Have you encountered ABA problems? Let's talk about the following in detail, how to avoid ABA problems

1、 There are two threads modifying the value of a variable at the same time , Threads 1、 Threads 2, Update variable values , Change variables from A Updated to B

2、 The first thread 1 To obtain CPU Time slice , Threads 2 Wait for some reason , Threads 1 Compare and update ,CAS
（CompareAndSwap）, Successfully changed the value of the variable from A Update to B

3、 After updating , There happens to be another thread 3 Come in and want to change the value of the variable from B Update to A, Threads 3 Compare and update , Successfully changed the value of the variable from B Updated to A

4、 Threads 2 To obtain CPU Time slice , Then compare and update , The value found is expected A, And then it's updated B, But threads 1 I don't know that the value has changed from A-->B-->A This process , This is it. ABA problem .

 

How to avoid ABA problem ？

It can be solved by adding version number or timestamp , Or guarantee one-way increasing or decreasing, there will be no such problems ,atomic Under bag AtomicStampedReference Class CompareAndSet Method first checks whether the current reference is equal to the expected reference , And whether the current flag is equal to the expected flag , If all are equal , Then set the value of this flag to the given update value in an atomic way

package com.ws.cas;

import java.util.concurrent.atomic.AtomicInteger;

public class CASDemo {

    //CAS compareAndSet： Compare and exchange ！！！
    public static void main(String[] args) {
        AtomicInteger atomicInteger = new AtomicInteger(2020);

        // For what we usually write SQL Come on ： Optimism lock ！
        // The first parameter is expectation 、 The second parameter is update 
        //public final boolean compareAndSet(int expect, int update)
        // If my expectations reach , Then update , Otherwise, it will not be updated  CAS yes CPU The concurrent primitives of 


        // Mess with threads 
        System.out.println(atomicInteger.compareAndSet(2020, 2021));
        System.out.println(atomicInteger.get());

        System.out.println(atomicInteger.compareAndSet(2021, 2020));
        System.out.println(atomicInteger.get());

        // Expected thread 
        System.out.println(atomicInteger.compareAndSet(2020, 6666));
        System.out.println(atomicInteger.get());



    }
}

CAS Other problems caused ？

1. Only one atomic operation of shared variables can be guaranteed
2. The spin CAS If it doesn't work for a long time , Will give CPU Bring the problem of high cost