填坑：Externalizable、readUnshared

public interface Externalizable extends java.io.Serializable {    /**     * The object implements the writeExternal method to save its contents     * by calling the methods of DataOutput for its primitive values or     * calling the writeObject method of ObjectOutput for objects, strings,     * and arrays.     *     * @serialData Overriding methods should use this tag to describe     *             the data layout of this Externalizable object.     *             List the sequence of element types and, if possible,     *             relate the element to a public/protected field and/or     *             method of this Externalizable class.     *     * @param out the stream to write the object to     * @exception IOException Includes any I/O exceptions that may occur     */    void writeExternal(ObjectOutput out) throws IOException;

    /**     * The object implements the readExternal method to restore its     * contents by calling the methods of DataInput for primitive     * types and readObject for objects, strings and arrays.  The     * readExternal method must read the values in the same sequence     * and with the same types as were written by writeExternal.     *     * @param in the stream to read data from in order to restore the object     * @exception IOException if I/O errors occur     * @exception ClassNotFoundException If the class for an object being     *              restored cannot be found.     */    void readExternal(ObjectInput in) throws IOException, ClassNotFoundException;}