MorrisTcpExample1Telnet.java

package MV3500Cohort2021JulySeptember.homework1;;

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

/**
 * The simplest possible TCP network program. It listens for
 * a connection, from telnet (telnet localhost 2317) or a program
 * you write, which we will do later. Right now the TcpExample simply
 * writes a string in response to a connection.
 * 
 * Modifying his program is the basis for Assignment 1.
 * 
 * Testing the running server program from telnet looks like this:
 * 
 * it154916:projects mcgredo$ telnet localhost 2317
 * Trying ::1...
 * Connected to localhost.
 * Escape character is '^]'.
 * This was written by the server
 * Connection closed by foreign host.
 * 
 * Notice that "This was written by the server" matches 
 * what is written by the code below, over the output stream.
 * 
 * After this first connection the program below drops out
 * the bottom of the program, and does not repeat itself.
 * The program exits.
 * 
 * @author mcgredo
 * @author brutzman
 */
public class MorrisTcpExample1Telnet 
{
    /**
     * Program invocation, execution starts here
     * @param args command-line arguments
     */
    public static void main(String[] args)
    {
        try
        {
            System.out.println("TcpExample1Telnet  has started and is waiting for a connection.");
            System.out.println("  help: https://savage.nps.edu/Savage/developers.html#telnet");
            System.out.println("  enter (telnet localhost 2317) or (nc localhost 2317)..." );
			
            // The ServerSocket waits for a connection from a client.
            // It returns a Socket object when the connection occurs.
            ServerSocket serverSocket = new ServerSocket(2317);
            
            // Use Java io classes to write text (as opposed to
            // unknown bytes of some sort) to the client
            
            // The Socket object represents the connection between
            // the server and client, including a full duplex connection
            try (Socket clientConnection = serverSocket.accept()) // wait here for a client to connect
            {
                // OK we got something, time to respond!
                // Use Java io classes to write text (as opposed to
                // unknown bytes of some sort) to the client
                OutputStream os = clientConnection.getOutputStream();
                PrintStream ps = new PrintStream(os);
                
                        ps.println("This client response was written by Johnny Smiles ");  // to remote client
                        ps.println("MOVES 2021 Cohort is full of badassery.");
                System.out.println("This server response was written by Johnny Smiles " ); // to server console

                // "flush()" in important in that it forces a write
                // across what is in fact a slow connection
                ps.flush();
            }
            System.out.println(" completed successfully.");
        }
        catch(IOException e)
        {
            System.err.println(" Pardon the interruption, networking issue"); // describe what is happening
            System.err.println(e);
            // Provide more helpful information to user if exception occurs due to running twice at one time
            
            // brute force exception checking, can be brittle if exception message changes
            // if (e.getMessage().equals("Address already in use: NET_Bind")) 
            if (e instanceof java.net.BindException)
                System.err.println("*** Be sure to stop any other running instances of programs using this port!");
        }
    }
}