CopyOnWriteArrayList和CopyOnWriteArraySet

CopyOnWriteArrayList和CopyOnWriteArraySet应用场合一般 是在读多写少的情况，比如黑名单，他们之间的区别就是list和set的区别，但是在实现上CopyOnWriteArraySet使用CopyOnWriteArrayList来实现的，就像set使用了hashmap，treeset使用treemap，下面先看CopyOnWriteArrayList的添加操作.

    /**     * Inserts the specified element at the specified position in this     * list. Shifts the element currently at that position (if any) and     * any subsequent elements to the right (adds one to their indices).     *     * @throws IndexOutOfBoundsException {@inheritDoc}     */    public void add(int index, E element) {        final ReentrantLock lock = this.lock;        //加锁，CopyOnWriteArrayList是线程安全的        lock.lock();        try {            Object[] elements = getArray();            int len = elements.length;            if (index > len || index < 0)                throw new IndexOutOfBoundsException("Index: "+index+                                                    ", Size: "+len);            Object[] newElements;            int numMoved = len - index;            //如果位移实在列表的尾部，那么直接复制数组，，使用Arrays.copyOf()            if (numMoved == 0)                newElements = Arrays.copyOf(elements, len + 1);            else {            //否则，以index为中心点，把原始数组分成2段，index左边一段，右边一段，index放要插入的数据元素，放进新的数组newElements。                newElements = new Object[len + 1];                System.arraycopy(elements, 0, newElements, 0, index);                System.arraycopy(elements, index, newElements, index + 1,                                 numMoved);            }            newElements[index] = element;            //覆盖原来的数组，原来的数组失去引用，被jvm回收            setArray(newElements);        } finally {         //解锁            lock.unlock();        }    }

再看remove方法：

    /**     * Removes the first occurrence of the specified element from this list,     * if it is present.  If this list does not contain the element, it is     * unchanged.  More formally, removes the element with the lowest index     * <tt>i</tt> such that     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>     * (if such an element exists).  Returns <tt>true</tt> if this list     * contained the specified element (or equivalently, if this list     * changed as a result of the call).     *     * @param o element to be removed from this list, if present     * @return <tt>true</tt> if this list contained the specified element     */    public boolean remove(Object o) {        final ReentrantLock lock = this.lock;        //加锁，        lock.lock();        try {            Object[] elements = getArray();            int len = elements.length;            if (len != 0) {                // Copy while searching for element to remove                // This wins in the normal case of element being present                int newlen = len - 1;                Object[] newElements = new Object[newlen];                for (int i = 0; i < newlen; ++i) {                    if (eq(o, elements[i])) {                        // found one;  copy remaining and exit                        //找到要删除的元素，然后将元素后面的数据复制到新的数组里边                        for (int k = i + 1; k < len; ++k)                            newElements[k-1] = elements[k];                        setArray(newElements);                        return true;                    } else                        //不是删除数据，直接进入新的数组                        newElements[i] = elements[i];                }                // special handling for last cell                //最后一个元素的单独处理                if (eq(o, elements[newlen])) {                    setArray(newElements);                    return true;                }            }            return false;        } finally {        //解锁            lock.unlock();        }    }

再看CopyOnWriteArraySet
内部维护了一个

    private final CopyOnWriteArrayList<E> al;

因为CopyOnWriteArraySet元素不能重复，所以CopyOnWriteArraySet的add用的是CopyOnWriteArrayList的addIfAbsent，

    public boolean add(E e) {        return al.addIfAbsent(e);    }

删除操作用的是CopyOnWriteArrayList的remove

    public boolean remove(Object o) {        return al.remove(o);    }