PughTcp2ConnectionCounting.java

package MV3500Cohort2021JulySeptember.homework1;

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 pugh
 * @author brutzman
 */
public class PughTcp2ConnectionCounting {

    /**
     * Program invocation, execution starts here
     *
     * @param args command-line arguments
     */
    public static void main(String[] args) {
        try {
            System.out.println("TcpExample2ConnectionCounting has started and is waiting for a connection.");
            System.out.println("Get Ready To Network!!!");
            System.out.println("  help: https://savage.nps.edu/Savage/developers.html#telnet");
            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) {
                // blocks! then proceeds once a connection is "accept"ed
                try ( Socket clientConnection = serverSocket.accept()) {
                    connectionCount++; // got another one!

                    OutputStream os = clientConnection.getOutputStream();
                    PrintStream ps = new PrintStream(os);

                    ps.println("This client response was written by server " + PughTcp2ConnectionCounting.class.getName()); // to remote client
                    System.out.println("This server response was written by server " + PughTcp2ConnectionCounting.class.getName() + "\n"); // to server console
                    if (connectionCount % 2 == 1) {
                        System.out.println("Go\n");
                    }
                    if (connectionCount % 2 == 0) {
                        System.out.println("Gators!\n");
                    }
                    if (connectionCount % 3 == 1 && connectionCount != 0) {
                        System.out.println("O'Doyle Rules!!! \n");
                    }
                    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 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();
                }
            }
        } catch (IOException e) {
            System.err.println("Problem with  " + PughTcp2ConnectionCounting.class.getName() + " networking:"); // describe what is happening
            System.err.println("Error: " + e);
            // Provide more helpful information to user if exception occurs due to running twice at one time
            if (e instanceof java.net.BindException) {
                System.err.println("*** Be sure to stop any other running instances of programs using this port!");
            }
        }
    }
}