JAVA面试-并发篇 06-ReentrantLock如何实现公平锁的以及可重入吗

ReentrantLock公平锁实现与可重入特性

一、文章概述

ReentrantLock是JUC并发包下基于AQS实现的独占锁，支持可重入，并提供公平锁、非公平锁两种模式。本文讲解可重入实现原理，明确公平锁获取失败后的入队规则，清晰对比两种锁的核心差异。

二、核心特性

特性	说明
可重入性	同一线程可多次获取锁，通过state计数实现
公平锁	严格遵循队列顺序，无抢占、无重试
非公平锁	支持直接抢占锁，性能更高

三、可重入性实现原理

依托AQS的state变量实现可重入：

1. 线程重入锁，state +1
2. 释放锁，state -1
3. state=0 代表锁完全释放

四、公平锁获取流程

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否
成功
失败
线程尝试获取锁
队列有等待线程?
加入队列队尾阻塞
CAS修改state
获取锁
被前驱唤醒

五、非公平锁获取流程

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失败
线程尝试获取锁
直接CAS抢占锁
获取锁执行代码
加入队列队尾阻塞
被唤醒

六、核心规则

1. 公平锁 ：CAS获取失败后，不重试、不插队，直接加入AQS队列队尾，严格遵守FIFO先来先服务
2. 非公平锁：CAS抢占失败后，同样加入队列队尾，但获取锁时允许直接抢占

总结

ReentrantLock通过state实现可重入；公平锁与非公平锁获取失败后，统一加入AQS队列队尾。公平锁全程禁止插队，保证执行顺序；非公平锁允许抢占，提升吞吐量，可按需选择。