A little friend confided in me , He said he met a man about CAS The problem of mechanism , He thought the interviewer was asking CAS Single sign on .

assume , I'm familiar with this question , Then follow the idea of single sign on to answer , As a result, the interviewer kept shaking his head .

He came to talk to me , At the end of the interview, I didn't want to answer so well , Why didn't you send it to me on the spot offer Well ？

actually , The interviewer asked about... In concurrent programming CAS Mechanism .

Now let's take a look at ordinary people and experts for CAS The answer of the mechanism

Ordinary people ：

CAS, It is an operation used to realize atomic functions in concurrent programming , Um. , It is similar to an optimistic lock mechanism , It can ensure the atomicity of changes to the value of shared variables in the case of concurrency .

Um. , image AtomicInteger In this class , It uses CAS Mechanism . Um. …

master ：

CAS yes Java in Unsafe Methods in class , Its full name is CompareAndSwap, Compare and exchange . Its main function is to ensure that in a multithreaded environment , Atomicity of modifications to shared variables .

Let me give you an example , For example, there is such a scene , There is a member variable state, The default value is 0,

Defines a method doSomething(), The logic of this method is , Judge state Is it 0 , If 0, Just change it to 1.

There seems to be no problem with this logic , But in a multithreaded environment , There will be atomic problems , Because this is a typical ,Read - Write The operation of .

In general , We will be in doSomething() This method adds a synchronization lock to solve the atomicity problem .

however , Add synchronization lock , It will bring performance loss , therefore , For such scenarios , We can use CAS Mechanism to optimize

This is the optimized code

stay doSomething() In the method , We call unsafe Class compareAndSwapInt() Methods to achieve the same purpose , This method has four parameters ,

Namely ： Current object instance 、 Member variables state Offset in memory address 、 Expected value 0、 Expected value after change 1.

CAS The mechanism will be more state The value corresponding to the memory address offset and the expected value passed in 0 Whether it is equal or not , If equal , Just modify the memory address directly state The value of is 1.

otherwise , return false, Indicates that the modification failed , And this process is atomic , There will be no thread safety issues .

CompareAndSwap It's a native Method , In fact, it will eventually face the same problem , Is to read from the memory address first state Value , And then compare , Finally, I will revise .

No matter at what level this process is realized , There will be atomic problems .

So ,CompareAndSwap In the underlying implementation of , In multicore CPU In the environment , One will be added Lock Instruction locks the cache or bus , So as to ensure the atomicity of comparing and replacing the two instructions .

CAS Mainly used in concurrent scenarios , There are two typical usage scenarios .

1. The first is J.U.C Inside Atomic Atomic realization of , such as AtomicInteger,AtomicLong.
2. The second is to realize the mutually exclusive nature of multi-threaded competition for shared resources , For example AQS、ConcurrentHashMap、ConcurrentLinkedQueue And so on .

The above is my understanding of this problem .

summary

Recently, you also found the change of the content of my article in the expert answer part .

Some of the lads said , How can you bring pictures to the interview , Obviously cheating .

In fact, the main reason is that many recent interview questions are at the bottom , The content of the bottom layer covers a wide range of knowledge , We hardly have any contact with .

therefore , If I want to pass this knowledge on to you , You have to do a lot of graphic and content structure design , Otherwise, everyone will still look confused after reading .

well , Ordinary people in this issue VS That's the end of the expert interview series , Remember to like your friends .

I am a Mic, A job 14 Year of Java The programmer , Let's see you next time .

 

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