1 /*
   2  * Copyright (c) 1996, 2004, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.  Oracle designates this
   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Oracle in the LICENSE file that accompanied this code.
  10  *
  11  * This code is distributed in the hope that it will be useful, but WITHOUT
  12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14  * version 2 for more details (a copy is included in the LICENSE file that
  15  * accompanied this code).
  16  *
  17  * You should have received a copy of the GNU General Public License version
  18  * 2 along with this work; if not, write to the Free Software Foundation,
  19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  20  *
  21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  22  * or visit www.oracle.com if you need additional information or have any
  23  * questions.
  24  */
  25 
  26 package java.io;
  27 
  28 import java.io.ObjectOutput;
  29 import java.io.ObjectInput;
  30 
  31 /**
  32  * Only the identity of the class of an Externalizable instance is
  33  * written in the serialization stream and it is the responsibility
  34  * of the class to save and restore the contents of its instances.
  35  *
  36  * The writeExternal and readExternal methods of the Externalizable
  37  * interface are implemented by a class to give the class complete
  38  * control over the format and contents of the stream for an object
  39  * and its supertypes. These methods must explicitly
  40  * coordinate with the supertype to save its state. These methods supersede
  41  * customized implementations of writeObject and readObject methods.<br>
  42  *
  43  * Object Serialization uses the Serializable and Externalizable
  44  * interfaces.  Object persistence mechanisms can use them as well.  Each
  45  * object to be stored is tested for the Externalizable interface. If
  46  * the object supports Externalizable, the writeExternal method is called. If the
  47  * object does not support Externalizable and does implement
  48  * Serializable, the object is saved using
  49  * ObjectOutputStream. <br> When an Externalizable object is
  50  * reconstructed, an instance is created using the public no-arg
  51  * constructor, then the readExternal method called.  Serializable
  52  * objects are restored by reading them from an ObjectInputStream.<br>
  53  *
  54  * An Externalizable instance can designate a substitution object via
  55  * the writeReplace and readResolve methods documented in the Serializable
  56  * interface.<br>
  57  *
  58  * @author  unascribed
  59  * @see java.io.ObjectOutputStream
  60  * @see java.io.ObjectInputStream
  61  * @see java.io.ObjectOutput
  62  * @see java.io.ObjectInput
  63  * @see java.io.Serializable
  64  * @since   JDK1.1
  65  */
  66 public interface Externalizable extends java.io.Serializable {
  67     /**
  68      * The object implements the writeExternal method to save its contents
  69      * by calling the methods of DataOutput for its primitive values or
  70      * calling the writeObject method of ObjectOutput for objects, strings,
  71      * and arrays.
  72      *
  73      * @serialData Overriding methods should use this tag to describe
  74      *             the data layout of this Externalizable object.
  75      *             List the sequence of element types and, if possible,
  76      *             relate the element to a public/protected field and/or
  77      *             method of this Externalizable class.
  78      *
  79      * @param out the stream to write the object to
  80      * @exception IOException Includes any I/O exceptions that may occur
  81      */
  82     void writeExternal(ObjectOutput out) throws IOException;
  83 
  84     /**
  85      * The object implements the readExternal method to restore its
  86      * contents by calling the methods of DataInput for primitive
  87      * types and readObject for objects, strings and arrays.  The
  88      * readExternal method must read the values in the same sequence
  89      * and with the same types as were written by writeExternal.
  90      *
  91      * @param in the stream to read data from in order to restore the object
  92      * @exception IOException if I/O errors occur
  93      * @exception ClassNotFoundException If the class for an object being
  94      *              restored cannot be found.
  95      */
  96     void readExternal(ObjectInput in) throws IOException, ClassNotFoundException;
  97 }