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<String> 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 < 20; 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<String> 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 < 20; 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
原文地址:https://www.cnblogs.com/pony1223/p/9567470.html
时间: 2024-10-24 02:06:51