談談原子類AtomicInteger的ABA問題？原子更新引用知道嗎？

ABA 問題怎麼產生的

CAS會導致"ABA問題"。

CAS算法實現一個重要前提需要取出內存中某時刻的數據並在當下時刻比較並替換，那麼在這個時間差類會導致數據的變化。

比如說一個線程one從內存位置V中取出A，這時候另一個線程two也從內存中取出A，並且線程two進行了一些操作將值變成了B，
然後線程two又將V位置的數據變成A，這時候線程one進行CAS操作發現內存中仍然是A，然後線程one操作成功。

儘管線程one的CAS操作成功，但是不代表這個過程就是沒有問題的。

原子引用

package com.brian.interview.study.thread;

import java.util.concurrent.atomic.AtomicReference;

/**
 * Copyright (c) 2020 ZJU All Rights Reserved
 * <p>
 * Project: JavaSomeDemo
 * Package: com.brian.interview.study.thread
 * Version: 1.0
 * <p>
 * Created by Brian on 2020/2/11 15:29
 */


class User{
    String userName;
    int age;

    public User(String userName, int age) {
        this.userName = userName;
        this.age = age;
    }

    @Override
    public String toString() {
        return "User{" +
                "userName='" + userName + '\'' +
                ", age=" + age +
                '}';
    }
}

public class AtomicReferenceDemo {

    public static void main(String[] args) {

        User z3 = new User("z3", 22);
        User li4 = new User("li4", 25);

        AtomicReference<User> atomicReference = new AtomicReference<>();
        atomicReference.set(z3);

        System.out.println(atomicReference.compareAndSet(z3, li4)+"\t"+atomicReference.get().toString());
        System.out.println(atomicReference.compareAndSet(z3, li4)+"\t"+atomicReference.get().toString());

    }

}

時間戳原子引用

package com.brian.interview.study.thread;

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

/**
 * Copyright (c) 2020 ZJU All Rights Reserved
 * <p>
 * Project: JavaSomeDemo
 * Package: com.brian.interview.study.thread
 * Version: 1.0
 * <p>
 * Created by Brian on 2020/2/11 16:58
 */
public class ABADemo {  // ABA問題的解決   AtomicStampedReference

    static AtomicReference<Integer> atomicReference = new AtomicReference<>(100);

    static AtomicStampedReference<Integer> atomicStampedReference = new AtomicStampedReference<>(100, 1);

    public static void main(String[] args) {
        System.out.println("======================以下是ABA問題的產生======================");
        new Thread(() -> {
            atomicReference.compareAndSet(100, 101);
            atomicReference.compareAndSet(101, 100);
        }, "t1").start();

        new Thread(() -> {
            // 暫停1秒鐘t2線程, 保證上面的t1線程完成了一次ABA操作
            try {
                TimeUnit.SECONDS.sleep(1);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(atomicReference.compareAndSet(100, 2020) + "\t" + atomicReference.get());
        }, "t2").start();

        System.out.println("======================以下是ABA問題的解決======================");

        new Thread(() -> {
            int stamp = atomicStampedReference.getStamp();
            System.out.println(Thread.currentThread().getName() + "\t第1次版本號：" + stamp);
            // 暫停1秒鐘t3線程
            try {
                TimeUnit.SECONDS.sleep(1);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            atomicStampedReference.compareAndSet(100, 101, atomicStampedReference.getStamp(), atomicStampedReference.getStamp() + 1);
            System.out.println(Thread.currentThread().getName() + "\t第2次版本號：" + atomicStampedReference.getStamp());
            atomicStampedReference.compareAndSet(101, 100, atomicStampedReference.getStamp(), atomicStampedReference.getStamp() + 1);
            System.out.println(Thread.currentThread().getName() + "\t第3次版本號：" + atomicStampedReference.getStamp());
        }, "t3").start();

        new Thread(() -> {
            int stamp = atomicStampedReference.getStamp();
            System.out.println(Thread.currentThread().getName() + "\t第1次版本號：" + stamp);
            // 暫停3秒鐘t4線程, 保證上面的t3線程完成了一次ABA操作
            try {
                TimeUnit.SECONDS.sleep(3);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            boolean result = atomicStampedReference.compareAndSet(100, 2020, stamp, stamp + 1);
            System.out.println(Thread.currentThread().getName() + "\t修改成功否：" + result+"\t當前最新實際版本號：" + atomicStampedReference.getStamp());
            System.out.println(Thread.currentThread().getName() + "\t當前實際最新值：" + atomicStampedReference.getReference());
        }, "t4").start();

    }
}