CopyOnWriteArrayList

ArrayList VS Vector VS CopyOnWriteArrayList

• ArrayList 线程不安全
• Vector 线程安全，但是用synchronized修饰的方法，性能不行
• CopyOnWriteArrayList 兼顾了线程安全和性能

继承关系

CopyOnWriteArrayList.png

解析：

• 属性

    final transient ReentrantLock lock = new ReentrantLock();
    // 元素
    private transient volatile Object[] array;

• 线程安全，操作元素都加了锁

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();
        }
    }
``