juejin.im%25252525253Ftarget%25252525253Dhttps%2525252525253A%2525252525252F%2525252525252Flink.juejin.im%2525252525252F%2525252525253Ftarget%2525252525253Dhttps%252525252525253A%252525252525252F%252525252525252Flink.juejin.im%252525252525252F%252525252525253Ftarget%252525252525253Dhttps%25252525252525253A%25252525252525252F%25252525252525252Fjuejin.im%25252525252525252Fpost%25252525252525252F5beaf02ce51d457e90196069) > > 6、[大规模集群下Hadoop NameNode如何承载每秒上千次的高并发访问](https://link.juejin.im?target=https%3A%2F%2Flink.juejin.im%3Ftarget%3Dhttps%253A%252F%252Flink.juejin.im%252F%253Ftarget%253Dhttps%25253A%25252F%25252Flink.juejin.im%25253Ftarget%25253Dhttps%2525253A%2525252F%2525252Flink.juejin.im%2525253Ftarget%2525253Dhttps%252525253A%252525252F%252525252Flink.juejin.im%252525252F%252525253Ftarget%252525253Dhttps%25252525253A%25252525252F%25252525252Flink.juejin.im%25252525253Ftarget%25252525253Dhttps%2525252525253A%2525252525252F%2525252525252Flink.juejin.im%2525252525252F%2525252525253Ftarget%2525252525253Dhttps%252525252525253A%252525252525252F%252525252525252Flink.juejin.im%252525252525252F%252525252525253Ftarget%252525252525253Dhttps%25252525252525253A%25252525252525252F%25252525252525252Fjuejin.im%25252525252525252Fpost%25252525252525252F5bec278c5188253e64332c76) > > 7、【[性能优化的秘密】Hadoop如何将TB级大文件的上传性能优化上百倍](https://link.juejin.im?target=https%3A%2F%2Flink.juejin.im%3Ftarget%3Dhttps%253A%252F%252Flink.juejin.im%252F%253Ftarget%253Dhttps%25253A%25252F%25252Flink.juejin.im%25253Ftarget%25253Dhttps%2525253A%2525252F%2525252Flink.juejin.im%2525253Ftarget%2525253Dhttps%252525253A%252525252F%252525252Flink.juejin.im%252525252F%252525253Ftarget%252525253Dhttps%25252525253A%25252525252F%25252525252Flink.juejin.im%25252525253Ftarget%25252525253Dhttps%2525252525253A%2525252525252F%2525252525252Flink.juejin.im%2525252525252F%2525252525253Ftarget%2525252525253Dhttps%252525252525253A%252525252525252F%252525252525252Flink.juejin.im%252525252525252F%252525252525253Ftarget%252525252525253Dhttps%25252525252525253A%25252525252525252F%25252525252525252Fjuejin.im%25252525252525252Fpost%25252525252525252F5bed82a9e51d450f9461cfc7) > > 8、[拜托,面试请不要再问我TCC分布式事务的实现原理](https://link.juejin.im?target=https%3A%2F%2Flink.juejin.im%3Ftarget%3Dhttps%253A%252F%252Flink.juejin.im%252F%253Ftarget%253Dhttps%25253A%25252F%25252Flink.juejin.im%25253Ftarget%25253Dhttps%2525253A%2525252F%2525252Flink.juejin.im%2525253Ftarget%2525253Dhttps%252525253A%252525252F%252525252Flink.juejin.im%252525252F%252525253Ftarget%252525253Dhttps%25252525253A%25252525252F%25252525252Flink.juejin.im%25252525253Ftarget%25252525253Dhttps%2525252525253A%2525252525252F%2525252525252Flink.juejin.im%2525252525252F%2525252525253Ftarget%2525252525253Dhttps%252525252525253A%252525252525252F%252525252525252Fjuejin.im%252525252525252Fpost%252525252525252F5bf201f7f265da610f63528a)[坑爹呀!](https://link.juejin.im?target=https%3A%2F%2Flink.juejin.im%3Ftarget%3Dhttps%253A%252F%252Flink.juejin.im%252F%253Ftarget%253Dhttps%25253A%25252F%25252Flink.juejin.im%25253Ftarget%25253Dhttps%2525253A%2525252F%2525252Flink.juejin.im%2525253Ftarget%2525253Dhttps%252525253A%252525252F%252525252Flink.juejin.im%252525252F%252525253Ftarget%252525253Dhttps%25252525253A%25252525252F%25252525252Flink.juejin.im%25252525253Ftarget%25252525253Dhttps%2525252525253A%2525252525252F%2525252525252Fjuejin.im%2525252525252Fpost%2525252525252F5bf2c6b6e51d456693549af4) > > 9、[【坑爹呀!】最终一致性分布式事务如何保障实际生产中99.99%高可用?](https://link.juejin.im?target=https%3A%2F%2Flink.juejin.im%3Ftarget%3Dhttps%253A%252F%252Flink.juejin.im%252F%253Ftarget%253Dhttps%25253A%25252F%25252Flink.juejin.im%25253Ftarget%25253Dhttps%2525253A%2525252F%2525252Flink.juejin.im%2525253Ftarget%2525253Dhttps%252525253A%252525252F%252525252Flink.juejin.im%252525252F%252525253Ftarget%252525253Dhttps%25252525253A%25252525252F%25252525252Flink.juejin.im%25252525253Ftarget%25252525253Dhttps%2525252525253A%2525252525252F%2525252525252Fjuejin.im%2525252525252Fpost%2525252525252F5bf2c6b6e51d456693549af4) > > 10、[拜托,面试请不要再问我Redis分布式锁的实现原理!](https://link.juejin.im?target=https%3A%2F%2Flink.juejin.im%3Ftarget%3Dhttps%253A%252F%252Flink.juejin.im%252F%253Ftarget%253Dhttps%25253A%25252F%25252Flink.juejin.im%25253Ftarget%25253Dhttps%2525253A%2525252F%2525252Flink.juejin.im%2525253Ftarget%2525253Dhttps%252525253A%252525252F%252525252Flink.juejin.im%252525252F%252525253Ftarget%252525253Dhttps%25252525253A%25252525252F%25252525252Flink.juejin.im%25252525253Ftarget%25252525253Dhttps%2525252525253A%2525252525252F%2525252525252Fjuejin.im%2525252525252Fpost%2525252525252F5bf3f15851882526a643e207) > > **11、****[【眼前一亮!】看Hadoop底层算法如何优雅的将大规模集群性能提升10倍以上?](https://link.juejin.im?target=https%3A%2F%2Flink.juejin.im%3Ftarget%3Dhttps%253A%252F%252Flink.juejin.im%252F%253Ftarget%253Dhttps%25253A%25252F%25252Flink.juejin.im%25253Ftarget%25253Dhttps%2525253A%2525252F%2525252Flink.juejin.im%2525253Ftarget%2525253Dhttps%252525253A%252525252F%252525252Flink.juejin.im%252525252F%252525253Ftarget%252525253Dhttps%25252525253A%25252525252F%25252525252Flink.juejin.im%25252525253Ftarget%25252525253Dhttps%2525252525253A%2525252525252F%2525252525252Fjuejin.im%2525252525252Fpost%2525252525252F5bf5396f51882509a768067e)** > > **12、****[亿级流量系统架构之如何支撑百亿级数据的存储与计算](https://link.juejin.im?target=https%3A%2F%2Flink.juejin.im%3Ftarget%3Dhttps%253A%252F%252Flink.juejin.im%252F%253Ftarget%253Dhttps%25253A%25252F%25252Flink.juejin.im%25253Ftarget%25253Dhttps%2525253A%2525252F%2525252Flink.juejin.im%2525253Ftarget%2525253Dhttps%252525253A%252525252F%252525252Flink.juejin.im%252525252F%252525253Ftarget%252525253Dhttps%25252525253A%25252525252F%25252525252Fjuejin.im%25252525252Fpost%25252525252F5bfab59fe51d4551584c7bcf)** > > 13、[亿级流量系统架构之如何设计高容错分布式计算系统](https://link.juejin.im?target=https%3A%2F%2Flink.juejin.im%3Ftarget%3Dhttps%253A%252F%252Flink.juejin.im%252F%253Ftarget%253Dhttps%25253A%25252F%25252Flink.juejin.im%25253Ftarget%25253Dhttps%2525253A%2525252F%2525252Flink.juejin.im%2525253Ftarget%2525253Dhttps%252525253A%252525252F%252525252Fjuejin.im%252525252Fpost%252525252F5bfbeeb9f265da61407e9679) > > 14、[亿级流量系统架构之如何设计承载百亿流量的高性能架构](https://link.juejin.im?target=https%3A%2F%2Flink.juejin.im%3Ftarget%3Dhttps%253A%252F%252Flink.juejin.im%252F%253Ftarget%253Dhttps%25253A%25252F%25252Flink.juejin.im%25253Ftarget%25253Dhttps%2525253A%2525252F%2525252Fjuejin.im%2525252Fpost%2525252F5bfd2df1e51d4574b133dd3a) > > 15、[亿级流量系统架构之如何设计每秒十万查询的高并发架构](https://link.juejin.im?target=https%3A%2F%2Flink.juejin.im%3Ftarget%3Dhttps%253A%252F%252Flink.juejin.im%252F%253Ftarget%253Dhttps%25253A%25252F%25252Fjuejin.im%25252Fpost%25252F5bfe771251882509a7681b3a) > > 16、[亿级流量系统架构之如何设计全链路99.99%高可用架构](https://link.juejin.im?target=https%3A%2F%2Flink.juejin.im%3Ftarget%3Dhttps%253A%252F%252Fjuejin.im%252Fpost%252F5bffab686fb9a04a102f0022) > > 17、[七张图彻底讲清楚ZooKeeper分布式锁的实现原理](https://link.juejin.im?target=https%3A%2F%2Fjuejin.im%2Fpost%2F5c01532ef265da61362232ed) > > 18、[大白话聊聊Java并发面试问题之volatile到底是什么?](https://juejin.cn/post/6844903730303746061) > > 19、[大白话聊聊Java并发面试问题之Java 8如何优化CAS性能?](https://juejin.cn/post/6844903731234865160)**