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   }