Forse ti puo' essere utile questo esempio che ho trovato in rete:
(http://www.planetsourcecode.com)
import java.io.*;
import java.net.*;
import java.util.*;
import java.awt.*;
import java.awt.event.*;


public class server extends Frame {
public static TextArea display;
public static server a;
public static int portNumber, maxClients;


public static void main (String [] args) {
BufferedReader stdin = new BufferedReader
(new InputStreamReader (System.in));


try {
System.out.print("Give the port number to listen for requests: ");
System.out.flush();
portNumber = Integer.parseInt(stdin.readLine());
System.out.print("Give the maximum number of clients for this server: ");
System.out.flush();
maxClients = Integer.parseInt(stdin.readLine());
} catch (IOException ioe) {System.out.println(ioe);}
a = new server();
a.show();
a.startServer();
}
public ServerSocket connect;
public Vector sockets;
public TextField input;


public server () {
setSize(780,550);
setTitle("server");
setLayout(null);
addWindowListener(new closeListener());
display = new TextArea();
display.setEditable(false);
input = new TextField();
input.addActionListener(new al());
add(input);
add(display);
display.setBounds(10,30,760,489);
input.setBounds(10,520,760,20);
sockets = new Vector();


try {
connect = new ServerSocket(portNumber,maxClients);
} catch (IOException ioe) {System.out.println(ioe);}
}


public void startServer () {


try {


while (true) {
sockets.addElement(new ThreadedSocket(connect.accept()));
Thread.yield();
}
} catch (Exception e) {System.out.println(e);}
}


public static synchronized void echoToAllClients (String s) {


try {


for (int i=0;i<A.sockets.size();i++) {
ThreadedSocket temp = (ThreadedSocket)a.sockets.elementAt(i);
temp.out.writeObject(s);
}
} catch (Exception e) {}
}


private class closeListener extends WindowAdapter {


public void windowClosing (WindowEvent e) {
System.exit(0);
}
}


private class al implements ActionListener {


public void actionPerformed (ActionEvent e) {
String message = "<Server> "+input.getText();
input.setText("");
display.append(message+"\n");
echoToAllClients(message);
}
}
}


class ThreadedSocket extends Thread {
public Socket socket;
public ObjectInputStream in;
public ObjectOutputStream out;
public String nick;


public ThreadedSocket (Socket s) {
socket = s;


try {
out = new ObjectOutputStream(socket.getOutputStream());
out.flush();
in = new ObjectInputStream(socket.getInputStream());
} catch (Exception e) {System.out.println(e);}
nick = "";
start();
}


public void run () {
String message = "";
int flag = 0;


while (!message.endsWith("quit")) {


try {
message = (String)in.readObject();
} catch (Exception e) {System.out.println(e);}


if (flag == 0) {
nick = message;
server.display.append("* "+nick+" ["+socket.getInetAddress().getHostName()+"] has joined the server *\n");
server.echoToAllClients("* "+nick+" ["+socket.getInetAddress().getHostName()+"] has joined the server *");
server.display.append("* "+nick+" welcome to Ezad's Chat Server (c) 2000-01 Ali Ezad Abbas *\n");
server.echoToAllClients("* "+nick+" welcome to Ezad's Chat Server (c) 2000-01 Ali Ezad Abbas *");
flag = 1;


} else {
server.display.append(message+"\n");
server.echoToAllClients(message);
}
Thread.yield();
}


try{
server.echoToAllClients("* Connection Closed by "+nick+" *");
in.close();
out.close();
socket.close();
server.display.append("* Connection Closed by "+nick+" *\n");
} catch (Exception e) {System.out.println(e);}
}
}