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 }