JUC（10）深入理解CAS和ABA

• 1、CAS
• 2、原子引用解决ABA问题，版本号。修改后，可以看到

package com.cas;

import java.util.concurrent.atomic.AtomicInteger;


public class CASDemo {
    public static void main(String[] args) {
        AtomicInteger atomicInteger = new AtomicInteger(2021);
        
        atomicInteger.compareAndSet(2021,2022);
        System.out.println(atomicInteger.get());
    }
}

CAS：比较当前工作内存中的值和主存中的值，如果这个值是期望的，那么执行操作！如果不是、一直循环

缺点：

• 1、循环会耗时
• 2、一次性只能保证一个共享变量的原子性
• 3、存在ABA问题
  ABA问题：狸猫换太子

举例子：A 打算让1变为2.在这个过程中，线程B已经对其进行过修改，从1，到3，再从3，到1。对线程A来讲，好像没啥大的变化。

package com.cas;

import java.util.concurrent.atomic.AtomicInteger;

public class ABADemo {
    public static void main(String[] args) {
        AtomicInteger atomicInteger = new AtomicInteger(2021);
        
        System.out.println(atomicInteger.compareAndSet(2021, 2022));
        System.out.println(atomicInteger.get());

        System.out.println(atomicInteger.compareAndSet(2022, 2021));
        System.out.println(atomicInteger.get());

        System.out.println(atomicInteger.compareAndSet(2021, 6666));
        System.out.println(atomicInteger.get());

    }
}

举例子（乐观锁）

package com.cas;

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


public class AtomicStampedReferenceDemo {
    public static void main(String[] args) {
        AtomicStampedReference atomicStampedReference = new AtomicStampedReference<>(66, 1);

        new Thread(()->{
         int stamp = atomicStampedReference.getStamp();//获得版本号
         System.out.println("A===>"+stamp);



            atomicStampedReference.compareAndSet(66,99,atomicStampedReference.getStamp(),atomicStampedReference.getStamp()+1);
            System.out.println("A2==>"+atomicStampedReference.getStamp());

            atomicStampedReference.compareAndSet(99,66,atomicStampedReference.getStamp(),atomicStampedReference.getStamp()+1);
            System.out.println("A3==>"+atomicStampedReference.getStamp());

        },"A").start();

        new Thread(()->{
            int stamp = atomicStampedReference.getStamp();//获得版本号
            System.out.println("B===>"+stamp);

            try {
                TimeUnit.SECONDS.sleep(6);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            atomicStampedReference.compareAndSet(66,6,stamp,stamp+1);
            System.out.println("B1==>"+atomicStampedReference.getStamp());

        },"B").start();
    }
}

测试结果