标签：cas 经历 reads tla wait ali 情况下 ted exti
CAS(Compare And Swap)导致的ABA问题
问题描述
多线程情况下，每个线程使用CAS操作欲将数据A修改成B，当然我们只希望只有一个线程能够正确的修改数据，并且只修改一次。当并发的时候，其中一个线程已经将A成功的改成了B，但是在线程并发调度过程中尚未被调度，在这个期间，另外一个线程(不在并发中的请求线程)将B又修改成了A，那么原来并发中的线程又可以通过CAS操作将A改成B
测试用例:
public class AbaPro {
private static final Random RANDOM = new Random();
private static final String B = "B";
private static final String A = "A";
public static final AtomicReference ATOMIC_REFERENCE = new AtomicReference(A);
public static void main(String[] args) throws InterruptedException {
final CountDownLatch startLatch = new CountDownLatch(1);
Thread[] threads = new Thread[20];
for (int i=0; i ){
threads[i] = new Thread(){
@Override
public void run() {
String oldValue = ATOMIC_REFERENCE.get();
try {
startLatch.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
try {
Thread.sleep(RANDOM.nextInt()&500);
} catch (InterruptedException e) {
e.printStackTrace();
}
if (ATOMIC_REFERENCE.compareAndSet(oldValue, B )){
System.out.println(Thread.currentThread().getName()+ " 已经对原始值进行了修改,此时值为: "+ ATOMIC_REFERENCE.get());
}
}
};
threads[i].start();
}
startLatch.countDown();
Thread.sleep(200);
new Thread(){
@Override
public void run() {
try {
Thread.sleep(RANDOM.nextInt() & 200);
} catch (InterruptedException e) {
e.printStackTrace();
}
String oldVal = ATOMIC_REFERENCE.get();
while (!ATOMIC_REFERENCE.compareAndSet(ATOMIC_REFERENCE.get(), A));
System.out.println(Thread.currentThread().getName() +" 已经将值 "+oldVal+" 修改成原始值: A");
}
}.start();
}
}
结果:
Thread-12 已经对原始值进行了修改,此时值为: B
Thread-20 已经将值 B 修改成原始值: A
Thread-14 已经对原始值进行了修改,此时值为: B
可以看到并发中的线程Thread-12已经成功的将A修改成B，其他线程Thread-20在某一时刻将B修改成A，而并发中的线程Thread-14又能再次成功的将A修改成B，虽然最终结果是B,但是中途经历了一次被修改的过程，在某些情况下是致使的
解决方案
java中提供了AtomicStampedReference来解决这个问题，它是基于版本或者是一种状态，在修改的过程中不仅对比值，也同时会对比版本号
public class AabProResolve {
private static final Random RANDOM = new Random();
private static final String B = "B";
private static final String A = "A";
private static final AtomicStampedReference ATOMIC_STAMPED_REFERENCE = new AtomicStampedReference(A,0);
public static void main(String[] args) throws InterruptedException {
final CountDownLatch startLatch = new CountDownLatch(1);
Thread[] threads = new Thread[20];
for (int i=0; i ){
threads[i] = new Thread(){
@Override
public void run() {
String oldValue = ATOMIC_STAMPED_REFERENCE.getReference();
int stamp = ATOMIC_STAMPED_REFERENCE.getStamp();
try {
startLatch.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
try {
Thread.sleep(RANDOM.nextInt() & 500);
} catch (InterruptedException e) {
e.printStackTrace();
}
if (ATOMIC_STAMPED_REFERENCE.compareAndSet(oldValue, B, stamp, stamp+1)){
System.out.println(Thread.currentThread().getName()+ " 已经对原始值: "+oldValue+" 进行了修改,此时值为: "+ ATOMIC_STAMPED_REFERENCE.getReference());
}
}
};
threads[i].start();
}
Thread.sleep(200);
startLatch.countDown();
new Thread(){
@Override
public void run() {
try {
Thread.sleep(RANDOM.nextInt() & 200);
} catch (InterruptedException e) {
e.printStackTrace();
}
int stamp = ATOMIC_STAMPED_REFERENCE.getStamp();
String oldVal = ATOMIC_STAMPED_REFERENCE.getReference();
while (!ATOMIC_STAMPED_REFERENCE.compareAndSet(
B,
A,stamp, stamp+1)){
stamp = ATOMIC_STAMPED_REFERENCE.getStamp();
}
System.out.println(Thread.currentThread().getName() +" 已经将值 "+oldVal+" 修改成原始值: A");
}
}.start();
}
}
结果:
Thread-1 已经对原始值: A 进行了修改,此时值为: B
Thread-20 已经将值 B 修改成原始值: A
可以看到并发期间的线程只有Thread-1对A进行了修改，保证了只有一个线程对数据的修改，短暂的并发时间之后的其他线程Thread-20对其修改自然也就没有影响、
优化方案,可以参考：
https://blog.csdn.net/wufaliang003/article/details/78797203
Java并发编程原理与实战四十三：CAS ---- ABA问题
标签：cas 经历 reads tla wait ali 情况下 ted exti
原文地址：https://www.cnblogs.com/pony1223/p/9567470.html