CopyOnWriteArrayList类set方法疑惑？

在淘宝内网有位同事提了一个很好的问题，大家能否帮忙解答下？

在CopyOnWriteArrayList类的set方法中有一段setArray(elements)代码，实际上这段代码并未对elements做任何改动，实现的volatile语意并不对CopyOnWriteArrayList实例产生任何影响，为什么还是要保留这行语句？见以下代码红体部分：

01/** The array, accessed only via getArray/setArray. */

02private volatile transient Object[] array;

03 

04/**

05 * Replaces the element at the specified position in this list with the

06 * specified element.

07 *

08 * @throws IndexOutOfBoundsException {@inheritDoc}

09 */

10public E set(int index, E element) {

11    final ReentrantLock lock = this.lock;

12    lock.lock();

13    try {

14        Object[] elements = getArray();

15        E oldValue = get(elements, index);

16 

17        if (oldValue != element) {

18            int len = elements.length;

19            Object[] newElements = Arrays.copyOf(elements, len);

20            newElements[index] = element;

21            setArray(newElements);

22        } else {

23            // Not quite a no-op; ensures volatile write semantics

24            setArray(elements);

25        }

26        return oldValue;

27    } finally {

28        lock.unlock();

29    }

30}

31 

32/**

33 * Sets the array.

34 */

35final void setArray(Object[] a) {

36    array = a;

37}

38 

39/**

40 * Gets the array.  Non-private so as to also be accessible

41 * from CopyOnWriteArraySet class.

42 */

43final Object[] getArray() {

44    return array;

45}

这个问题在concurrency-interest邮件列表里也有人讨论：

http://cs.oswego.edu/pipermail/concurrency-interest/2010-February/006886.html

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