Home Accessibility Courses Diary The Mouth Forum Resources Site Map About Us Contact
For 2023 (and 2024 ...) - we are now fully retired from IT training.
We have made many, many friends over 25 years of teaching about Python, Tcl, Perl, PHP, Lua, Java, C and C++ - and MySQL, Linux and Solaris/SunOS too. Our training notes are now very much out of date, but due to upward compatability most of our examples remain operational and even relevant ad you are welcome to make us if them "as seen" and at your own risk.

Lisa and I (Graham) now live in what was our training centre in Melksham - happy to meet with former delegates here - but do check ahead before coming round. We are far from inactive - rather, enjoying the times that we are retired but still healthy enough in mind and body to be active!

I am also active in many other area and still look after a lot of web sites - you can find an index ((here))
A multithreaded server in Java

Java is a great language for network programming, and for handling applications where you want to do several things at the same time.

This article shows you how we've used Java to provide a simple "chatroom" facility on our local area network - all controlled by a daemon process written in Java.

Users who want to visit the chatroom can simply telnet to port 1357 on the computer that's running the daemon, and then whatever they type in will be copied to everyone else who's logged in. The daemon will work just as well over a wide area; indeed, if you want to try it out, it's usually running on chat.wellho.net on the internet!


Planning your application is vital in any object oriented language such as Java - for smaller applications you may choose to use informal methods, but for larger applications you'll need something more formal such as UML, and perhaps even tools such as Rational Rose to manage your model.

Informally, our plans for the talker are as follows:

 * A single parent thread will await new connections, and
 will create a new child thread when it receives a connection

 * A child thread will exist for each live connection; it will
 wait for input from the user of the connection, and when input
 is received it will echo it to all live users.

 * Threads will be managed in a Vector from which they will be
 deleted when the connection is closed

 * A few special directives (such as ".q" to quit) will be
 provided by the application to allow users to log out, and to
 enquire as to who is logged on.

Have a look through our Java code - see if you can work out how the elements fit together.

import java.net.*;
import java.io.*;
import java.util.*;

public class talker implements Runnable {

// A simple talker - Sample written by Graham Ellis, April 2001
// No login needed. Connect on port 1357.
// commands:
// .q quit
// .w who

// Variables for each thread
Socket linkto; // the socket
PrintWriter out; // the output and input streams
BufferedReader in;
int id; // i.d. of the connection
String from_name; // name of host connecting

// Class Variables

static Vector connectiontable; // current connections
static int nextid = 1; // Increasing i.d.

/////////////////////////////// Main method ////////////////////////

public static void main(String [] args) {
// Parent thread - create a server socket and await a connection
ServerSocket ss = null;
Socket s = null;
connectiontable = new Vector();

try {
ss = new ServerSocket(1357);
while ((s=ss.accept())!= null) {

// Connection received - create a thread
talker now;
Thread current = new Thread(now = new talker(s));
connectiontable.addElement(now); // Save talker into vector ..
current.start(); // start the user's thread
catch (Exception e) {

// should add finally block to close down
///////////////// Constructor for a new thread //////////////////////

talker (Socket from) {
 id = nextid++;
 linkto = from;
 InetAddress source = linkto.getInetAddress();
 from_name = source.getHostName();
 try {
 out = new PrintWriter(new OutputStreamWriter
 in = new BufferedReader(new InputStreamReader
 catch (Exception e) {}

public void run () {

 String line = " [User has just logged in] ";

 while (true) {
  boolean done=false;

  // read a line from the user

  if (line == null) {
   try {
    out.print(">: "); //prompt,flush,read
    line = in.readLine();
   } catch (Exception e) {
    done = true; // force exit if there's a problem

  // Handle special cases - user input starts with "."

  if (line.startsWith(".q")) done = true;
  if (line.startsWith(".w")) { // build up list of host names
   int k;
   StringBuffer fred = new StringBuffer("WHO HERE?\n");
   for (k=0;k<connectiontable.size();k++) {
   talker person = (talker)connectiontable.elementAt(k);
   fred.append(" ");
   line = fred.toString();

  // echo the line (with a header) to all users

  String outline = from_name+" "+id+": "+line;
  int k;
  for (k=0;k<connectiontable.size();k++) {
   talker person = (talker)connectiontable.elementAt(k);
   person.out.flush(); // Vital - ensure it is sent!

  // clear out the user if they're done

  if (done) {
   try {
   out.close(); // closes needed to terminate connection
   in.close(); // otherwise user's window goes mute
   } catch (Exception e) {}
  line = null;

See also Training exercise - a talk server and client in Java

Please note that articles in this section of our web site were current and correct to the best of our ability when published, but by the nature of our business may go out of date quite quickly. The quoting of a price, contract term or any other information in this area of our website is NOT an offer to supply now on those terms - please check back via our main web site

Related Material

Java - Putting it all together
  [2525] - ()
  [2653] - ()
  [4305] - ()

resource index - Java
Solutions centre home page

You'll find shorter technical items at The Horse's Mouth and delegate's questions answered at the Opentalk forum.

At Well House Consultants, we provide training courses on subjects such as Ruby, Lua, Perl, Python, Linux, C, C++, Tcl/Tk, Tomcat, PHP and MySQL. We're asked (and answer) many questions, and answers to those which are of general interest are published in this area of our site.

You can Add a comment or ranking to this page

© WELL HOUSE CONSULTANTS LTD., 2024: Well House Manor • 48 Spa Road • Melksham, Wiltshire • United Kingdom • SN12 7NY
PH: 01144 1225 708225 • FAX: 01144 1225 793803 • EMAIL: info@wellho.net • WEB: http://www.wellho.net • SKYPE: wellho

PAGE: http://www.wellho.net/solutions/java-a-m ... -java.html • PAGE BUILT: Wed Mar 28 07:47:11 2012 • BUILD SYSTEM: wizard