Enhance Java Network Programming code snippets

Author: 100yo

11-network-ii/snippets/echoclientserver/src/bg/sofia/uni/fmi/mjt/echo/net/multithreaded/ClientRequestHandler.java

@@ -8,7 +8,7 @@
 
 public class ClientRequestHandler implements Runnable {
 
-    private Socket socket;
+    private final Socket socket;
 
     public ClientRequestHandler(Socket socket) {
         this.socket = socket;
@@ -20,7 +20,8 @@ public void run() {
         Thread.currentThread().setName("Client Request Handler for " + socket.getRemoteSocketAddress());
 
         try (PrintWriter out = new PrintWriter(socket.getOutputStream(), true); // autoflush on
-             BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()))) {
+             BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
+             socket) { // resources created elsewhere can also be declared here and will be auto-closed
 
             String inputLine;
             while ((inputLine = in.readLine()) != null) { // read the message from the client
@@ -30,12 +31,6 @@ public void run() {
 
         } catch (IOException e) {
             System.out.println(e.getMessage());
-        } finally {
-            try {
-                socket.close();
-            } catch (IOException e) {
-                e.printStackTrace();
-            }
         }
 
     }

11-network-ii/snippets/echoclientserver/src/bg/sofia/uni/fmi/mjt/echo/net/multithreaded/EchoServer.java

@@ -11,12 +11,16 @@ public class EchoServer {
 
     private static final int SERVER_PORT = 4444;
     private static final int MAX_EXECUTOR_THREADS = 10;
+    // In production, consider:
+    // - Runtime.getRuntime().availableProcessors() for CPU-bound tasks
+    // - Larger pool (e.g., availableProcessors() * 2-4) for I/O-bound tasks,
+    //   since threads block waiting for I/O and don't consume CPU
 
     static void main() {
         Thread.currentThread().setName("Echo Server Thread");
 
         try (ServerSocket serverSocket = new ServerSocket(SERVER_PORT);
-             ExecutorService executor = Executors.newFixedThreadPool(MAX_EXECUTOR_THREADS);) {
+             ExecutorService executor = Executors.newFixedThreadPool(MAX_EXECUTOR_THREADS)) {
 
             // Get the local IP address of the server
             InetAddress serverAddress = InetAddress.getLocalHost();

11-network-ii/snippets/echoclientserver/src/bg/sofia/uni/fmi/mjt/echo/net/simple/EchoClient.java

@@ -13,13 +13,11 @@ public class EchoClient {
 
     static void main() {
 
-        try (Socket socket = new Socket("192.168.0.3", SERVER_PORT);
+        try (Socket socket = new Socket("localhost", SERVER_PORT);
              PrintWriter writer = new PrintWriter(socket.getOutputStream(), true); // autoflush on
              BufferedReader reader = new BufferedReader(new InputStreamReader(socket.getInputStream()));
              Scanner scanner = new Scanner(System.in)) {
 
-            System.out.println("Connected to the server.");
-
             while (true) {
                 System.out.print("Enter message: ");
                 String message = scanner.nextLine(); // read a line from the console
@@ -39,6 +37,6 @@ static void main() {
         } catch (IOException e) {
             throw new RuntimeException("There is a problem with the network communication", e);
         }
-        
+
     }
 }

11-network-ii/snippets/echoclientserver/src/bg/sofia/uni/fmi/mjt/echo/net/simple/EchoServer.java

@@ -21,13 +21,14 @@ static void main() {
                 " and listening for connection requests on port " + SERVER_PORT);
 
             try (Socket clientSocket = serverSocket.accept();
-                 BufferedReader br = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
+                 BufferedReader in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
                  PrintWriter out = new PrintWriter(clientSocket.getOutputStream(), true)) { // autoflush on
 
                 String inputLine;
-                while ((inputLine = br.readLine()) != null) {
+                while ((inputLine = in.readLine()) != null) {
                     System.out.println("Message received from client: " + inputLine);
                     out.println("Echo " + inputLine);
+                    //out.flush();
                 }
             }
 

11-network-ii/snippets/echoclientserver/src/bg/sofia/uni/fmi/mjt/echo/nio/EchoClientNio.java

@@ -4,6 +4,7 @@
 import java.net.InetSocketAddress;
 import java.nio.ByteBuffer;
 import java.nio.channels.SocketChannel;
+import java.nio.charset.StandardCharsets;
 import java.util.Scanner;
 
 // NIO, blocking
@@ -13,10 +14,10 @@ public class EchoClientNio {
     private static final String SERVER_HOST = "localhost";
     private static final int BUFFER_SIZE = 512;
 
-    private static ByteBuffer buffer = ByteBuffer.allocateDirect(BUFFER_SIZE);
-
     public static void main() {
 
+        ByteBuffer buffer = ByteBuffer.allocateDirect(BUFFER_SIZE);
+
         try (SocketChannel socketChannel = SocketChannel.open();
              Scanner scanner = new Scanner(System.in)) {
 
@@ -35,7 +36,7 @@ public static void main() {
                 System.out.println("Sending message <" + message + "> to the server...");
 
                 buffer.clear(); // switch to writing mode
-                buffer.put(message.getBytes()); // buffer fill
+                buffer.put(message.getBytes(StandardCharsets.UTF_8)); // buffer fill
                 buffer.flip(); // switch to reading mode
                 socketChannel.write(buffer); // buffer drain
 

11-network-ii/snippets/echoclientserver/src/bg/sofia/uni/fmi/mjt/echo/nio/EchoServer.java

@@ -13,7 +13,12 @@
 
 public class EchoServer {
     public static final int SERVER_PORT = 7777;
-    private static final String SERVER_HOST = "localhost";
+    private static final String SERVER_HOST = "0.0.0.0";
+
+    // 0.0.0.0 is a special address meaning "listen on all network interfaces"
+    // This allows clients to connect via localhost, your local IP address,
+    // or any other IP address your machine has
+
     private static final int BUFFER_SIZE = 1024;
 
     public static void main() {
@@ -52,16 +57,17 @@ public static void main() {
                         if (r < 0) {
                             System.out.println("Client has closed the connection");
                             sc.close();
+                            keyIterator.remove();
                             continue;
                         }
                         buffer.flip();
                         sc.write(buffer);
 
                     } else if (key.isAcceptable()) {
                         ServerSocketChannel sockChannel = (ServerSocketChannel) key.channel();
-                        SocketChannel accept = sockChannel.accept();
-                        accept.configureBlocking(false);
-                        accept.register(selector, SelectionKey.OP_READ);
+                        SocketChannel remoteClientChannel = sockChannel.accept();
+                        remoteClientChannel.configureBlocking(false);
+                        remoteClientChannel.register(selector, SelectionKey.OP_READ);
                     }
 
                     keyIterator.remove();