Merge origin/master

assignments/src/MV3500Cohort2021JulySeptember/homework1/McNeelyTCPExample2.java

+package MV3500Cohort2021JulySeptember.homework1;
+
+import java.io.IOException;
+import java.io.OutputStream;
+import java.io.PrintStream;
+import java.net.InetAddress;
+import java.net.ServerSocket;
+import java.net.Socket;
+
+/**
+ * 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
+ * @author brutzman
+ * @author McNeely
+ */
+public class McNeelyTCPExample2
+{
+ public static void main(String[] args)
+    {
+        try
+        {
+            System.out.println("Justin McNeely's TcpExample2 has started and is waiting for a connection...");
+            System.out.println("  help: https://savage.nps.edu/Savage/developers.html#telnet");
+            System.out.println("  enter (nc localhost 2317) or (telnet localhost 2317) to win..." );
+			
+            // 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
+            int totalEntrantCount = 31; // spoofed entrants
+            
+            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 " + McNeelyTCPExample2.class.getName()); // to remote client
+                    System.out.println("This server response was written by server " + McNeelyTCPExample2.class.getName()); // to server console
+                    
+                            ps.println("You have attempted " + connectionCount + "times, you are now aplicant number " + totalEntrantCount + " to win. Keep trying!");
+                    
+                    totalEntrantCount = (totalEntrantCount + 24);
+                    // 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! WINNING!, #" + 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  " + McNeelyTCPExample2.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!");
+        }
+    }
+    
+}