jdk并发包 CopyOnWriteArrayList源码分析

CopyOnWriteArrayList是jdk1.5并法包里面用于处理高并发下，读多写少的情况下，降低锁等待的集合类。下面对该类实现做一个简要的分析

1，首先CopyOnWriteArrayList是实现了List接口，对=List接口的相关方法进行了实现。

2，下面的它的add方法，会首先加锁，然后copy原List内部的数组，然后对新数组长度加1后释放锁。由于数组copy速度很快，切在读多写少的情况下锁开销比较少

    public boolean add(E e) {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            Object[] elements = getArray();
            int len = elements.length;
            Object[] newElements = Arrays.copyOf(elements, len + 1);
            newElements[len] = e;
            setArray(newElements);
            return true;
        } finally {
            lock.unlock();
        }
    }

2，它的迭代器COWIterator不是快速失败的，下面是它的源码

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++];
        }

3，下面是它的remove方法，加锁原理同add方法

    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);
            int numMoved = len - index - 1;
            if (numMoved == 0)
                setArray(Arrays.copyOf(elements, len - 1));
            else {
                Object[] newElements = new Object[len - 1];
                System.arraycopy(elements, 0, newElements, 0, index);
                System.arraycopy(elements, index + 1, newElements, index,
                                 numMoved);
                setArray(newElements);
            }
            return oldValue;
        } finally {
            lock.unlock();
        }
    }