Cheetim’s Blog

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 publicly 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 “.”.

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

http://www.kieser.net/linux/java_server.html

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

/**
 * 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;

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

}

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

 doComms(Socket server) {
 this.server=server;
 }

 public void run () {

 input="";

 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);

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