CAS（比较并交换），它是一条CPU并发原语，它的功能是判断内存中某个位置的值是否和期望值一致，如果一致的话就去更该为新的值，这个过程是原子的。

 

 

 

Unsafe类中的compareAndSwapInt方法，是native方法能够实现原子操作的本质是硬件层面的。

 

 CAS不需要加锁，但是也有缺点：

 

有些业务场景，仅仅只关注值是否一样是不够的，需要监控中间的值有没有变过，这时候就需要在修改之后也修改版本号了 

ABA问题的复现和解决代码

package com.jin.javaLearning220114;

import java.util.concurrent.atomic.AtomicReference;
import java.util.concurrent.atomic.AtomicStampedReference;

/**
 * @author JinJian
 * @version 1.0
 * @date 2023/8/27 11:03
 */

public class ABATest01 {
    private static AtomicReference<Integer> atomicReference = new AtomicReference(1);
    //加上了时间戳
    private static AtomicStampedReference<Integer> atomicStampedReference = new AtomicStampedReference<>(1,1);

    public static void main(String[] args) {
        System.out.println("+++++++++ABA问题的产生++++++++++++");
        new Thread(() ->{
            boolean b = atomicReference.compareAndSet(1, 2);
            System.out.println(Thread.currentThread().getName() + "修改" + b + " atomicReference的值为" + atomicReference.get());
            boolean b1 = atomicReference.compareAndSet(2, 1);
            System.out.println(Thread.currentThread().getName() + "修改" + b1 + " atomicReference的值为" + atomicReference.get());
        },"T1").start();

        new Thread(() ->{
            try {
                Thread.sleep(1);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            boolean b3 = atomicReference.compareAndSet(1, 6);
            System.out.println(Thread.currentThread().getName() + "修改" + b3 + " atomicReference的值为" + atomicReference.get());
        },"T2").start();

        System.out.println("+++++++++ABA问题的解决++++++++++++");

        new Thread(() ->{
            System.out.println(Thread.currentThread().getName() + " 拿到" + " atomicStampedReference的版本号 " + atomicStampedReference.getStamp());
            try {
                Thread.sleep(1);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            boolean b4 = atomicStampedReference.compareAndSet(1, 2, 1, atomicStampedReference.getStamp() + 1);
            System.out.println(Thread.currentThread().getName() + " 修改" + b4 + " atomicStampedReference值 " + atomicStampedReference.getReference()+ " atomicStampedReference版本号是 " + atomicStampedReference.getStamp());
            boolean b5 = atomicStampedReference.compareAndSet(2, 1, 2, atomicStampedReference.getStamp() + 1);
            System.out.println(Thread.currentThread().getName() + " 修改" + b5 + " atomicStampedReference值 " + atomicStampedReference.getReference()+ " atomicStampedReference版本号是 " + atomicStampedReference.getStamp());
        },"T3").start();

        new Thread(() ->{
            System.out.println(Thread.currentThread().getName() + " 拿到" + " atomicStampedReference的版本号 " + atomicStampedReference.getStamp());
            try {
                Thread.sleep(3);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            boolean b6 = atomicStampedReference.compareAndSet(1, 2, 1, atomicStampedReference.getStamp() + 1);
            System.out.println(Thread.currentThread().getName() + " 修改" + b6 + " atomicStampedReference值 " + atomicStampedReference.getReference()+ " atomicStampedReference版本号是 " + atomicStampedReference.getStamp());
        },"T4").start();

    }
}