Java底层类和源码分析系列-AtomicStampedReference解决ABA问题

在原子类持续累加或累减时，比如AtomicInteger的incrementAndGet时，是不存在ABA问题的，但compareAndSet或者updateAndGet是可能存在ABA问题，像AtomicBoolean或AtomicLong等这样的变量在多线程修改时，也都存在ABA的问题。
为了理解ABA，下面的例子，针对线程1来说，第一次的A也就是1和第二次的A是另外修改过的1，实际上并不是同一个A（1）。

public static void main(String[] args) {
        AtomicInteger atomicInteger = new AtomicInteger(1);
        new Thread(()->{
            int value = atomicInteger.get();
            System.out.println("thread 1 read value: " + value);
            // 阻塞1s
            LockSupport.parkNanos(1000000000L);
            if (atomicInteger.compareAndSet(value, 3)) {
                System.out.println("thread 1 update from " + value + " to 3");
            } else {
                System.out.println("thread 1 update fail!");
            }
        }).start();
        new Thread(()->{
            int value = atomicInteger.get();
            System.out.println("thread 2 read value: " + value);
            if (atomicInteger.compareAndSet(value, 2)) {
                System.out.println("thread 2 update from " + value + " to 2");
                // do sth
                value = atomicInteger.get();
                System.out.println("thread 2 read value: " + value);
                if (atomicInteger.compareAndSet(value, 1)) {
                    System.out.println("thread 2 update from " + value + " to 1");
                }
            }
        }).start();
    }

为了解决这个问题，引入了AtomicStampedReference。

内部类

    private static class Pair<T> {
    /**
         * 目标对象引用
         */
        final T reference;
        /**
         * 整形标记
         */
        final int stamp;
        private Pair(T reference, int stamp) {
            this.reference = reference;
            this.stamp = stamp;
        }
        static <T> Pair<T> of(T reference, int stamp) {
            return new Pair<T>(reference, stamp);
        }
    }

属性

private volatile Pair<V> pair;
private static final sun.misc.Unsafe UNSAFE = sun.misc.Unsafe.getUnsafe();
private static final long pairOffset =
    objectFieldOffset(UNSAFE, "pair", AtomicStampedReference.class);

构造方法

       public AtomicStampedReference(V initialRef, int initialStamp) {
        pair = Pair.of(initialRef, initialStamp);
    }

compareAndSet()方法

public boolean compareAndSet(V   expectedReference,
                             V   newReference,
                             int expectedStamp,
                             int newStamp) {
    // 获取当前的（元素值，版本号）对
    Pair<V> current = pair;
    return
        // 引用没变
        expectedReference == current.reference &&
        // 版本号没变
        expectedStamp == current.stamp &&
        // 新引用等于旧引用
        ((newReference == current.reference &&
        // 新版本号等于旧版本号
          newStamp == current.stamp) ||
          // 构造新的Pair对象并CAS更新
         casPair(current, Pair.of(newReference, newStamp)));
}
private boolean casPair(Pair<V> cmp, Pair<V> val) {
    // 调用Unsafe的compareAndSwapObject()方法CAS更新pair的引用为新引用
    return UNSAFE.compareAndSwapObject(this, pairOffset, cmp, val);
}

解决上个ABA的问题的版本：

private static void testStamp() {
        AtomicStampedReference<Integer> atomicStampedReference = new AtomicStampedReference<>(1, 1);
        new Thread(()->{
            int[] stampHolder = new int[1];
            int value = atomicStampedReference.get(stampHolder);
            int stamp = stampHolder[0];
            System.out.println("thread 1 read value: " + value + ", stamp: " + stamp);
            // 阻塞1s
            LockSupport.parkNanos(1000000000L);
            if (atomicStampedReference.compareAndSet(value, 3, stamp, stamp + 1)) {
                System.out.println("thread 1 update from " + value + " to 3");
            } else {
                System.out.println("thread 1 update fail!");
            }
        }).start();
        new Thread(()->{
            int[] stampHolder = new int[1];
            int value = atomicStampedReference.get(stampHolder);
            int stamp = stampHolder[0];
            System.out.println("thread 2 read value: " + value + ", stamp: " + stamp);
            if (atomicStampedReference.compareAndSet(value, 2, stamp, stamp + 1)) {
                System.out.println("thread 2 update from " + value + " to 2");
                // do sth
                value = atomicStampedReference.get(stampHolder);
                stamp = stampHolder[0];
                System.out.println("thread 2 read value: " + value + ", stamp: " + stamp);
                if (atomicStampedReference.compareAndSet(value, 1, stamp, stamp + 1)) {
                    System.out.println("thread 2 update from " + value + " to 1");
                }
            }
        }).start();
    }