ConnectException thrown when sockets are created at the same time

Question

Hi everyone,
I have written a simple server and a simple client as follows:
SERVER:
import java.net.*;
public class Server
     public static void main(String[] args)
          try
               ServerSocket ss = new ServerSocket(1234);
               while (true)
                    final Socket s = ss.accept();
                    Thread t = new Thread() {
                         public void run() {
                              System.out.println(s.toString());
                    t.start();
          catch (Exception e)
               e.printStackTrace();
CLIENT:
import java.net.*;
public class Client
     public static void main(String[] args)
          try
               int numSocket = 10;
               Socket[] s = new Socket[numSocket];
               for (int i = 0; i < numSocket; i++) {
                    s[i] = new Socket("localhost", 1234);
          catch (Exception e)
               e.printStackTrace();
}When I run the client, the following stack trace was printed after 4 or 5 sockets are created successfully.
java.net.ConnectException: Connection refused
        at java.net.PlainSocketImpl.socketConnect(Native Method)
        at java.net.PlainSocketImpl.doConnect(PlainSocketImpl.java:305)
        at java.net.PlainSocketImpl.connectToAddress(PlainSocketImpl.java:171)
        at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:158)
        at java.net.Socket.connect(Socket.java:426)
        at java.net.Socket.connect(Socket.java:376)
        at java.net.Socket.<init>(Socket.java:291)
        at java.net.Socket.<init>(Socket.java:119)
        at Client.main(Client.java:12)If I add Thread.sleep(500) after a socket was created, then it's fine. All sockets are created successfullly.
I have tested this on both RedHat linux 7.2 and RHEL 3.0 AS and both fails.
However I run on Windows XP, it's ok even without Thread.sleep
It seems that RH linux cannot handle clients connecting to localhost that quickly. It works well even if I create 100 sockets in a row for WinXP.
I have searched the web for similiar problems, but I couldn't find any.
Please reply if you have the same problem or know how to solve it.
Thanks in advanced!
Joseph

Answer 1

Hello,
Your code is correct and should work, i'm actually quite positive it will work on at least a Windows OS. Also, it is possible you're running into backlog problems but i find it somewhat unlikely that since in most cases the backlog is larger than 10, and you're only testing 10 connections.
Regardless of the actual problem, you seem to be running into issues with a VM implementation that isnt behaving as it should. ServerSocket.accept() is a blocking IO call, and there should be no problem with doing it in a tight loop like yours. It's worth noting that you're not testing a practical solution anyway, since in real life you'll need to do some stream setup and such to initialise your newly accepted Socket which you could do in the same thread that's accepting the connections. If you change your code accordingly there's a good change it will work right away. Also, your problem will be fixed by having your accept thread wait a number of milliseconds before accepting a new connection. Although this isnt the most clean approach imaginable it does give other threads some time to breathe if you get a large amount of simultaneous connection requests. It's a fair bet a sleep(1) will already fix your problem.
Perhaps it's a good idea to add a little more code and move closer to your goal application, and see if the problem fixes itself. You'll need a work-around anyway since it's unexpected behaviour.
Good luck.