https://openjdk.java.net/ 
/*
 * Copyright (c) 1996, 2004, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

package java.io;

import java.io.ObjectOutput;
import java.io.ObjectInput;


Only the identity of the class of an Externalizable instance is
written in the serialization stream and it is the responsibility
of the class to save and restore the contents of its instances.
The writeExternal and readExternal methods of the Externalizable
interface are implemented by a class to give the class complete
control over the format and contents of the stream for an object
and its supertypes. These methods must explicitly
coordinate with the supertype to save its state. These methods supersede
customized implementations of writeObject and readObject methods.

Object Serialization uses the Serializable and Externalizable
interfaces.  Object persistence mechanisms can use them as well.  Each
object to be stored is tested for the Externalizable interface. If
the object supports Externalizable, the writeExternal method is called. If the
object does not support Externalizable and does implement
Serializable, the object is saved using
ObjectOutputStream. 
 When an Externalizable object is
reconstructed, an instance is created using the public no-arg
constructor, then the readExternal method called.  Serializable
objects are restored by reading them from an ObjectInputStream.

An Externalizable instance can designate a substitution object via
the writeReplace and readResolve methods documented in the Serializable
interface.


Author: unascribed
See Also:
Since:  JDK1.1
/**
 * Only the identity of the class of an Externalizable instance is
 * written in the serialization stream and it is the responsibility
 * of the class to save and restore the contents of its instances.
 *
 * The writeExternal and readExternal methods of the Externalizable
 * interface are implemented by a class to give the class complete
 * control over the format and contents of the stream for an object
 * and its supertypes. These methods must explicitly
 * coordinate with the supertype to save its state. These methods supersede
 * customized implementations of writeObject and readObject methods.<br>
 *
 * Object Serialization uses the Serializable and Externalizable
 * interfaces.  Object persistence mechanisms can use them as well.  Each
 * object to be stored is tested for the Externalizable interface. If
 * the object supports Externalizable, the writeExternal method is called. If the
 * object does not support Externalizable and does implement
 * Serializable, the object is saved using
 * ObjectOutputStream. <br> When an Externalizable object is
 * reconstructed, an instance is created using the public no-arg
 * constructor, then the readExternal method called.  Serializable
 * objects are restored by reading them from an ObjectInputStream.<br>
 *
 * An Externalizable instance can designate a substitution object via
 * the writeReplace and readResolve methods documented in the Serializable
 * interface.<br>
 *
 * @author  unascribed
 * @see java.io.ObjectOutputStream
 * @see java.io.ObjectInputStream
 * @see java.io.ObjectOutput
 * @see java.io.ObjectInput
 * @see java.io.Serializable
 * @since   JDK1.1
 */
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.

Params:
  • out – the stream to write the object to
Throws:
  • IOException – Includes any I/O exceptions that may occur
@serialDataOverriding 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.
/**
     * 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.

Params:
  • in – the stream to read data from in order to restore the object
Throws:
/**
     * 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;
}