Externalizable


1   /*
2    * @(#)Externalizable.java  1.19 04/01/12
3    *
4    * Copyright 2004 Sun Microsystems, Inc. All rights reserved.
5    * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
6    */
7   
8   package java.io;
9   
10  import java.io.ObjectOutput  ;
11  import java.io.ObjectInput  ;
12  
13  /**
14   * Only the identity of the class of an Externalizable instance is
15   * written in the serialization stream and it is the responsibility
16   * of the class to save and restore the contents of its instances.
17   *
18   * The writeExternal and readExternal methods of the Externalizable
19   * interface are implemented by a class to give the class complete
20   * control over the format and contents of the stream for an object
21   * and its supertypes. These methods must explicitly
22   * coordinate with the supertype to save its state. These methods supersede
23   * customized implementations of writeObject and readObject methods.<br>
24   *
25   * Object Serialization uses the Serializable and Externalizable
26   * interfaces.  Object persistence mechanisms can use them as well.  Each
27   * object to be stored is tested for the Externalizable interface. If
28   * the object supports Externalizable, the writeExternal method is called. If the
29   * object does not support Externalizable and does implement
30   * Serializable, the object is saved using
31   * ObjectOutputStream. <br> When an Externalizable object is
32   * reconstructed, an instance is created using the public no-arg
33   * constructor, then the readExternal method called.  Serializable
34   * objects are restored by reading them from an ObjectInputStream.<br>
35   *
36   * An Externalizable instance can designate a substitution object via
37   * the writeReplace and readResolve methods documented in the Serializable
38   * interface.<br>
39   *
40   * @author  unascribed
41   * @version 1.19, 01/12/04
42   * @see java.io.ObjectOutputStream
43   * @see java.io.ObjectInputStream
44   * @see java.io.ObjectOutput
45   * @see java.io.ObjectInput
46   * @see java.io.Serializable
47   * @since   JDK1.1
48   */
49  public interface Externalizable extends java.io.Serializable   {
50      /**
51       * The object implements the writeExternal method to save its contents
52       * by calling the methods of DataOutput for its primitive values or
53       * calling the writeObject method of ObjectOutput for objects, strings,
54       * and arrays.
55       *
56       * @serialData Overriding methods should use this tag to describe
57       *             the data layout of this Externalizable object.
58       *             List the sequence of element types and, if possible,
59       *             relate the element to a public/protected field and/or
60       *             method of this Externalizable class.
61       *
62       * @param out the stream to write the object to
63       * @exception IOException Includes any I/O exceptions that may occur
64       */
65      void writeExternal(ObjectOutput   out) throws IOException  ;
66  
67      /**
68       * The object implements the readExternal method to restore its
69       * contents by calling the methods of DataInput for primitive
70       * types and readObject for objects, strings and arrays.  The
71       * readExternal method must read the values in the same sequence
72       * and with the same types as were written by writeExternal.
73       *
74       * @param in the stream to read data from in order to restore the object
75       * @exception IOException if I/O errors occur
76       * @exception ClassNotFoundException If the class for an object being
77       *              restored cannot be found.
78       */
79      void readExternal(ObjectInput   in) throws IOException  , ClassNotFoundException  ;
80  }
81
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