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：
一、写在前面
上篇文章（大白话聊聊Java并发面试问题之谈谈你对AQS的理解？）聊了一下java并发包中的AQS的工作原理，也间接说明了ReentrantLock的工作原理。
这篇文章接着来聊一个话题，java并发包中的公平锁与非公平锁有啥区别？
二、什么是非公平锁？
先来聊聊非公平锁是啥，现在大家先回过头来看下面这张图。
如上图，现在线程1加了锁，然后线程2尝试加锁，失败后进入了等待队列，处于阻塞中。然后线程1释放了锁，准备来唤醒线程2重新尝试加锁。
注意一点，此时线程2可还停留在等待队列里啊，还没开始尝试重新加锁呢！
然而，不幸的事情发生了，这时半路杀出个程咬金，来了一个线程3！线程3突然尝试对ReentrantLock发起加锁操作，此时会发生什么事情？
很简单！线程2还没来得及重新尝试加锁呢。也就是说，还没来得及尝试重新执行CAS操作将state的值从0变为1呢！线程3冲上来直接一个CAS操作，尝试将state的值从0变为1，结果还成功了！
一旦CAS操作成功，线程3就会将“加锁线程”这个变量设置为他自己。给大家来一张图，看看这整个过程：
明明人家线程2规规矩矩的排队领锁呢，结果你线程3不守规矩，线程1刚释放锁，不分青红皂白，直接就跑过来抢先加锁了。
这就导致线程2被唤醒过后，重新尝试加锁执行CAS操作，结果毫无疑问，失败！
原因很简单啊！因为加锁CAS操作，是要尝试将state从0变为1，结果此时state已经是1了，所以CAS操作一定会失败！
一旦加锁失败，就会导致线程2继续留在等待队列里不断的等着，等着线程3释放锁之后，再来唤醒自己，真是可怜！先来的线程2居然加不到锁！
同样给大家来一张图，体会一下线程2这无助的过程：
上述的锁策略，就是所谓的非公平锁！
如果你用默认的构造函数来创建ReentrantLock对象，默认的锁策略就是非公平的。
在非公平锁策略之下，不一定说先来排队的线程就就先会得到机会加锁，而是出现各种线程随意抢占的情况。
那如果要实现公平锁的策略该怎么办呢？也很简单，在构造ReentrantLock对象的时候传入一个true即可：
ReentrantLock lock = new ReentrantLock(true)
此时就是说让他使用公平锁的策略，那么公平锁具体是什么意思呢？
三、什么是公平锁？
咱们重新回到第一张图，就是线程1刚刚释放锁之后，线程2还没来得及重新加锁的那个状态。
同样，这时假设来了一个线程3，突然杀出来，想要加锁。
如果是公平锁的策略，那么此时线程3不会跟个愣头青一样盲目的直接加锁。
他会先判断一下：咦？AQS的等待队列里，有没有人在排队啊？如果有人在排队的话，说明我前面有兄弟正想要加锁啊！
如果AQS的队列里真的有线程排着队，那我线程3就不能跟个二愣子一样直接抢占加锁了。
因为现在咱们是公平策略，得按照先来后到的顺序依次排队，谁先入队，谁就先从队列里出来加锁！
所以，线程3此时一判断，发现队列里有人排队，自己就会乖乖的排到队列后面去，而不会贸然加锁！
同样，整个过程我们用下面这张图给大家直观的展示一下：
上面的等待队列中，线程3会按照公平原则直接进入队列尾部进行排队。
接着，线程2不是被唤醒了么？他就会重新尝试进行CAS加锁，此时没人跟他抢，他当然可以加锁成功了。
然后呢，线程2就会将state值变为1，同时设置“加锁线程”是自己。最后，线程2自己从等待队列里出队。
整个过程，参见下图：
这个就是公平锁的策略，过来加锁的线程全部是按照先来后到的顺序，依次进入等待队列中排队的，不会盲目的胡乱抢占加锁，非常的公平。
四、小结
好了，通过画图和文字分析，相信大家都明白什么是公平锁，什么是非公平锁了！
不过要知道java并发包里很多锁默认的策略都是非公平的，也就是可能后来的线程先加锁，先来的线程后加锁。
而一般情况下，非公平的策略都没什么大问题，但是大家要对这个策略做到心里有数，在开发的时候，需要自己来考虑和权衡是要用公平策略还是非公平策略。
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？》
《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)
>
> 20、[大白话聊聊Java并发面试问题之谈谈你对AQS的理解？](https://juejin.cn/post/6844903732061159437)**
