CAS 的ABA 问题

CAS

CAS:Compare and Swap, 翻译成比较并交换。
java.util.concurrent包中借助CAS实现了区别于synchronized同步锁的一种乐观锁。
其原理是CAS有3个操作数，内存值V，旧的预期值A，要修改的新值B。当且仅当预期值A和内存值V相同时，将内存值V修改为B，否则什么都不做。

public final boolean compareAndSet(int expect, int update) { return unsafe.compareAndSwapInt(this, valueOffset, expect, update); } 

ABA问题

    private static AtomicInteger atomicInt = new AtomicInteger(100); public static void main(String[] args) throws InterruptedException { Thread intT1 = new Thread(new Runnable() { @Override public void run() { atomicInt.compareAndSet(100, 101); log("thread intT1:" + atomicInt.get()); atomicInt.compareAndSet(101, 100); log("thread intT1:" + atomicInt.get()); } }); Thread intT2 = new Thread(new Runnable() { @Override public void run() { try { TimeUnit.SECONDS.sleep(1); } catch (InterruptedException e) { e.printStackTrace(); } boolean c3 = atomicInt.compareAndSet(100, 101); log("thread intT2:" + atomicInt.get() + ",c3 is:" + c3); //true } }); intT1.start(); intT2.start(); 

上面程序的打印结果如下：

thread intT1:101
thread intT1:100
thread intT2:101,c3 is:true 

线程intT2获取到的变量值A，尽管和当前的实际值相同，但内存地址V中的变量已经经历了A->B->A的改变。实际应用中有可能会导致漫画：什么是CAS机制？（进阶篇）中所提到的提款问题。

解决方案

JDK的atomic包里提供了一个类AtomicStampedReference来解决ABA问题。如果当前引用 == 预期引用，并且当前标志等于预期标志，则以原子方式将该引用和该标志的值设置为给定的更新值。源码如下：

/**
 *expectedReference - 该引用的预期值
 *newReference - 该引用的新值
 *expectedStamp - 该标志的预期值
 *newStamp - 该标志的新值
 */
public boolean compareAndSet(V   expectedReference, V newReference, int expectedStamp, int newStamp) { Pair> current = pair; return expectedReference == current.reference && expectedStamp == current.stamp && ((newReference == current.reference && newStamp == current.stamp) || casPair(current, Pair.of(newReference, newStamp))); } 

最佳实践

    private static AtomicStampedReference> atomicStampedRef = new AtomicStampedReference>(100, 0); Thread refT1 = new Thread(new Runnable() { @Override public void run() { try { TimeUnit.SECONDS.sleep(1); } catch (InterruptedException e) { e.printStackTrace(); } atomicStampedRef.compareAndSet(100, 101, atomicStampedRef.getStamp(), atomicStampedRef.getStamp() + 1); log("thread refT1:" + atomicStampedRef.getReference()); atomicStampedRef.compareAndSet(101, 100, atomicStampedRef.getStamp(), atomicStampedRef.getStamp() + 1); log("thread refT1:" + atomicStampedRef.getReference()); } }); Thread refT2 = new Thread(new Runnable() { @Override public void run() { int stamp = atomicStampedRef.getStamp(); log("before sleep : stamp = " + stamp); // stamp = 0 try { TimeUnit.SECONDS.sleep(2); } catch (InterruptedException e) { e.printStackTrace(); } log("after sleep : stamp = " + atomicStampedRef.getStamp());//stamp = 1 boolean c3 = atomicStampedRef.compareAndSet(100, 101, stamp, stamp + 1); log("thread refT2:" + atomicStampedRef.getReference() + ",c3 is " + c3); //true } }); refT1.start(); refT2.start(); } private static void log(String logString) { System.out.println(logString); } 

输出结果如下：

before sleep : stamp = 0
thread refT1:101 thread refT1:100 after sleep : stamp = 2 thread refT2:100,c3 is false 

可以看到refT2的值和expect是相同的，但是由于版本号发生了变化，所以更新失败。

作者：时光之刃51y
链接：https://www.jianshu.com/p/8de8c6a839e8
来源：简书
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