java.io

Interface Externalizable

All Superinterfaces:
Serializable
Known Subinterfaces:
RemoteRef, ServerRef
Known Implementing Classes:
ActivationDataFlavor, DataFlavor, MimeType

public interface Externalizable
extends Serializable

This interface provides a way that classes can completely control how the data of their object instances are written and read to and from streams. It has two methods which are used to write the data to a stream and to read the data from a stream. The read method must read the data in exactly the way it was written by the write method.

Note that classes which implement this interface must take into account that all superclass data must also be written to the stream as well. The class implementing this interface must figure out how to make that happen.

This interface can be used to provide object persistence. When an object is to be stored externally, the writeExternal method is called to save state. When the object is restored, an instance is created using the default no-argument constructor and the readExternal method is used to restore the state.

Method Summary

void
readExternal(ObjectInput in)
This method restores an object's state by reading in the instance data for the object from the passed in stream.
void
writeExternal(ObjectOutput out)
This method is responsible for writing the instance data of an object to the passed in stream.

Method Details

readExternal

public void readExternal(ObjectInput in)
            throws ClassNotFoundException,
                   IOException
This method restores an object's state by reading in the instance data for the object from the passed in stream. Note that this stream is not a subclass of InputStream, but rather is a class that implements the ObjectInput interface. That interface provides a mechanism for reading in Java data types from a stream.

Note that this method must be compatible with writeExternal. It must read back the exact same types that were written by that method in the exact order they were written.

If this method needs to read back an object instance, then the class for that object must be found and loaded. If that operation fails, then this method throws a ClassNotFoundException

Parameters:
in - An ObjectInput instance for reading in the object state
Throws:
ClassNotFoundException - If the class of an object being restored cannot be found
IOException - If any other error occurs

writeExternal

public void writeExternal(ObjectOutput out)
            throws IOException
This method is responsible for writing the instance data of an object to the passed in stream. Note that this stream is not a subclass of OutputStream, but rather is a class that implements the ObjectOutput interface. That interface provides a number of methods for writing Java data values to a stream.

Not that the implementation of this method must be coordinated with the implementation of readExternal.

Parameters:
out - An ObjectOutput instance for writing the object state
Throws:
IOException - If an error occurs

Externalizable.java -- Interface for saving and restoring object data Copyright (C) 1998 Free Software Foundation, Inc. This file is part of GNU Classpath. GNU Classpath is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2, or (at your option) any later version. GNU Classpath is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with GNU Classpath; see the file COPYING. If not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA. Linking this library statically or dynamically with other modules is making a combined work based on this library. Thus, the terms and conditions of the GNU General Public License cover the whole combination. As a special exception, the copyright holders of this library give you permission to link this library with independent modules to produce an executable, regardless of the license terms of these independent modules, and to copy and distribute the resulting executable under terms of your choice, provided that you also meet, for each linked independent module, the terms and conditions of the license of that module. An independent module is a module which is not derived from or based on this library. If you modify this library, you may extend this exception to your version of the library, but you are not obligated to do so. If you do not wish to do so, delete this exception statement from your version.