CopyOnWriteArrayList类set方法疑惑？

2021-09-04 09:43:35

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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01 /** The array, accessed only via getArray/setArray. */
02 private 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  */
10 public 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  */
35 final 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  */
43 final 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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