libjava/java/io/Externalizable.java

   1 /* Externalizable.java -- Interface for saving and restoring object data
   2    Copyright (C) 1998 Free Software Foundation, Inc.
   3
   4 This file is part of GNU Classpath.
   5
   6 GNU Classpath is free software; you can redistribute it and/or modify
   7 it under the terms of the GNU General Public License as published by
   8 the Free Software Foundation; either version 2, or (at your option)
   9 any later version.
  10
  11 GNU Classpath is distributed in the hope that it will be useful, but
  12 WITHOUT ANY WARRANTY; without even the implied warranty of
  13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  14 General Public License for more details.
  15
  16 You should have received a copy of the GNU General Public License
  17 along with GNU Classpath; see the file COPYING.  If not, write to the
  18 Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
  19 02111-1307 USA.
  20
  21 Linking this library statically or dynamically with other modules is
  22 making a combined work based on this library.  Thus, the terms and
  23 conditions of the GNU General Public License cover the whole
  24 combination.
  25
  26 As a special exception, the copyright holders of this library give you
  27 permission to link this library with independent modules to produce an
  28 executable, regardless of the license terms of these independent
  29 modules, and to copy and distribute the resulting executable under
  30 terms of your choice, provided that you also meet, for each linked
  31 independent module, the terms and conditions of the license of that
  32 module.  An independent module is a module which is not derived from
  33 or based on this library.  If you modify this library, you may extend
  34 this exception to your version of the library, but you are not
  35 obligated to do so.  If you do not wish to do so, delete this
  36 exception statement from your version. */
  37
  38
  39 package java.io;
  40
  41 /**
  42  * This interface provides a way that classes can completely control how
  43  * the data of their object instances  are written and read to and from
  44  * streams.  It has two methods which are used to write the data to a stream
  45  * and to read the data from a stream.  The read method must read the data
  46  * in exactly the way it was written by the write method.
  47  * <p>
  48  * Note that classes which implement this interface must take into account
  49  * that all superclass data must also be written to the stream as well.
  50  * The class implementing this interface must figure out how to make that
  51  * happen.
  52  * <p>
  53  * This interface can be used to provide object persistence.  When an
  54  * object is to be stored externally, the <code>writeExternal</code> method is
  55  * called to save state.  When the object is restored, an instance is
  56  * created using the default no-argument constructor and the
  57  * <code>readExternal</code> method is used to restore the state.
  58  *
  59  * @author Aaron M. Renn (arenn@urbanophile.com)
  60  */
  61 public interface Externalizable extends Serializable
  62 {
  63   /**
  64    * This method restores an object's state by reading in the instance data
  65    * for the object from the passed in stream.  Note that this stream is not
  66    * a subclass of <code>InputStream</code>, but rather is a class that
  67    * implements
  68    * the <code>ObjectInput</code> interface.  That interface provides a
  69    * mechanism for
  70    * reading in Java data types from a stream.
  71    * <p>
  72    * Note that this method must be compatible with <code>writeExternal</code>.
  73    * It must read back the exact same types that were written by that
  74    * method in the exact order they were written.
  75    * <p>
  76    * If this method needs to read back an object instance, then the class
  77    * for that object must be found and loaded.  If that operation fails,
  78    * then this method throws a <code>ClassNotFoundException</code>
  79    *
  80    * @param in An <code>ObjectInput</code> instance for reading in the object
  81    * state
  82    *
  83    * @exception ClassNotFoundException If the class of an object being
  84    * restored cannot be found
  85    * @exception IOException If any other error occurs
  86    */
  87   void readExternal(ObjectInput in)
  88     throws ClassNotFoundException, IOException;
  89
  90   /**
  91    * This method is responsible for writing the instance data of an object
  92    * to the passed in stream.  Note that this stream is not a subclass of
  93    * <code>OutputStream</code>, but rather is a class that implements the
  94    * <code>ObjectOutput</code> interface.  That interface provides a
  95    * number of methods
  96    * for writing Java data values to a stream.
  97    * <p>
  98    * Not that the implementation of this method must be coordinated with
  99    * the implementation of <code>readExternal</code>.
 100    *
 101    * @param out An <code>ObjectOutput</code> instance for writing the
 102    * object state
 103    *
 104    * @exception IOException If an error occurs
 105    */
 106   void writeExternal(ObjectOutput out) throws IOException;
 107 }