1 /*
  2  * Copyright (c) 1996, 2020, 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 object can designate a substitution object via
 55  * the writeReplace and readResolve methods documented in the Serializable
 56  * interface. To control the serialized form of Records and Externalizable value classes
 57  * the record or class uses {@link Serializable} {@code writeReplace}
 58  * to delegate to another serializable or externalizable class and that delegate
 59  * uses {@code readResolve} to supply the replacement on deserialization.
 60  * Value classes implementing {@link Externalizable}
 61  * and not using {@code writeReplace} are not supported.
 62  * <br>
 63  *
 64  * @see java.io.ObjectOutputStream
 65  * @see java.io.ObjectInputStream
 66  * @see java.io.ObjectOutput
 67  * @see java.io.ObjectInput
 68  * @see java.io.Serializable
 69  * @since   1.1
 70  */
 71 public interface Externalizable extends java.io.Serializable {
 72     /**
 73      * The object implements the writeExternal method to save its contents
 74      * by calling the methods of DataOutput for its primitive values or
 75      * calling the writeObject method of ObjectOutput for objects, strings,
 76      * and arrays.
 77      *
 78      * @serialData Overriding methods should use this tag to describe
 79      *             the data layout of this Externalizable object.
 80      *             List the sequence of element types and, if possible,
 81      *             relate the element to a public/protected field and/or
 82      *             method of this Externalizable class.
 83      *
 84      * @param     out the stream to write the object to
 85      * @throws    IOException Includes any I/O exceptions that may occur
 86      */
 87     void writeExternal(ObjectOutput out) throws IOException;
 88 
 89     /**
 90      * The object implements the readExternal method to restore its
 91      * contents by calling the methods of DataInput for primitive
 92      * types and readObject for objects, strings and arrays.  The
 93      * readExternal method must read the values in the same sequence
 94      * and with the same types as were written by writeExternal.
 95      *
 96      * @param     in the stream to read data from in order to restore the object
 97      * @throws    IOException if I/O errors occur
 98      * @throws    ClassNotFoundException If the class for an object being
 99      *            restored cannot be found.
100      */
101     void readExternal(ObjectInput in) throws IOException, ClassNotFoundException;
102 }
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