Hi,
It took me ages to get this working, so I'm posting it as an example of how to use a MulticastSocket.
It's chat client and server, which works all on the one machine, if not over the network. The existing examples using MulticastSocket all seem to oversimplify the problem by not enabling their clients to talk and listen at the same time.
I also found few examples of multi-threaded chat servers and client which use ServerSocket... so here's the cross breed.
My apologies for the awful-netbeans-generated-code in the ClientForm GUI.
-----
krc/chat/Config.java
package krc.chat;
public abstract class Config {
public static final int BUF_SIZE = 256;
// see: http://java.sun.com/javase/6/docs/api/java/net/MulticastSocket.html
public static final String MULTICAST_IP = "224.0.0.1";
public static final int MULTICAST_INBOUND_PORT = 4447;
public static final int MULTICAST_OUTBOUND_PORT = 4448;
}
-----
krc/chat/client/Client.java
package krc.chat.client;
import javax.swing.JOptionPane;
public class Client
{
// create the GUI and show it. Invoke me only on the EDT.
private static void createAndShowGUI(String username) {
new ClientForm(username).setVisible(true);
}
public static void main(String[] args) {
// get the username from command line args, else ask for it, else "guest".
final String username = getUsername(args);
//create and show the GUI on the EDT
javax.swing.SwingUtilities.invokeLater(
new Runnable() {
public void run() {
createAndShowGUI(username);
}
}
);
}
private static String getUsername(String[] args) {
if (args.length>0) return args[0];
String username = JOptionPane.showInputDialog(
null, "Username ?", "Enter username", JOptionPane.QUESTION_MESSAGE
);
if ( username!=null && username.trim().length()>0 ) return username;
return "guest";
}
}
-----
krc/chat/client/ClientForm.java
package krc.chat.client;
import java.io.IOException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;
import java.net.MulticastSocket;
import java.net.SocketException;
import java.net.UnknownHostException;
import java.util.List;
import java.util.concurrent.ExecutionException;
import javax.swing.JOptionPane;
import javax.swing.SwingWorker;
import krc.chat.Config;
/**
* krc.chat.client.ClientForm - A wee GUI for a wee chat app.
* @author Keith
*/
public class ClientForm extends javax.swing.JFrame {
private static final long serialVersionUID = 1675435212L;
// <awful-netbeans-generated-code>
private javax.swing.JTextArea displayTextArea;
private javax.swing.JTextField inputTextField;
private javax.swing.JScrollPane scrollPane;
private javax.swing.JButton sendButton;
// </awful-netbeans-generated-code>
private Sender sender = null;
private class Listener extends SwingWorker<Void, String>
// The 1st arg is the type of the threads return value.
// The 2nd arg is the type of intermediate results to be "published".
{
// doInBackground is invoked in the background thread, it publishes messages
// as they arive to a List of Strings which is then picked up and displayed
// by the process method which runs periodically on the EDT.
@Override
public Void doInBackground() {
MulticastSocket socket = null;
InetAddress address = null;
try {
socket = new MulticastSocket(Config.MULTICAST_OUTBOUND_PORT);
address = InetAddress.getByName(Config.MULTICAST_IP);
socket.joinGroup(address);
while (!isCancelled()) {
byte[] buf = new byte[Config.BUF_SIZE];
DatagramPacket packet = new DatagramPacket(buf, buf.length);
System.out.println("DEBUG: Listener.doInBackground: receiving on "+address);
socket.receive(packet); //thread blocks here
//String response = new String(packet.getData(), 0, packet.getLength());
String response = new String(packet.getData()).trim();
System.out.println("DEBUG: Listener.doInBackground: Received: "+packet.getAddress()+" "+packet.getPort()+" "+response);
publish(response);
}
} catch (Exception ex) {
ex.printStackTrace();
} finally {
if(socket!=null) try{socket.leaveGroup(address);}catch(Exception e){e.printStackTrace();}
socket.close();
socket = null;
}
return null;
}
// process is invoked in the event dispatch thread whenever swing feels like
// it so it could process several messages at once, but it's safe to mutate
// the swing controls on the EDT.
@Override
protected void process(List<String> messages) {
System.out.println("DEBUG: Listener.process: processing: "+messages.size()+" messages");
for(String message : messages) {
System.out.println("DEBUG: Listener.process: displayTextArea.append("+message+")");
displayTextArea.append(message+"\n");
}
}
}
private class Sender {
private String username = null;
private DatagramSocket socket = null;
private InetAddress address = null;
Sender(String username) throws SocketException, UnknownHostException {
this.username = username.toLowerCase();
this.socket = new DatagramSocket();
this.address = InetAddress.getByName("localhost");
}
public void send(String message) throws IOException {
// send the message
message = username+": "+message;
byte[] bytes = message.getBytes();
DatagramPacket packet = new DatagramPacket(bytes, bytes.length, this.address, Config.MULTICAST_INBOUND_PORT);
String data = new String(packet.getData(), 0, packet.getLength());
System.out.println("DEBUG: Sender.send: sending "+packet.getAddress()+" "+packet.getPort()+" "+data);
this.socket.send(packet);
System.out.println("DEBUG: Sender.send: sent.");
}
public void close() {
if(socket!=null)socket.close();
}
}
/**
* ClientForm constructor
*/
public ClientForm(String username) {
this.setTitle("Chat - "+username);
// build the GUI
initComponents();
// select all the text in the input field and give it focus
inputTextField.setSelectionStart(0);
inputTextField.setSelectionEnd(inputTextField.getText().length());
inputTextField.requestFocus();
try {
sender = new Sender(username);
sender.send(username+" joins the chat.");
} catch (Exception ex) {
ex.printStackTrace();
}
// setup a background thread which recieves messages from the server
// and appends them to the TextArea on the form.
new Listener().execute();
}
/**
* initComponents is called by the constructor to initialize the form.
*/
// <awful-netbeans-generated-code>
private void initComponents() {
scrollPane = new javax.swing.JScrollPane();
displayTextArea = new javax.swing.JTextArea();
inputTextField = new javax.swing.JTextField();
sendButton = new javax.swing.JButton();
setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
scrollPane.setVerticalScrollBarPolicy(javax.swing.ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS);
displayTextArea.setColumns(20);
displayTextArea.setFont(new java.awt.Font("Arial", 0, 10));
displayTextArea.setRows(5);
displayTextArea.setTabSize(4);
displayTextArea.setFocusable(false);
scrollPane.setViewportView(displayTextArea);
inputTextField.setText("... type your message here ...");
inputTextField.setName("messageText");
inputTextField.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
inputTextFieldActionPerformed(evt);
}
});
sendButton.setText("Send");
sendButton.setName("sendButton");
sendButton.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
sendButtonActionPerformed(evt);
}
});
javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
getContentPane().setLayout(layout);
layout.setHorizontalGroup(
layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
.addContainerGap()
.addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.TRAILING)
.addComponent(scrollPane, javax.swing.GroupLayout.Alignment.LEADING, javax.swing.GroupLayout.DEFAULT_SIZE, 507, Short.MAX_VALUE)
.addGroup(layout.createSequentialGroup()
.addComponent(inputTextField, javax.swing.GroupLayout.DEFAULT_SIZE, 427, Short.MAX_VALUE)
.addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
.addComponent(sendButton, javax.swing.GroupLayout.PREFERRED_SIZE, 74, javax.swing.GroupLayout.PREFERRED_SIZE)))
.addContainerGap())
);
layout.setVerticalGroup(
layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(layout.createSequentialGroup()
.addContainerGap()
.addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
.addComponent(sendButton, javax.swing.GroupLayout.PREFERRED_SIZE, 23, javax.swing.GroupLayout.PREFERRED_SIZE)
.addComponent(inputTextField, javax.swing.GroupLayout.PREFERRED_SIZE, 20, javax.swing.GroupLayout.PREFERRED_SIZE))
.addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
.addComponent(scrollPane, javax.swing.GroupLayout.DEFAULT_SIZE, 273, Short.MAX_VALUE)
.addContainerGap())
);
pack();
}// </awful-netbeans-generated-code>
private void inputTextFieldActionPerformed(java.awt.event.ActionEvent evt) {//GEN-FIRST:event_inputTextFieldActionPerformed
sendButtonActionPerformed(evt);
}
private void sendButtonActionPerformed(java.awt.event.ActionEvent evt) {//GEN-FIRST:event_sendButtonActionPerformed
try {
String message = inputTextField.getText();
inputTextField.setText("");
if ("".equals(message.trim())) return;
sender.send(message);
} catch (Exception e) {
e.printStackTrace();
JOptionPane.showConfirmDialog(this, e.toString(), "Exception", JOptionPane.ERROR_MESSAGE);
}
}
}
-----
krc/chat/server/Server.java
package krc.chat.server;
public class Server {
public static void main(String[] args) {
try {
new ServerThread().start();
} catch (Exception e) {
e.printStackTrace();
}
}
}
-----
krc/chat/server/ServerThread.java
package krc.chat.server;
import java.util.Map;
import java.util.HashMap;
import java.util.Iterator;
import java.net.InetAddress;
import java.net.DatagramSocket;
import java.net.DatagramPacket;
import krc.chat.Config;
public class ServerThread extends Thread {
protected DatagramSocket socket = null;
//private static Map<Integer, InetAddress> ports = new HashMap<Integer, InetAddress>();
public ServerThread() throws java.net.SocketException {
super("ServerThread");
this.socket = new DatagramSocket(Config.MULTICAST_INBOUND_PORT);
}
public void run() {
while (true) { // until killed
try {
byte[] buf = new byte[Config.BUF_SIZE];
DatagramPacket packet = new DatagramPacket(buf, buf.length);
String data = null;
// recieve a packet
System.out.println("DEBUG: receiving on: "+this.socket.getLocalAddress());
socket.receive(packet); // thread blocks here
data = new String(packet.getData(), 0, packet.getLength());
System.out.println("DEBUG: recieved: "+packet.getAddress()+" "+packet.getPort()+" "+data);
// broadcast it
InetAddress group = InetAddress.getByName(Config.MULTICAST_IP);
packet = new DatagramPacket(buf, buf.length, group, Config.MULTICAST_OUTBOUND_PORT);
socket.send(packet);
} catch (Exception e) {
e.printStackTrace();
break;
}
}
socket.close();
}
}
I hope this helps someone else understand datagrams and multi-casting.
Cheers. Keith.