Java 7之多线程并发容器 - CopyOnWriteArrayList

    private volatile transient Object[] array;

    final Object[] getArray() {
        return array;
    }
    final void setArray(Object[] a) {
        array = a;
    }
    // -----开始构造函数----------------------------
    public CopyOnWriteArrayList() {
        setArray(new Object[0]);
    }
    public CopyOnWriteArrayList(Collection<? extends E> c) {
        Object[] elements = c.toArray();
        // c.toArray might (incorrectly) not return Object[] (see 6260652)
        if (elements.getClass() != Object[].class)
            elements = Arrays.copyOf(elements, elements.length, Object[].class);
        setArray(elements);
    }
    // Creates a list holding a copy of the given array.
    public CopyOnWriteArrayList(E[] toCopyIn) {
        setArray(Arrays.copyOf(toCopyIn, toCopyIn.length, Object[].class));
    }

 public boolean add(E e) {
        final ReentrantLock lock = this.lock;                       // 获取独占锁
        lock.lock();
        try {
            Object[] elements = getArray();
            int len = elements.length;
            Object[] newElements = Arrays.copyOf(elements, len + 1);// 重新生成一个新的数组实例，并将原始数组的元素拷贝到新数组中
            newElements[len] = e;                                   // 添加新的元素到新数组的末尾
            setArray(newElements);                                  // 更新底层数组
            return true;
        } finally {
            lock.unlock();
        }
    }

有两点必须清楚：

 第一，在”添加操作“开始前，获取独占锁(lock)，若此时有需要线程要获取锁，则必须等待；在操作完毕后，释放独占锁(lock)，此时其它线程才能获取锁。通过独占锁，来防止多线程同时修改数据！

 第二，操作完毕时，会通过setArray()来更新volatile数组。对一个volatile变量的读，总是能看到（任意线程）对这个volatile变量最后的写入；这样，每次添加元素之后，其它线程都能看到新添加的元素。

public E get(int index) {
    return get(getArray(), index);
}

private E get(Object[] a, int index) {
    return (E) a[index];
}

public E remove(int index) {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        Object[] elements = getArray();
        int len = elements.length;
        E oldValue = get(elements, index); // 获取volatile数组中指定索引处的元素值
        int numMoved = len - index - 1;
        if (numMoved == 0) // 如果被删除的是最后一个元素，则直接通过Arrays.copyOf()进行处理，而不需要新建数组
            setArray(Arrays.copyOf(elements, len - 1));
        else {
            Object[] newElements = new Object[len - 1];
            System.arraycopy(elements, 0, newElements, 0, index);    // 拷贝删除元素前半部分数据到新数组中
            System.arraycopy(elements, index + 1, newElements, index, numMoved);// 拷贝删除元素后半部分数据到新数组中
            setArray(newElements); // 更新volatile数组
        }
        return oldValue;
    } finally {
        lock.unlock();
    }
}

    public Iterator<E> iterator() {
        return new COWIterator<E>(getArray(), 0);
    }
    public ListIterator<E> listIterator() {
        return new COWIterator<E>(getArray(), 0);
    }
    public ListIterator<E> listIterator(final int index) {
        Object[] elements = getArray();
        int len = elements.length;
        if (index<0 || index>len)
            throw new IndexOutOfBoundsException("Index: "+index);

        return new COWIterator<E>(elements, index);
    }

    private static class COWIterator<E> implements ListIterator<E> {
        private final Object[] snapshot; // 保存数组的快照，是一个不可变的对象
        private int cursor;

        private COWIterator(Object[] elements, int initialCursor) {
            cursor = initialCursor;
            snapshot = elements;
        }

        public boolean hasNext() {
            return cursor < snapshot.length;
        }

        public boolean hasPrevious() {
            return cursor > 0;
        }

        @SuppressWarnings("unchecked")
        public E next() {
            if (! hasNext())
                throw new NoSuchElementException();
            return (E) snapshot[cursor++];
        }

        @SuppressWarnings("unchecked")
        public E previous() {
            if (! hasPrevious())
                throw new NoSuchElementException();
            return (E) snapshot[--cursor];
        }

        public int nextIndex() {
            return cursor;
        }

        public int previousIndex() {
            return cursor-1;
        }
        public void remove() {
            throw new UnsupportedOperationException();
        }
        public void set(E e) {
            throw new UnsupportedOperationException();
        }
        public void add(E e) {
            throw new UnsupportedOperationException();
        }
    }