Listening to multiple ports(socets) for data?

Question

I know that this topic has been covered before, but is either simplistic questions or overly complex things (that I don't understand... yet)
Here is my issue: I have data coming from two ports (TCP/IP) an array of 210 bytes and I'm not sure which port will be activated first. The data is asyncronus. So basically I want to figure out how to listen to the ports. I have this code...
import java.io.*;
import java.util.*;
import java.net.*;
public class SocketListener{
    public static void main(String[] argv){
        ServerSocket mySocket = null;
        ServerSocket mySocket2 = null;
        try {
            mySocket=new ServerSocket(12000);
            mySocket2= new ServerSocket (13000);
        catch (IOException e){
            System.err.println("error");
            System.exit(-1);
        try {
            Socket client = mySocket.accept();
            System.out.println("The server has connected to 12000\n");
            // do what I need it to do a.k.a. get data array
            client.close();
        catch(IOException e){
            System.err.println("error");
            System.exit(-1);
        try {
            Socket client2 = mySocket2.accept();
            System.out.println("The server has connected to 13000\n");
            // do what I need it to do a.k.a. get data array
            client2.close();
        catch (IOException e){
            System.err.println("error");
            System.exit(-1);
}I'm not sure if it works or not... but thats what I got. Is this the best way to go about this or is there something else that I should look up in documentation / look for examples / someone might have a link to something.
Thanks a lot!!!!
Edited by: KTLightning on Sep 12, 2007 1:25 PM

Answer 1

There are two issues at hand here, one being networking and the second being threading. Threading is an extensive subject and I don't want to shortchange you here, so you may want to read this tutorial if you are not familiar with Java threading concepts: http://java.sun.com/docs/books/tutorial/essential/concurrency/index.html
That being said, if you want to concurrently listen to the sockets, you will need a thread for each socket you are listening. Otherwise you can use Java NIO, but unless you are going to be listening to many sockets at once, this complication is not necessary since the learning curve of Java NIO can be quite steep.
You might want to start out creating a new class that will handle the task of listening to a socket and retrieving the data in its own thread. You can do this by either extending the Thread class or implementing the Runnable interface. From your main class, you can then spawn as many of those objects as you need. The trick to this approach is knowing when the data has been read from the socket and stored in the object. For only two objects this is simple as you can have a hasData() method on your worker objects and then just put a loop in your main class checking to see if the condition has been met on the objects.
Here's some code fragments as an example...
public class SocketReader extends Thread ...
ServerSocket ss = null;
boolean hasData = false;
SocketReader(int port) {
ss = new ServerSocket(port);
public void run() {
ss.accept();
// do your stuff
hasData =true;
public boolean hasData() {
return hasData;
main() {
SocketReader sr1 = new SocketReader(12000);
SocketReader sr2 = new SocketReader(12000);
sr1.start();
sr2.start();
// keep looping until we have data in both
while (!sr1.hasData() && !sr2.hasData()) {
try {
Thread.sleep(100);
} catch (InterrruptedException ie) {/ do something}
} // end main
Keep in mind that this is a very simplistic example to demonstrate the concepts and would not be a good design for pretty much any other case.