Java Sockets: One Server and Multiple Clients

Question

So I created a basic client-server program in java. It starts out like this:

Client connects to Server
Server asks for Client's name
Client responds with name
Server greets Client
After this, Client speaks and the Server repeats the words back

I got this to work without too much trouble using this tutorial. The problem comes whenever I try to introduce multiple clients. I thought that it would work because I'm using multiple threads, however, the second clients just hangs until the first client quits and then it starts it work (the server does accept input from the second client, but it doesn't respond with anything until the first client quits).

Here is the code I'm using:

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

public class Server extends Thread {
  private ServerSocket listener;

  public Server(int port) throws IOException {
    listener = new ServerSocket(port);
  }

  public void run() {
    while(true) {
      try {
        Socket server = listener.accept();
        DataOutputStream out = new DataOutputStream(server.getOutputStream());
        out.writeUTF("What is your name?");
        DataInputStream in = new DataInputStream(server.getInputStream());
        String user_name = in.readUTF();
        out.writeUTF("Hello "+user_name);
        while(true) {
          String client_message = in.readUTF();
          out.writeUTF(client_message);
        }
      }
      catch(IOException e) {
        e.printStackTrace();
      }
    }
  }

  public static void main(String[] args) {
    int port = 6006;
    try {
      Thread t = new Server(port);
      t.start();
    } catch(IOException e) {
      e.printStackTrace();
    }
  }
}

Can someone explain what I'm doing wrong?

I have looked at the using Runnable instead of Extends Thread, but I ran into even more problems there, so I want to try and work with this first.

everything after listener.accept() needs to be in a different class that extends thread or implements runnable. You then at that point start that thread, and your loop will then accept the next connection — Cruncher
– Cruncher, Commented Sep 3, 2013 at 1:21

dst · Accepted Answer · 2013-09-03 04:04:11Z

4

Incoming connections are only handled by the line listener.accept();. But after you got a client connected, you're stuck in the while loop. You need to create a new Thread (or Runnable executed on an ExecutorService if you expect high load), and start it, then immediately accept the next connection.

edited Sep 3, 2013 at 4:04

answered Sep 3, 2013 at 1:19

dst

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1 Comment

dst Over a year ago

@AlexeiKaigorodov indeed easy to misunderstand that, edited that word now.

Neeraj · Accepted Answer · 2013-09-03 01:23:00Z

In a nutshell, this is what is going wrong.

You are using exactly ONE thread as the server.
Blocking this thread when you call listener.accept()

This is what you need to do:

Create two classes 1: Server - Similar to what you have now, but instead of doing the actual work of acting as an echo server, it just spawns a new Thread which starts listening on a NEW PORT (which you can select randomly), and sends the client the address for this new port. The client will then get the new port number and would try to connect to the server on the new port. 2: The Echo thread - This starts a new listener on the port passed, and does the job of echoing to whoever is listening.

OR:

You start a UDP server rather than a TCP server, and all this will not matter then, but that is out of the purview of this specific question.

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