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<V> 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<Integer> atomicStampedRef =
new AtomicStampedReference<Integer>(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。
