Zajecia 10. - Obsluga sieci w Java - cwiczenia demonstracyjne (hands-on)

• Czas: 20-30 min
• Forma: prowadzacy koduje na zywo, grupa odtwarza kroki
• Cel: od prostego ECHO TCP do mini HTTP API

• Demo 1 (latwe): ECHO TCP klient-serwer
• Demo 2 (latwe/srednie): UDP ping (datagram)
• Demo 3 (srednie): HTTP klient z HttpClient
• Demo 4 (srednie+): mini serwer HTTP
• Demo 5 (opcjonalne): podstawowe retry + timeout

Cel:

• poznac ServerSocket, Socket, strumienie tekstowe

Wskazowki:

• serwer: petla accept()
• klient: println + readLine
• uruchom serwer przed klientem

Rozwiazanie - serwer:

import java.io.*;
import java.net.*;
 
public class EchoTcpServer {
    public static void main(String[] args) throws IOException {
        try (ServerSocket server = new ServerSocket(5000)) {
            System.out.println("Serwer TCP: 5000");
            while (true) {
                Socket client = server.accept();
                new Thread(() -> handle(client)).start();
            }
        }
    }
 
    private static void handle(Socket client) {
        try (client;
             BufferedReader in = new BufferedReader(new InputStreamReader(client.getInputStream()));
             PrintWriter out = new PrintWriter(client.getOutputStream(), true)) {
 
            String line = in.readLine();
            out.println("ECHO: " + line);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Rozwiazanie - klient:

import java.io.*;
import java.net.*;
 
public class EchoTcpClient {
    public static void main(String[] args) throws IOException {
        try (Socket socket = new Socket("127.0.0.1", 5000);
             BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
             PrintWriter out = new PrintWriter(socket.getOutputStream(), true)) {
 
            out.println("Ala ma kota");
            System.out.println(in.readLine());
        }
    }
}

Cel:

• poznac DatagramSocket i DatagramPacket

Wskazowki:

• bufor odbiorczy ustaw np. na 1024 bajty
• uzyj packet.getLength() przy odczycie
• UDP nie gwarantuje dostarczenia

Rozwiazanie - odbiorca:

import java.net.*;
import java.nio.charset.StandardCharsets;
 
public class UdpReceiver {
    public static void main(String[] args) throws Exception {
        try (DatagramSocket socket = new DatagramSocket(6000)) {
            byte[] buffer = new byte[1024];
            DatagramPacket packet = new DatagramPacket(buffer, buffer.length);
            socket.receive(packet);
 
            String text = new String(packet.getData(), 0, packet.getLength(), StandardCharsets.UTF_8);
            System.out.println("Odebrano: " + text);
        }
    }
}

Rozwiazanie - nadawca:

import java.net.*;
import java.nio.charset.StandardCharsets;
 
public class UdpSender {
    public static void main(String[] args) throws Exception {
        try (DatagramSocket socket = new DatagramSocket()) {
            byte[] data = "ping-udp".getBytes(StandardCharsets.UTF_8);
            DatagramPacket packet = new DatagramPacket(
                data,
                data.length,
                InetAddress.getByName("127.0.0.1"),
                6000
            );
            socket.send(packet);
        }
    }
}

Cel:

• wykonac zapytanie GET przez HttpClient

Wskazowki:

• ustaw timeout po stronie klienta
• wypisz statusCode i fragment odpowiedzi
• przy bledach sieciowych obsluz IOException

Rozwiazanie:

import java.net.URI;
import java.net.http.*;
import java.time.Duration;
 
public class HttpGetDemo {
    public static void main(String[] args) throws Exception {
        HttpClient client = HttpClient.newBuilder()
            .connectTimeout(Duration.ofSeconds(5))
            .build();
 
        HttpRequest request = HttpRequest.newBuilder()
            .uri(URI.create("https://api.github.com"))
            .header("Accept", "application/json")
            .GET()
            .build();
 
        HttpResponse<String> response = client.send(request, HttpResponse.BodyHandlers.ofString());
        System.out.println("Status: " + response.statusCode());
        System.out.println(response.body().substring(0, Math.min(200, response.body().length())));
    }
}

Cel:

• przygotowac endpoint /time zwracajacy JSON

Wskazowki:

• uzyj com.sun.net.httpserver.HttpServer
• ustaw Content-Type i kod odpowiedzi 200
• zamknij OutputStream po zapisie odpowiedzi

Rozwiazanie:

import com.sun.net.httpserver.HttpServer;
import java.io.OutputStream;
import java.net.InetSocketAddress;
import java.nio.charset.StandardCharsets;
import java.time.Instant;
 
public class MiniHttpServer {
    public static void main(String[] args) throws Exception {
        HttpServer server = HttpServer.create(new InetSocketAddress(8080), 0);
        server.createContext("/time", exchange -> {
            String body = "{\"time\":\"" + Instant.now() + "\"}";
            exchange.getResponseHeaders().add("Content-Type", "application/json; charset=utf-8");
            byte[] bytes = body.getBytes(StandardCharsets.UTF_8);
            exchange.sendResponseHeaders(200, bytes.length);
            try (OutputStream os = exchange.getResponseBody()) {
                os.write(bytes);
            }
        });
        server.start();
        System.out.println("HTTP server: http://localhost:8080/time");
    }
}

Cel:

• pokazac podejscie do niestabilnej sieci

Wskazowki:

• ogranicz liczbe prob (np. 3)
• roznicuj opoznienie miedzy probami
• retry glownie dla bledow 5xx i timeout

Rozwiazanie:

for (int attempt = 1; attempt <= 3; attempt++) {
    try {
        HttpResponse<String> r = client.send(request, HttpResponse.BodyHandlers.ofString());
        if (r.statusCode() < 500) {
            System.out.println("OK w probie " + attempt);
            break;
        }
    } catch (Exception e) {
        if (attempt == 3) throw e;
    }
    Thread.sleep(200L * attempt);
}

• Wariant 20 min: Demo 1 + Demo 3
• Wariant 30 min: Demo 1 + Demo 2 + Demo 4
• Dla zaawansowanej grupy: dorzuc Demo 5