View Javadoc
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  }