[ Java ]ArrayList 里面 elementData 为什么要设成 transient

看了下 ArrayList 的源码

    /**
    * The array buffer into which the elements of the ArrayList are stored.
    * The capacity of the ArrayList is the length of this array buffer. Any
    * empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
    * will be expanded to DEFAULT_CAPACITY when the first element is added.
    */
    transient Object[] elementData; // non-private to simplify nested class access

看到 elementData 是 transient 的，于是又去看了下序列化的实现。

    /**
     * Save the state of the <tt>ArrayList</tt> instance to a stream (that
     * is, serialize it).
     *
     * @serialData The length of the array backing the <tt>ArrayList</tt>
     *             instance is emitted (int), followed by all of its elements
     *             (each an <tt>Object</tt>) in the proper order.
     */
    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException{
        // Write out element count, and any hidden stuff
        int expectedModCount = modCount;
        s.defaultWriteObject();

        // Write out size as capacity for behavioural compatibility with clone()
        s.writeInt(size);

        // Write out all elements in the proper order.
        for (int i=0; i<size; i++) {
            s.writeObject(elementData[i]);
        }

        if (modCount != expectedModCount) {
            throw new ConcurrentModificationException();
        }
    }

我觉得这里没有必要把 elementData 设成 transient，然后再一个一个遍历输出。直接用 defaultWriteObject 把 elementData 输出就可以了。

我自己写了个 Dummy 的 ArrayList 测试，是可以直接序列化+反序列化数组的。

求大神指点。