A multi-threaded socket-based server

The problem is old - How to implement a multi-threaded, socket-based server that will let you read and write to the client (for example a telnet terminal.

There are several problems with constructing such a server:

  1. You have to use threads because otherwise clients will be queued up waiting for a connection.
  2. Many of the publically available examples (even some in some books) do not work for I/O servers. They are mostly developed around reading from a socket not writing to the socket as well. In our example we give a working read/write code example.
  3. You need to avoid race conditions when starting up a thread at times when connections are coming through thick and fast.
The code below will implement just such a server. It is intended for a head start, so is a template rather than a tutorial on how to write a server in Java. We have decided on port 4444 to listen on in the example, but you need to decide on a suitable port yourself.

The server will run, establishing connections from the port and echoing input received until it receives a line of input that is a full stop only ".".

Have fun!

The Kieser.net team.
Honeypot: spam@kieser.net

package sample_server;

import java.io.*;
import java.net.*;
import java.security.*;

 * Title:        Sample Server
 * Description:  This utility will accept input from a socket, posting back to the socket before closing the link.
 * It is intended as a template for coders to base servers on. Please report bugs to brad at kieser.net
 * Copyright:    Copyright (c) 2002
 * Company:      Kieser.net
 * @author B. Kieser
 * @version 1.0

public class sample_server {

  private static int port=4444, maxConnections=0;
  // Listen for incoming connections and handle them
  public static void main(String[] args) {
    int i=0;

      ServerSocket listener = new ServerSocket(port);
      Socket server;

      while((i++ < maxConnections) || (maxConnections == 0)){
        doComms connection;

        server = listener.accept();
        doComms conn_c= new doComms(server);
        Thread t = new Thread(conn_c);
    } catch (IOException ioe) {
      System.out.println("IOException on socket listen: " + ioe);


class doComms implements Runnable {
    private Socket server;
    private String line,input;

    doComms(Socket server) {

    public void run () {


      try {
        // Get input from the client
        DataInputStream in = new DataInputStream (server.getInputStream());
        PrintStream out = new PrintStream(server.getOutputStream());

        while((line = in.readLine()) != null && !line.equals(".")) {
          input=input + line;
          out.println("I got:" + line);

        // Now write to the client

        System.out.println("Overall message is:" + input);
        out.println("Overall message is:" + input);

      } catch (IOException ioe) {
        System.out.println("IOException on socket listen: " + ioe);