After 4 years of developing two-sided meituan, we finally lost: the interview question of volatile keyword function and principle

a 6 A little partner with years of working experience , I was asked when I was in a certain place “volatile” keyword . then , No, then …

Again , And one more 4 Year's little friend , I was also asked about the interview in a certain group “volatile  keyword “. then , There was no more …

volatile Keywords are Java An important keyword in concurrent programming , This question is really asked at the bottom . Is it true that big factories are so busy now ？ What about today , Let me share my understanding , I hope I can help you .

in addition , I prepared a copy of 500 page PDF Supporting documents for interview question analysis ,

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1、volatile The role of

volatile  Key words? , It does two things ：

The first 1 individual , It can ensure the visibility of shared variables in a multithreaded environment .

The first 2 individual , It can be shielded in a multithreaded environment CPU The command rearrangement of .

below , Let me give you a detailed introduction to visibility and shielding instruction rearrangement and volatile How it works .

2、 Principle analysis

Let's first look at the visibility of variables , Simply speaking , It refers to the modification of shared variables by a thread , Other threads can immediately see the modified value . In fact, this visibility problem , I think it is essentially caused by the following two aspects .

First of all ,CPU The cache of . stay  CPU  A three-level cache is designed to solve  CPU  Operational efficiency and memory  IO  The efficiency problem , But there are cache consistency problems , In the case of multi-threaded parallel execution , Cache consistency can lead to visibility problems .

therefore , For added  volatile  Keyword decorated shared variables ,JVM  A virtual opportunity is automatically added #Lock  Assembly instruction , This instruction will be based on  CPU  The model automatically adds a bus lock or / Buffer lock .

Let me briefly introduce these two kinds of locks ：

The bus lock is locked  CPU  Front end bus , As a result, only one thread can communicate with memory at the same time , This avoids the visibility caused by multithreading concurrency .

Cache lock is the optimization of bus lock , Because the bus lock caused  CPU  The use efficiency of has decreased significantly , So the cache lock is only for  CPU  Lock the target data in the L3 cache , Cache lock is used  MESI  Cache consistency .

then , Is to shield instruction rearrangement , It means shielding CPU Instruction reordering . It means in a multithreaded environment ,CPU The writing order of instructions is inconsistent with the execution order , This leads to visibility problems , In order to improve  CPU  Utilization ratio ,CPU Introduced StoreBuffer  Mechanism , And this kind of optimization mechanism will lead to  CPU  To execute in disorder . Of course, in order to avoid such problems ,CPU  Provides memory barrier instructions , Upper layer applications can insert memory barriers in appropriate places to avoid  CPU  Command reordering problem . and volatile It is to set a memory barrier to prevent instruction rearrangement .

volatile The implementation principle of memory barrier is mainly analyzed from the following two aspects ：

The first 1 Yes ：volatile Two memory barriers are added before and after the variable write operation , To ensure that the previous write instructions and the subsequent read instructions are in order , As shown in the figure ：

The first 2 Yes ：volatile Insert two instructions after the read operation of the variable , Prevent subsequent read and write reordering ., As shown in the figure ：

volatile In fact, it can be regarded as lightweight synchronized, Although I say volatile Atomicity is not guaranteed , But if the operation under multithreading itself is atomic （ For example, assignment operation ）, So use volatile Better than synchronized. So that's me right volatile  Keyword understanding .

Concurrent programming is a field that every programmer must master , It covers the design ideas 、 And concurrency problems 、 And as a concurrency tool , Are very worthy of in-depth study .

Last , I sorted all the videos I shared in previous periods into 1 Share 20W Document of words , I hope this can improve the passing rate of all fans

I was delayed by programming Tom, Only dry goods without water ！ Your support is my biggest motivation ！ Pay attention to me , The interview is no longer difficult ！

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