JUC concurrent programming (XX) - Atomic references AtomicReference and AtomicStampedReferenc

20, reference atom: AtomicReference

ABA Atomic AtomicInteger class to talk about? Atomic update references know?

CAS cause problems with the ABA!

CAS algorithm is the premise: Remove the data in memory of a moment, compare and exchange! It is possible within this time difference data has been changed!

The following example:

package com.interview.concurrent.cas;

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

/**
 * @author yangxj
 * @description 描述:CAS带来的ABA问题
 * @date 2020/2/25 17:10
 */
public class CasAbaQuestionDemo {

    public static void main(String[] args) throws InterruptedException {
        AtomicInteger atomicInteger = new AtomicInteger(5);

        /**
         *  @description:A线程已经将5修改为其他数
         *  @author yangxj
         *  @date 2020/2/25 17:23
         */
        new Thread(() -> {
            atomicInteger.compareAndSet(5,200);
        },"A").start();


        /**
         *  @description:C线程是小偷，偷偷的改动数据，然后又改回原来的5
         *  @author yangxj
         *  @date 2020/2/25 17:23
         */
        new Thread(() -> {
            atomicInteger.compareAndSet(200,210);
            atomicInteger.compareAndSet(210,5);
        },"C").start();

        /**
         *  @description:B线程希望将5修改为1024，
         *  按照原理，期望值已经被A修改为200，B线程应该要修改失败，
         *  但是出现了小偷C线程，它将期望值又改成了5，导致B线程能成功的修改，还发现不了C的存在
         *  @author yangxj
         *  @date 2020/2/25 17:21
         */
        new Thread(() -> {
            atomicInteger.compareAndSet(5,1024);
        },"B").start();

        //确保上面的三个线程都执行完
        TimeUnit.SECONDS.sleep(5);
        System.out.println(atomicInteger.get());  //1024
    }
}

Despite the success of CAS operation! But this does not mean that the process is not a problem!

Solve this problem, we can use optimistic locking (add a version number) to solve, we can be resolved by reference to atomic time-stamped! Then explain for atomic reference.

Atomic reference AtomicReference:

With a version number, time stamp of atomic references, similar optimistic locking!

The following example:

package com.interview.concurrent.cas;

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

/**
 * @author yangxj
 * @description 描述
 *  AtomicReference 原子引用
 *  AtomicStampedReference 加了时间戳  类似于乐观锁！ 通过版本号
 * @date 2020/2/25 17:43
 */
public class CasAbaAtomicStampedReference {

    public static void main(String[] args) throws InterruptedException {
        AtomicStampedReference<Integer> atomicStampedReference = new AtomicStampedReference<>(5, 1);

        /**
         *  @description:A线程已经将5修改为其他数
         *  @author yangxj
         *  @date 2020/2/25 17:23
         */
        new Thread(() -> {
            //获得版本号
            System.out.println("A stamp 01=>"+atomicStampedReference.getStamp());

            atomicStampedReference.compareAndSet(5,200,atomicStampedReference.getStamp(),atomicStampedReference.getStamp() + 1);
        },"A").start();


        /**
         *  @description:C线程是小偷，偷偷的改动数据，然后又改回原来的5
         *  @author yangxj
         *  @date 2020/2/25 17:23
         */
        new Thread(() -> {
            // 保证上面的线程先执行完毕！
            try {
                TimeUnit.SECONDS.sleep(3);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            //获得版本号
            System.out.println("C stamp 01=>"+atomicStampedReference.getStamp());

            atomicStampedReference.compareAndSet(200,210,atomicStampedReference.getStamp(),atomicStampedReference.getStamp() + 1);
            System.out.println("C stamp 02=>"+atomicStampedReference.getStamp());
            atomicStampedReference.compareAndSet(210,5,atomicStampedReference.getStamp(),atomicStampedReference.getStamp() + 1);
            System.out.println("C stamp 03=>"+atomicStampedReference.getStamp());
        },"C").start();

        /**
         *  @description:B线程希望将5修改为1024，
         *  期望值已经被A修改为200，由于添加了时间戳，B线程修改失败
         *  @author yangxj
         *  @date 2020/2/25 17:21
         */
        new Thread(() -> {

            // 保证上面的线程先执行完毕！
            try {
                TimeUnit.SECONDS.sleep(7);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            //获得版本号
            System.out.println("B stamp 01=>"+atomicStampedReference.getStamp());

            boolean isSuccess = atomicStampedReference.compareAndSet(5, 1024, atomicStampedReference.getStamp(), atomicStampedReference.getStamp() + 1);
            System.out.println("线程B执行成功了吗？" + isSuccess);
            System.out.println("B 最新的stamp："+atomicStampedReference.getStamp());
            System.out.println("B 当前的最新值："+atomicStampedReference.getReference());
        },"B").start();
    }
}

Thread B will fail to modify, run the effect is as follows:

ABA solve the problem: AtomicStampedReference