一个基于RMI的网络聊天室

				/*
				 * Copyright 1999 by dreamBean Software,
				 * All rights reserved.
				 */
				
				
				import java.rmi.Remote;
				import java.rmi.RemoteException;
				
				/**
				 *   This is the Message object. A Message is the core object in the chat system.
				 *   The system can be extended by introducing new message types. Since the content
				 *   is an Object it can be anything including images. You must also extend the ChatGUI
				 *   if you want to present new types Messages in the user interface.
				 *      
				 *   @author Rickard 謆erg (rickard@dreambean.com)
				 *   @version $Revision:$
				 */
				public final class Message
				   implements java.io.Externalizable
				{
				   // Constants -----------------------------------------------------
				   private static final long serialVersionUID = 1L;
				   
				   // Pre-defined name for messages sent from the system
				   public static final String SYSTEM = "System";
				   
				   // Pre-defined types of messages
				   // This is the most common type: a text message
				   public static final String TEXT = "Text"; 
				      
				   // Pre-defined types for control messages
				   public static final String TOPIC_CREATED = "TopicCreated";
				   public static final String TOPIC_REMOVED = "TopicRemoved";
				   
				   public static final String USER_JOINED = "UserJoined";
				   public static final String USER_LEFT = "UserLeft";
				   
				   public static final String GREETING  = "Greeting";
				   
				   // Attributes ----------------------------------------------------
				   String sender;
				   String type;
				   Object content;
				   
				   // Constructors --------------------------------------------------
				   public Message()
				   {
				      // Used by Externalizable
				   }
				   
				   public Message(String sender, String type, Object content)
				   {
				      this.sender = sender;
				      this.type = type;
				      this.content = content;
				   }
				   
				   // Public --------------------------------------------------------
				   public String getSender() 
				   { 
				      return sender; 
				   }
				   
				   public String getType() 
				   { 
				      return type; 
				   }
				   
				   public Object getContent() 
				   { 
				      return content; 
				   }
				   
				   // Private -------------------------------------------------------
				   // We use externalization instead of serialization since it is 
				   // significantly faster in this case, and we are not worried about
				   // inheritance hierarchies
				   public void writeExternal(java.io.ObjectOutput out)
				      throws java.io.IOException
				   {
				      out.writeUTF(sender);
				      out.writeUTF(type);
				      out.writeObject(content);
				   }
				   
				   public void readExternal(java.io.ObjectInput in)
				      throws java.io.IOException, ClassNotFoundException
				   {
				      sender = in.readUTF();
				      type   = in.readUTF();
				      content = in.readObject();
				   }
				}