FrankTCPExample3Server.java

package MV3500Cohort2021JulySeptember.homework1;

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

/**
 * Very slightly more complex than example1, further modifying example2. 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.
 *
 * telnet (nc) localhost 2318
 *
 * If you're sophisticated you can contact the instructor's computer while
 * running this program.
 *
 * telnet (nc) [ipNumberOfServerLaptop] 2317
 *
 * and have the instructor display the socket pairs received.
 *
 * @author mcgredo
 * @author brutzman
 */
public class FrankTCPExample3Server {

    /**
     * Program invocation, execution starts here If already compiled, can run
     * using console in directory ../../build/classes/ by invoking \ java
     * -classpath . TcpExamples.TcpExample3Server
     *
     * @param args command-line arguments
     */
    public static void main(String[] args) throws InterruptedException {
        try {

            // ServerSocket waits for a connection from a client. 
            // Notice that it is outside the loop; ServerSocket
            // needs to be made only once.
            System.out.println(FrankTCPExample3Server.class.getName() + " has started..."); // it helps debugging to put this on console first

            ServerSocket serverSocket = new ServerSocket(2318); // changed from 2317 to 2318
            OutputStream os;
            PrintStream ps;
            InetAddress localAddress, remoteAddress;
            int localPort, remotePort;
            int serverLoopCount = 0;

            // Server is up and waiting (i.e. "blocked" or paused)
            // Loop, infinitely, waiting for client connections.
            // Stop the program somewhere else.
            while (true) {

                // block until connected to a client
                try ( Socket clientConnectionSocket = serverSocket.accept()) {
                    serverLoopCount++; // increment at beginning of loop for reliability

                    // Now hook everything up (i.e. set up the streams), Java style:
                    os = clientConnectionSocket.getOutputStream();
                    ps = new PrintStream(os);
                    if (serverLoopCount <= 20) { // checking if the loop count <= 20
                        ps.println("This is response " + serverLoopCount + " produced by the server."); // this gets sent back to client!
                    } else {
                        ps.println("this is good bye message from the server"); // termination after 20 messages
                        break; // Stop server
                    }
                    // Print some information locally about the Socket connection.
                    // This includes the port and IP numbers on both sides (the socket pair).
                    localAddress = clientConnectionSocket.getLocalAddress();
                    remoteAddress = clientConnectionSocket.getInetAddress();
                    localPort = clientConnectionSocket.getLocalPort();
                    remotePort = clientConnectionSocket.getPort();

                    System.out.print("Server loop " + serverLoopCount + ": ");

                    // 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 ))
                    // 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(FrankTCPExample3Server.class.getName() + " socket pair showing host name, address, port:");
                    System.out.println("  (( "
                            + localAddress.getHostName() + "=" + localAddress.getHostAddress() + ", " + localPort + " ), ( "
                            + remoteAddress.getHostName() + "=" + remoteAddress.getHostAddress() + ", " + remotePort + " ))");

                    if (localAddress.getHostName().equals(localAddress.getHostAddress())
                            || remoteAddress.getHostName().equals(remoteAddress.getHostAddress())) {
                        System.out.println("  note HostName matches address if host has no DNS name");
                    }

                    // Not/*i*/ce the use of flush() and try w/ resources. Without
                    // the try w/ resources the Socket object may stay open for
                    // a while after the client has stopped needing this
                    // connection. try w/ resources explicitly ends the connection.
                    ps.flush();
                    // like it or not, you're outta here!
                }
            }
        } catch (IOException e) {
            System.err.println("Problem with " + FrankTCPExample3Server.class.getName() + " networking: " + 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!");
            }
        }
    }
}