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Multithreaded tcp/ip server in Java
Network Programming example from a Well House Consultants training course
More on Network Programming [link]

This example is described in the following article(s):
   • Writing a server in Java - [link]

This example references the following resources:

Source code: Leanne.java Module: J808
/* This is an example of a multithreaded server in Java.
It listens for connections on port 1357 (use telnet to make such connections
once you have the server running) and echos any information sent to it to
anyone else who's logged in at the time - a very simple talker application.

See also http://www.wellho.net/solutions/java-a-multithreaded-server-in-java.html

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

public class Leanne implements Runnable {

// A simple talker - Sample written by Graham Ellis, July 2010 (update)
// 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
Leanne now;
Thread current = new Thread(now = new Leanne(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 //////////////////////

Leanne (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 "."
                // You get a NULL back if connection lost and we should
                // pick up that exception!

                try {
                        if (line.startsWith(".q")) done = true;
        } catch (Exception e) { // e.g. null
        done = true;
        line = "[exiting]";

                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++) {
                                Leanne person = (Leanne)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++) {
                        Leanne person = (Leanne)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;
Learn about this subject
This module and example are covered on our public Java Extra course. If you have a group of three or more trainees who need to learn the subject, we can also arrange a private or on site course for you.

Books covering this topic
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Other Examples
This example comes from our "Network Programming" training module. You'll find a description of the topic and some other closely related examples on the "Network Programming" module index page.

Full description of the source code
You can learn more about this example on the training courses listed on this page, on which you'll be given a full set of training notes.

Many other training modules are available for download (for limited use) from our download centre under an Open Training Notes License.

Other resources
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