Chat Application

The chat application allows users to exchange simple text messages with other users. In this example, we exploit the JMS pub/sub model to exchange messages between chat applications. The sender of chat messages is a string property is the message header to allow selectors that block certain users.
Below is the source code for the Chat class:
package chat;
                                                                           
import java.awt.Dimension;
import java.awt.BorderLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
                                                                           
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JButton;
import javax.swing.JMenuBar;
import javax.swing.JMenuItem;
import javax.swing.JSplitPane;
import javax.swing.JTextArea;
import javax.swing.JOptionPane;
                                                                           
import javax.naming.InitialContext;
                                                                           
import javax.jms.Topic;
import javax.jms.Session;
import javax.jms.Message;
import javax.jms.TextMessage;
import javax.jms.TopicSession;
import javax.jms.JMSException;
import javax.jms.TopicPublisher;
import javax.jms.MessageListener;
import javax.jms.TopicSubscriber;
import javax.jms.TopicConnection;
import javax.jms.TopicConnectionFactory;
                                                                           
/**
   This class ilustrates a very simple chat application. Users will
   connect to "topic0" and exchange messages. Please note that this
   program does not perform any error checking.
 */
                                                                           
public class Chat
{
    String _block = "";
    TopicSubscriber subscriber;
                                                                           
    JFrame    frame    = new JFrame("Chat");
    JButton   send     = new JButton("Send");
    JMenuBar  menu     = new JMenuBar();
    JMenuItem block    = new JMenuItem("Block...");
    JMenuItem clear    = new JMenuItem("Clear");
    JTextArea incoming = new JTextArea();
    JTextArea outgoing = new JTextArea();
                                                                           
    public static void main(String[] args) throws Exception
    {
    |   Chat chat = new Chat();
    }
                                                                           
    public Chat() throws Exception
    {
    |   // create publisher and subscriber
    |   InitialContext ctx = new InitialContext();
    |   TopicConnectionFactory factory = (TopicConnectionFactory)
    |       ctx.lookup("topic/connectionFactory");
    |   final Topic topic = (Topic) ctx.lookup("topic/topic0");
    |   TopicConnection conn = factory.createTopicConnection();
    |   final TopicSession session = conn.createTopicSession(false,
    |       Session.AUTO_ACKNOWLEDGE);
    |   final TopicPublisher publisher = session.createPublisher(topic); 
    |   subscriber = session.createSubscriber(topic);
    |                                                                      
    |   // create the gui
    |   incoming.setEditable(false);
    |                                                                      
    |   // add menu items
    |   menu.add(block);
    |   menu.add(clear);
    |                                                                      
    |   // replace subscriber according to blocked user
    |   block.addActionListener(new ActionListener() {
    |   |   public void actionPerformed(ActionEvent ev) {
    |   |   |   String user = (String) JOptionPane.showInputDialog(frame,
    |   |   |       "Block user:", "Select user name",
    |   |   |       JOptionPane.QUESTION_MESSAGE, null,
    |   |   |       null, _block);
    |   |   |   if (user == null || _block.equals(user)) return;
    |   |   |   _block = user;
    |   |   |   try {
    |   |   |   |   subscriber.close();
    |   |   |   |   String sel = "sender <> '" + _block + "'";
    |   |   |   |   subscriber = session.createSubscriber(topic, sel, false);
    |   |   |   |   setMessageListener();
    |   |   |   |   incoming.append("blocking " + _block);
    |   |   |   } catch (JMSException ex) { ex.printStackTrace(); }
    |   |   }
    |   });
    |                                                                      
    |   // clear text area
    |   clear.addActionListener(new ActionListener() {
    |   |   public void actionPerformed(ActionEvent ev) {
    |   |   |   incoming.setText("");
    |   |   }
    |   });
    |                                                                      
    |   setMessageListener();
    |                                                                      
    |   // now start the connection
    |   conn.start();
    |                                                                      
    |   // send message when user clicks the button
    |   send.addActionListener(new ActionListener() {
    |   |   public void actionPerformed(ActionEvent e) {
    |   |   |   String msg = outgoing.getText();
    |   |   |   if (msg.length() > 0) {
    |   |   |   |   try {
    |   |   |   |   |   String sender = System.getProperty("user.name");
    |   |   |   |   |   TextMessage m = session.createTextMessage(msg);
    |   |   |   |   |   m.setStringProperty("sender", sender);
    |   |   |   |   |   publisher.publish(m);
    |   |   |   |   |   outgoing.setText("");
    |   |   |   |   } catch (JMSException ex) { ex.printStackTrace(); }
    |   |   |   }
    |   |   }
    |   });
    |                                                                      
    |   // setup rest of gui
    |   JPanel panel = new JPanel(new BorderLayout());
    |   panel.add(outgoing);
    |   panel.add(send, BorderLayout.EAST);
    |   JSplitPane split = new JSplitPane(JSplitPane.VERTICAL_SPLIT,
    |       incoming, panel);
    |   split.setOneTouchExpandable(true);
    |   split.setDividerLocation(205);
    |                                                                      
    |   frame.getContentPane().setLayout(new BorderLayout());
    |   frame.getContentPane().add(split);
    |   frame.getContentPane().add(menu, BorderLayout.NORTH);
    |   frame.setSize(new Dimension(400,300));
    |   frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    |   frame.setVisible(true);
    }
                                                                           
    public void setMessageListener() throws JMSException
    {
    |   // add asynchronous message listener
    |   subscriber.setMessageListener(new MessageListener() {
    |   |   public void onMessage(Message m) {
    |   |   |   try {
    |   |   |   |   TextMessage msg = (TextMessage) m;
    |   |   |   |   String sender = msg.getStringProperty("sender");
    |   |   |   |   incoming.append(sender + "> " + msg.getText() + "\n");
    |   |   |   } catch (Exception ex) { ex.printStackTrace(); }
    |   |   }
    |   });
    }
}
The Chat application creates a GUI with an area for chat messages and an area for typing new messages to be sent. To send a message, you simply click the "send" button. Users are blocked by create a new subscription where the selector corresponds to the user types into an input dialog.