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：
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/** The array, accessed only via getArray/setArray. */ |
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private volatile transient Object[] array; |
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/** |
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* Replaces the element at the specified position in this list with the |
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* specified element. |
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* |
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* @throws IndexOutOfBoundsException {@inheritDoc} |
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*/ |
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public E set(int index, E element) { |
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final ReentrantLock lock = this.lock; |
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lock.lock(); |
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try { |
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Object[] elements = getArray(); |
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E oldValue = get(elements, index); |
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if (oldValue != element) { |
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int len = elements.length; |
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Object[] newElements = Arrays.copyOf(elements, len); |
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newElements[index] = element; |
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setArray(newElements); |
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} else { |
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// Not quite a no-op; ensures volatile write semantics |
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setArray(elements); |
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} |
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return oldValue; |
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} finally { |
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lock.unlock(); |
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} |
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} |
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/** |
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* Sets the array. |
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*/ |
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final void setArray(Object[] a) { |
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array = a; |
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} |
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/** |
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* Gets the array. Non-private so as to also be accessible |
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* from CopyOnWriteArraySet class. |
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*/ |
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final Object[] getArray() { |
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return array; |
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} |
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
