public E set(int index, E element) {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        Object[] elements = getArray();
        E oldValue = get(elements, index);

        if (oldValue != element) {
            int len = elements.length;
            Object[] newElements = Arrays.copyOf(elements, len);
            newElements[index] = element;
            setArray(newElements);
        } else {
            // Not quite a no-op; ensures volatile write semantics
            setArray(elements);
        }
        return oldValue;
    } finally {
        lock.unlock();
    }
}

http://cs.oswego.edu/pipermail/concurrency-interest/2010-February/006886.html
https://stackoverflow.com/questions/28772539/why-setarray-method-call-required-in-copyonwritearraylist

The reason this code calls setArray even when it's apparently unnecessary is so that it establishes an invariant for this method that it always performs a volatile write to this array. This establishes happens-before semantics with other threads that perform volatile reads from this array. This is important because the volatile write-read semantics apply to reads and writes other than those of the volatile field itself. Specifically, writes to other (non-volatile) fields before a volatile write happen-before reads from those other fields after a volatile read of the same volatile variable.

第17-18行的意义：
这个hb意义何在？如下例子：a为非volatile的某基本类型变量,coal为CopyOnWriteArrayList对象

t1: x:a = calValue; y:coal.set…. ——————— t2: m:coal.get… n:int tmp = a;

假设存在以上场景，如果能保证只会存在这样的轨迹：x,y,m,n.根据上述java API文档中的约定有hb(y,m),根据线程内的操作相关规定有hb(x,y),hb(m,n)，根据hb的传递性读写a变量就有hb(x,n)，所以t1对a的写操作对t2中a的读操作可见。如果CopyOnWriteArrayList的set的else里没有setArray(elements)的话，hb(y,m)就不再有了，上述的可见性也就无法保证。

我们讨论运行线程t1的状态。中断t1的线程是t2

1、如果t1在运行阻塞方法（sleep/wait/poll），那么会产生InterruptedException。并且isInterrupted状态已经清空。 2、如果t1仅仅在跑计算，那么会设置isInterrupted会设置为true

1、可以简单的往上抛。 2、如果不能继续往上层抛异常的话，那么应该手动恢复状态Thread.currentThread().interrupt（）。其他其他线程t2、t3能通过t1.isInterrpted来判断，t1是成功跑完任务了呢，还是被啥坏家伙异常中断了。