# 04-锁内存语义的实现

锁一般式基于AQS实现的，AQS维护了一个volatile的state， 这个state式实现锁内存语义的关键

## CAS

如果当前状态值等于预期值，则以原子方式将同步状态设置为给定的更新值。此操作具有volatile读和写的内存语义。

编译器不会对volatile读与volatile读后面的任意内存操作重排序；编译器不会对volatile写与volatile写前面的任意内存操作重排序。组合这两个条件，意味着为了同时实现volatile读和volatile写的内存语义，编译器不能对CAS与CAS前面和后面的任意内存操作重排序

## 内存语义总结

* 公平锁和非公平锁释放时，最后都要写一个volatile变量state。
* 公平锁获取时，首先会去读volatile变量。
* 非公平锁获取时，首先会用CAS更新volatile变量，这个操作同时具有volatile读和volatile写的内存语义。


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