[volatile principle] volatile principle

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One 、 What is? volatile

volatile yes Java A kind of Lightweight synchronization mechanism .Java Language contains two internal synchronization mechanisms ： Synchronized block （ Or method ） and volatile Variable , comparison synchronized To solve the problem of shared memory ,volatile It's a lighter choice , He There is no additional overhead cost of context switching . Use volatile Declared variables , Yes When the hedge is updated, it is immediately visible to other threads , In a nutshell , It can lead to other CPU The corresponding cache value in is invalid .volatile Use memory barrier to ensure that instruction reordering does not occur , Solved the problem of memory visibility . however volatile Variable Poor synchronization （ Sometimes it's simpler and less expensive ）, And its use is also more error prone .

Two 、volatile principle

Threads read shared variables from main memory to working memory for operation , Write the result to main memory after completion , But that brings visibility issues . for instance , Now let's say we're a two-level cache with dual cores CPU framework , contain L1、L2 Two level cache .
（1） Threads 1 First get the variable X Value , Because the first two levels of cache are empty , So read directly from main memory X, hypothesis X The initial value is 0, Threads 1 After reading, put X Values are changed to 1, Write back to main memory at the same time . At this time, the cache and main memory are shown in the following figure .

（2） Threads 2 Also read variables X Value , because L2 The cache already has a cache X=1, So directly from L2 Cache reads , After that thread 2 hold X It is amended as follows 2, At the same time, write back L2 And main memory . By this time X The value is shown in the figure below .

At this point, the thread 1 If you want to get a variable again X Value , because L1 The cache already has x=1 了 , So at this point Variable memory invisibility problem It came into being. , Threads 2 take X It is amended as follows 2 For threads 1 Come on No perception , The thread 1 I don't know at this time X The value of has become 2 了 .
however , If X Variable usage volatile Embellished words , When a thread 1 Read variables again X Words ,CPU The thread will be forced according to the cache consistency protocol 1 Reload the latest value from main memory to your working memory , Instead of just using the values in the cache .

stay JVM Bottom volatile Is to use Memory barrier To achieve . The memory barrier will provide 3 Features ：
（1） It ensures that instructions are reordered without putting the instructions behind them in front of the memory barrier , It doesn't put the previous instructions behind the memory barrier ; That is, when the instruction of memory barrier is executed , All the operations before it have been completed ; In a nutshell , Execute to volatile variable , In front of the All statements have been executed , None of the following statements are executed . And the result of the previous statement is right volatile Variables and statements after them can be seen .
（2） It will Force changes to the cache to be written to main memory immediately ;
（3） If it's a write operation , it It can lead to other CPU The corresponding cache row in is invalid . Make sure When the value is updated, it is immediately visible to other threads .

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This article is a study note , It is the understanding of others' knowledge , Together with some personal understandings of myself . If there is an infringement connection, delete . Writing is not easy to , I hope you guys can support me for three times . Be deeply grateful
notes ： The source of the original text is https://mp.weixin.qq.com/s/-xFSHf7Gz3FUcafTJUIGWQ