TcpExample2ConnectionCounting.java

package TcpExamples;

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

/**
 * Very slightly more complex than example1. The only thing this does
 * differently is introduce a loop into the response, so you don't
 * have to restart the program after one response. Also, it prints
 * out the socket pair the server sees. Run the program via telnet
 * several times and compare the socket pairs.
 * 
 * <code>telnet localhost 2317</code>
 * 
 * If you're sophisticated you can contact the instructor's computer
 * while running this program.
 * 
 * <code>telnet ipOfServersLaptop 2317</code>
 * 
 * And have that machine display the socket pairs received.
 * @author mcgredo
 */
public class TcpExample2ConnectionCounting 
{

    public static void main(String[] args) 
    {
        try
        {
            System.out.println("TcpExample2ConnectionCounting has started and is waiting for a connection.");
            System.out.println("  enter (nc localhost 2317) or (telnet localhost 2317)..." );
			
            // ServerSocket waits for a connection from a client. 
            // Notice that it is outside the loop; ServerSocket
            // needs to be made only once.
			
			int connectionCount = 0; // state variable
            
            ServerSocket serverSocket = new ServerSocket(2317); // server decides here what port to listen on.
			// of interest: often client doesn't care what port it uses locally when connecting to that server port.

            // Loop, infinitely, waiting for client connections.
            // Stop the program somewhere else.
            while(true)
            {
                Socket clientConnection = serverSocket.accept(); // blocks! then proceeds once a connection is "accept"ed
				
				connectionCount++; // got another one!
				
                OutputStream os = clientConnection.getOutputStream();
                PrintStream ps = new PrintStream(os);

				        ps.println("This client response was written by server TcpExample2ConnectionCounting"); // to remote client
				System.out.println("This server response was written by server TcpExample2ConnectionCounting"); // to server console
				
				ps.println("You were connection #" + connectionCount + ", by my count");
                
                // Print some information locally about the Socket
                // connection. This includes the port and IP numbers
                // on both sides (the socket pair.)
                
                InetAddress  localAddress = clientConnection.getLocalAddress();
                InetAddress remoteAddress = clientConnection.getInetAddress();
                
                int  localPort = clientConnection.getLocalPort();
                int remotePort = clientConnection.getPort();       // remember the prior question, why are 2 ports different?
                
                // My socket pair connection looks like this, to localhost:
                // Socket pair: (( /0:0:0:0:0:0:0:1, 2317 ), ( /0:0:0:0:0:0:0:1, 54876 )) note IPv6
                // Socket pair: (( /0:0:0:0:0:0:0:1, 2317 ), ( /0:0:0:0:0:0:0:1, 54881 ))
                //
                // Why is the first IP/port the same, while the second set has
                // different ports?

                System.out.println("Socket pair: (( " + localAddress.toString() + ", " + localPort + " ), ( " + 
                                    remoteAddress.toString() + ", " + remotePort + " ))");
                
                System.out.println("got another connection, #" + connectionCount); // report progress
				
                // Notice the use of flush() and close(). Without
                // the close() to Socket object may stay open for 
                // a while after the client has stopped needing this
                // connection. Close() explicitly ends the connection.
                ps.flush();
                clientConnection.close();
            }
       }
        catch(Exception e)
        {
            System.out.println("problem with networking: " + e);
        }
    }
}