Homework 1

assignments/src/MV3500Cohort2024JulySeptember/homework1/Lennon/LennonNetworkHW1.java

+package MV3500Cohort2024JulySeptember.homework1.Lennon;
+
+import java.io.*;
+import java.net.*;
+
+/**
+ * <p>
+ * The simplest possible TCP network program, opening a socket and waiting for a reply. 
+ * It listens for any socket connection response, either from telnet (telnet localhost 2317)
+ * or another program that you write (which we will do later). 
+ * Once a socket connection is established, TcpExample1Telnet simply
+ * writes a string in response to that connection and finishes, exiting.
+ * 
+ * <p>
+ * As an alternative to running the Windows (or other operating system) console, 
+ * you can instead run the NetBeans terminal window.  If you are on Windows,
+ * NetBeans is looking for Cygwin installation (for Unix-like compatibility)
+ * with details at <a href="https://savage.nps.edu/Savage/developers.html#Cygwin" target="blank">Savage Developers Guide: Cygwin</a>.
+ * Modifying this program is the basis for Assignment 1.
+ * </p>
+ * 
+ * <p>
+ * Testing the running server program from telnet looks like this:
+ * </p>
+ * <pre>
+ * it154916:projects mcgredo$ <b>telnet localhost 2317</b>
+ * Trying ::1...
+ * Connected to localhost.
+ * Escape character is '^]'.
+ * This was written by the server
+ * Connection closed by foreign host.
+ * </pre>
+ * <p>
+ * Notice that "This was written by the server" matches 
+ * what is written by the code below, over the output stream.
+ * </p>
+ * 
+ * <p>
+ * After this first connection the program below drops to
+ * the bottom of the method, and does not repeat itself.
+ * In other words, the program exits.
+ * </p>
+ * 
+ * @see <a href="../../../src/TcpExamples/TcpExample1TerminalLog.txt" target="_blank">TcpExample1TerminalLog.txt</a>
+ * @see <a href="../../../src/TcpExamples/TcpExample1NetBeansConsoleTelnet.png" target="_blank">TcpExample1NetBeansConsoleTelnet.png</a>
+ * @see <a href="../../../src/TcpExamples/TcpExample1NetBeansConsoleTelnet.pdf" target="_blank">TcpExample1NetBeansConsoleTelnet.pdf</a>
+ * @see <a href="../../../src/TcpExamples/TcpExample1ScreenshotNetcat.png" target="_blank">TcpExample1ScreenshotNetcat.png</a>
+ * @see <a href="../../../src/TcpExamples/TcpExample1ScreenshotTelnet.png" target="_blank">TcpExample1ScreenshotTelnet.png</a>
+ * @see <a href="https://savage.nps.edu/Savage/developers.html#Cygwin" target="blank">Savage Developers Guide: Cygwin</a>
+ * @see <a href="https://savage.nps.edu/Savage/developers.html#telnet" target="blank">Savage Developers Guide: telnet</a>
+ * 
+ * @author mcgredo
+ * @author brutzman@nps.edu
+ */
+public class LennonNetworkHW1 
+{
+    /** Default constructor */
+    public LennonNetworkHW1()
+    {
+        // default constructor
+    }
+    /**
+     * Program invocation, execution starts here
+     * @param args command-line arguments
+     */
+    public static void main(String[] args)
+    {
+        try
+        {
+            System.out.println(LennonNetworkHW1.class.getName() + " Server has started and is waiting for a connection.");
+            System.out.println("  help:     https://savage.nps.edu/Savage/developers.html#telnet");
+            System.out.println("  terminal: enter (telnet localhost 2317) or, for macOS (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()) // listen, 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("Welcome to server " + LennonNetworkHW1.class.getName()); // to remote client
+                System.out.println("This server response was written by server " + LennonNetworkHW1.class.getName()); // 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(LennonNetworkHW1.class.getName() + " completed successfully.");
+        }
+        catch(IOException ioe)
+        {
+            System.err.println("Exception with " + LennonNetworkHW1.class.getName() + " networking:"); // describe what is happening
+            System.err.println(ioe);
+            // 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 (ioe instanceof java.net.BindException)
+                System.err.println("*** Be sure to stop any other running instances of programs using this port!");
+        }
+    }
+}