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大白话聊聊Java并发面试问题之公平锁与非公平锁是啥?【石杉的架构笔记】

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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

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