Socket Programming in Java

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.List;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
 
public class EchoServer {
    // Server side: one virtual thread per connected client.
    static void serve(ServerSocket server) {
        try (ExecutorService pool =
                 Executors.newVirtualThreadPerTaskExecutor()) {
            while (true) {
                Socket client = server.accept();
                pool.submit(() -> handle(client));
            }
        } catch (IOException e) {
            // accept() fails when the server socket is closed: time to stop.
        }
    }
 
    static void handle(Socket client) {
        try (client;
             BufferedReader in = new BufferedReader(
                 new InputStreamReader(client.getInputStream()));
             PrintWriter out =
                 new PrintWriter(client.getOutputStream(), true)) {
            String line;
            while ((line = in.readLine()) != null) {
                if (line.equals("bye")) {
                    out.println("Goodbye!");
                    break;
                }
                out.println("ECHO: " + line.toUpperCase());
            }
        } catch (IOException e) {
            System.out.println("Client problem: " + e.getMessage());
        }
    }
 
    // Client side: send two lines, read two replies, then say bye.
    static String talk(int port, String name) throws IOException {
        try (Socket socket = new Socket("localhost", port);
             BufferedReader in = new BufferedReader(
                 new InputStreamReader(socket.getInputStream()));
             PrintWriter out =
                 new PrintWriter(socket.getOutputStream(), true)) {
            socket.setSoTimeout(3000);
            out.println("hello from " + name);
            String first = in.readLine();
            out.println("bye");
            String last = in.readLine();
            return name + " got [" + first + "] then [" + last + "]";
        }
    }
 
    public static void main(String[] args) throws Exception {
        try (ServerSocket server = new ServerSocket(0)) {
            int port = server.getLocalPort();
            Thread.ofVirtual().start(() -> serve(server));
 
            try (ExecutorService clients =
                     Executors.newVirtualThreadPerTaskExecutor()) {
                List<Future<String>> results = List.of(
                    clients.submit(() -> talk(port, "Navika")),
                    clients.submit(() -> talk(port, "Srihitha")),
                    clients.submit(() -> talk(port, "Advitha")));
                for (Future<String> r : results) {
                    System.out.println(r.get());
                }
            }
        }
        System.out.println("Server stopped");
    }
}

import java.io.IOException;
import java.net.InetSocketAddress;
import java.net.ServerSocket;
import java.net.Socket;
 
public class Fail {
    public static void main(String[] args) throws Exception {
        // 1. Nothing is listening on this port.
        int freePort;
        try (ServerSocket probe = new ServerSocket(0)) {
            freePort = probe.getLocalPort();
        }
        try (Socket s = new Socket()) {
            s.connect(new InetSocketAddress("localhost", freePort), 2000);
        } catch (IOException e) {
            System.out.println("1. " + e.getClass().getSimpleName());
        }
 
        // 2. The server accepts the connection but never sends anything.
        try (ServerSocket silent = new ServerSocket(0);
             Socket s = new Socket("localhost", silent.getLocalPort())) {
            s.setSoTimeout(300);
            s.getInputStream().read();
        } catch (IOException e) {
            System.out.println("2. " + e.getClass().getSimpleName());
        }
 
        // 3. Two servers try to use the same port.
        try (ServerSocket first = new ServerSocket(0);
             ServerSocket second = new ServerSocket(first.getLocalPort())) {
            System.out.println("should not happen");
        } catch (IOException e) {
            System.out.println("3. " + e.getClass().getSimpleName());
        }
    }
}