CopyOnWriteArrayList

[diaosichengxuyuan]

CopyOnWriteArrayList是线程安全的List，数据结构是数组，线程安全体现在每次修改会重新copy一份数据。

public class CopyOnWriteArrayList<E>
    implements List<E>, RandomAccess, Cloneable, java.io.Serializable {
    private static final long serialVersionUID = 8673264195747942595L;

    /** The lock protecting all mutators */
    final transient ReentrantLock lock = new ReentrantLock();

    /** The array, accessed only via getArray/setArray. */
    private transient volatile Object[] array;

    public CopyOnWriteArrayList() {
        setArray(new Object[0]);
    }

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

    //添加时，加锁，重新copy一份数组，用新数组覆盖原来的数组
    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();
        }
    }    

思考一下，为什么add时要重新copy一份数组，只加锁不行吗？如果只加锁不copy，那就相当于变成了ArrayList修改时加锁，读取时不加锁，假如有一个线程在for循环，另一个线程在修改，就会产生ConcurrentModificationException。