【多线程】原子引用ABA问题

news2024/11/26 19:56:15

package com.learning.atomic;

import lombok.extern.slf4j.Slf4j;
import java.util.concurrent.atomic.AtomicReference;
/**
 * @Author wangyouhui
 * @Description ABA问题
 **/
@Slf4j
public class AtomicReferenceABA {
    static AtomicReference<String> reference = new AtomicReference<>("A");

    public static void main(String[] args) {
        String previous = reference.get();
        otherThreadExecute();
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        log.info("A->C:{}",reference.compareAndSet(previous, "C"));
    }

    private static void otherThreadExecute() {
        new Thread(()->{
            log.info("A->B:{}", reference.compareAndSet(reference.get(), "B"));
        }).start();
        try {
            Thread.sleep(500);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        new Thread(()->{
            log.info("B->A:{}",reference.compareAndSet(reference.get(), "A"));
        }).start();
    }
}

二、执行结果截图

在这里插入图片描述

三、说明

1.主线程准备将共享变量由A改为C
2.在这期间，线程1将共享变量由A改为B，线程2再将共享变量B改为A
3.对于主线程仅能判断出共享变量的值与最初值A是否相同，不能感知到A改为B又改回A的情况
4.如果希望主线程能感知到其它线程动过该共享变量，则cas失败，所以比较值是不能解决问题的，需要再加一个版本号
5.AtomicStampedReferennce

四、AtomicStampedReference使用

4.1 代码示例

package com.learning.atomic;

import lombok.extern.slf4j.Slf4j;

import java.util.concurrent.atomic.AtomicStampedReference;

/**
 * @Author wangyouhui
 * @Description AtomicStampedReference解决ABA问题
 **/
@Slf4j
public class AtomicStampedReferenceABA {
    static AtomicStampedReference<String> reference = new AtomicStampedReference("A", 0);

    public static void main(String[] args) {
        // 主线程获取值
        String previous = reference.getReference();
        // 主线程获取版本号
        int stamp = reference.getStamp();
        log.info("主线程获取版本号：{}", stamp);
        otherThreadExecute();
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        log.info("主线程获取版本号：{}", reference.getStamp());
        log.info("A->C:{}",reference.compareAndSet(previous, "C", stamp, stamp+1));
    }

    private static void otherThreadExecute() {
        new Thread(()->{
            int stamp = reference.getStamp();
            log.info("线程1获取版本号：{}", stamp);
            log.info("A->B:{}", reference.compareAndSet(reference.getReference(), "B", stamp, stamp + 1));
        }).start();
        try {
            Thread.sleep(500);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        new Thread(()->{
            int stamp = reference.getStamp();
            log.info("线程2获取版本号：{}", stamp);
            log.info("B->A:{}",reference.compareAndSet(reference.getReference(), "A", stamp, stamp + 1));
        }).start();
    }
}