CAS的ABA问题描述

• 在CAS操作的时候，其他线程将当前变量的值从A改成B，又改回A；
• CAS线程用期望值A与当前变量比较的时候，发现当前变量没有变，于是CAS就将当前变量进行了交换操作，但其实当前变量改变过，这与设计思想是不符合的；
• ABA问题的解决思路：每次当前变量更新的时候，将当前变量的版本号加1；

AtomicStampReference示例

• public boolean compareAndSet(
  V expectedReference,
  V newReference,
  int expectedStamp,
  int newStamp)
  • 如果当前值和expectedReference相等，并且当前stamp和expectedStamp相等，把当前值更新为newReference，当前stamp更新为newStamp，并返回true；
  • 如果不满足条件，不更新，并返回false；

import java.util.concurrent.TimeUnit;import java.util.concurrent.atomic.AtomicInteger;import java.util.concurrent.atomic.AtomicStampedReference;public class ABA {private static AtomicInteger atomicInt= new AtomicInteger(100);private static AtomicStampedReference atomicStampedRef= new AtomicStampedReference(100, 0);public static void main(String[] args) throws InterruptedException {Thread intABA = new Thread(new Runnable() {@Overridepublic void run() {atomicInt.compareAndSet(100, 101);atomicInt.compareAndSet(101, 100);}});Thread intCAS = new Thread(new Runnable() {@Overridepublic void run() {try {TimeUnit.SECONDS.sleep(1);} catch (InterruptedException e) {}boolean isUpdated = atomicInt.compareAndSet(100, 101);System.out.println("Thread AtomicInteger CAS isUpdated: " + isUpdated); // true}});intABA.start();intCAS.start();intABA.join();intCAS.join();Thread refABA = new Thread(() -> {try {TimeUnit.SECONDS.sleep(1);} catch (InterruptedException e) {}atomicStampedRef.compareAndSet(100, 101, atomicStampedRef.getStamp(), atomicStampedRef.getStamp() + 1);atomicStampedRef.compareAndSet(101, 100, atomicStampedRef.getStamp(), atomicStampedRef.getStamp() + 1);});Thread refCAS = new Thread(() -> {int stamp = atomicStampedRef.getStamp(); // 0try {TimeUnit.SECONDS.sleep(2);} catch (InterruptedException e) {}boolean isUpdated = atomicStampedRef.compareAndSet(100, 101, stamp, stamp + 1);System.out.println("Thread AtomicStampedReference CAS isUpdated: " + isUpdated); // false});refABA.start();refCAS.start();}}

输出：

Thread AtomicInteger CAS isUpdated: true
Thread AtomicStampedReference CAS isUpdated: false