Atomic class and CAS

Atomic classes

Why use atomic classes ？

Performance issues ,synchronized Keyword will let the thread that does not get the subelement enter BLOCKED state , Recover to... After contention for lock resources RUNNABLE state , In this process, the conversion between user mode and internal mode is required , The cost is high . although jdk 6 after ,synchronized With the transition from biased lock to lightweight lock to heavyweight lock , But after finally becoming a heavyweight lock , Performance is still low , In this case, we use java The atomic class in .

Atomic operation class ：java.util.concurrent.atomic The series under the package is Atomic The first packaging class , Such as AtomicInteger、AtomicBoolean、AtomicLong.

The atomicity of atomic class is through volatile + CAS The realization of atomic operation .

CAS

CAS(Compare and swap)： Compare and replace , It's an implementation of optimistic lock , Using the idea of spin , Is a lightweight locking mechanism .

Three values in memory

V： Memory value

A： Expected value , The value read again from memory during comparison

B： Update value , Updated value

If and only if expected A==V, Put the memory value V=B, Otherwise, do nothing .

Spin thought

The expected value is different from the value in memory every time , The difference is because the memory value has been modified by other threads , Therefore, we need to get the latest value again as the expected value , Judge again . And the thread constantly circulates to determine whether the memory value has been modified by other threads , This is the idea of spin . Through the bottom layer Unsafe Class compareAndSwapInt And so on .

shortcoming

1.cpu It's expensive ,CAS It's unlocked , Using the idea of spin , Threads don't block , If there are a large number of threads trying , It costs more .

2. There's no guarantee that the code block is atomic

3.ABA problem

ABA Problems and solutions

A thread transfers the memory value from A Change it to B, Again by B Change it to A, When another thread uses the expected value to judge , The expected value is the same as the memory value , Of the current thread CAS The operation cannot tell whether the memory value has been modified .

resolvent ： Add version numbers by using classes . As before The memory value of is （A,1）, Thread will （A,1） Revised to （B,2）, Again by （B,2） It is amended as follows （A,3）. At this point, another thread uses the expected value （A,1） And memory value （A,3） Compare , Just compare the version number 1 and 3, You can find that the data in the memory has been updated .