import java.net.InetAddress;

public class InetAddressExample {
    public static void main(String[] args) throws Exception {
        // 로컬 호스트 정보
        InetAddress local = InetAddress.getLocalHost();
        System.out.println("호스트 이름: " + local.getHostName());
        System.out.println("IP 주소: " + local.getHostAddress());

        // 도메인으로 IP 조회
        InetAddress addr = InetAddress.getByName("www.google.com");
        System.out.println("Google IP: " + addr.getHostAddress());

        // 하나의 도메인에 여러 IP가 매핑된 경우
        InetAddress[] all = InetAddress.getAllByName("www.google.com");
        for (InetAddress a : all) {
            System.out.println("IP: " + a.getHostAddress());
        }
    }
}

import java.net.URL;

public class URLExample {
    public static void main(String[] args) throws Exception {
        URL url = new URL("https://example.com:8080/api/data?type=json#result");

        System.out.println("프로토콜: " + url.getProtocol());   // https
        System.out.println("호스트: " + url.getHost());         // example.com
        System.out.println("포트: " + url.getPort());           // 8080
        System.out.println("경로: " + url.getPath());           // /api/data
        System.out.println("쿼리: " + url.getQuery());          // type=json
        System.out.println("참조: " + url.getRef());            // result
    }
}

import java.io.*;
import java.net.URL;

public class URLReadExample {
    public static void main(String[] args) throws Exception {
        URL url = new URL("https://www.example.com");

        try (BufferedReader br = new BufferedReader(
                new InputStreamReader(url.openStream()))) {
            String line;
            while ((line = br.readLine()) != null) {
                System.out.println(line);
            }
        }
    }
}

import java.io.*;
import java.net.*;

public class URLConnectionExample {
    public static void main(String[] args) throws Exception {
        URL url = new URL("https://www.example.com");
        HttpURLConnection conn = (HttpURLConnection) url.openConnection();

        // 응답 정보 확인
        System.out.println("응답 코드: " + conn.getResponseCode());      // 200
        System.out.println("콘텐츠 타입: " + conn.getContentType());     // text/html
        System.out.println("콘텐츠 길이: " + conn.getContentLength());

        // 응답 본문 읽기
        try (BufferedReader br = new BufferedReader(
                new InputStreamReader(conn.getInputStream()))) {
            String line;
            while ((line = br.readLine()) != null) {
                System.out.println(line);
            }
        }

        conn.disconnect();
    }
}

// Socket — 프로세스 간 통신을 담당
//   InputStream과 OutputStream을 통해 데이터 송수신

// ServerSocket — 포트에 바인딩되어 연결 요청을 대기
//   연결 요청이 오면 새로운 Socket을 생성하여 통신
//   한 포트에 하나의 ServerSocket만 연결 가능

import java.io.*;
import java.net.*;

public class TcpServer {
    public static void main(String[] args) {
        try (ServerSocket serverSocket = new ServerSocket(9000)) {
            System.out.println("[서버] 연결 대기 중...");

            // 클라이언트 연결 요청 대기 (블로킹)
            Socket socket = serverSocket.accept();
            System.out.println("[서버] 클라이언트 연결됨: "
                    + socket.getInetAddress());

            // 스트림 생성
            BufferedReader in = new BufferedReader(
                    new InputStreamReader(socket.getInputStream()));
            PrintWriter out = new PrintWriter(
                    socket.getOutputStream(), true);

            // 메시지 수신
            String received = in.readLine();
            System.out.println("[서버] 수신: " + received);

            // 응답 전송
            out.println("Hello from Server!");

            socket.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

import java.io.*;
import java.net.*;

public class TcpClient {
    public static void main(String[] args) {
        try (Socket socket = new Socket("localhost", 9000)) {
            System.out.println("[클라이언트] 서버에 연결됨");

            // 스트림 생성
            PrintWriter out = new PrintWriter(
                    socket.getOutputStream(), true);
            BufferedReader in = new BufferedReader(
                    new InputStreamReader(socket.getInputStream()));

            // 메시지 전송
            out.println("Hello from Client!");

            // 응답 수신
            String response = in.readLine();
            System.out.println("[클라이언트] 수신: " + response);

        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

import java.io.*;
import java.net.*;

public class MultiClientServer {
    public static void main(String[] args) throws IOException {
        ServerSocket serverSocket = new ServerSocket(9000);
        System.out.println("[서버] 다중 클라이언트 대기 중...");

        while (true) {
            Socket socket = serverSocket.accept();
            System.out.println("[서버] 새 클라이언트: "
                    + socket.getInetAddress());

            // 클라이언트마다 쓰레드 생성
            new Thread(() -> handleClient(socket)).start();
        }
    }

    private static void handleClient(Socket socket) {
        try (BufferedReader in = new BufferedReader(
                new InputStreamReader(socket.getInputStream()));
             PrintWriter out = new PrintWriter(
                socket.getOutputStream(), true)) {

            String msg;
            while ((msg = in.readLine()) != null) {
                System.out.println("[수신] " + msg);
                out.println("에코: " + msg);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

import java.io.*;
import java.net.*;
import java.util.concurrent.*;

public class ThreadPoolServer {
    public static void main(String[] args) throws IOException {
        ServerSocket serverSocket = new ServerSocket(9000);
        ExecutorService pool = Executors.newFixedThreadPool(10);

        System.out.println("[서버] 쓰레드 풀 서버 시작 (최대 10 쓰레드)");

        while (true) {
            Socket socket = serverSocket.accept();
            pool.execute(() -> handleClient(socket));
        }
    }

    private static void handleClient(Socket socket) {
        try (BufferedReader in = new BufferedReader(
                new InputStreamReader(socket.getInputStream()));
             PrintWriter out = new PrintWriter(
                socket.getOutputStream(), true)) {

            String msg;
            while ((msg = in.readLine()) != null) {
                out.println("응답: " + msg);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

import java.net.*;

public class UdpReceiver {
    public static void main(String[] args) throws Exception {
        DatagramSocket socket = new DatagramSocket(9000);
        System.out.println("[수신] 데이터 대기 중...");

        byte[] buffer = new byte[1024];
        DatagramPacket packet = new DatagramPacket(buffer, buffer.length);

        // 데이터 수신 (블로킹)
        socket.receive(packet);

        String received = new String(
                packet.getData(), 0, packet.getLength());
        System.out.println("[수신] " + received);
        System.out.println("[송신자] " + packet.getAddress()
                + ":" + packet.getPort());

        socket.close();
    }
}

import java.net.*;

public class UdpSender {
    public static void main(String[] args) throws Exception {
        DatagramSocket socket = new DatagramSocket();

        String message = "Hello UDP!";
        byte[] data = message.getBytes();

        // 수신 대상 주소 지정
        InetAddress address = InetAddress.getByName("localhost");
        DatagramPacket packet = new DatagramPacket(
                data, data.length, address, 9000);

        // 데이터 전송
        socket.send(packet);
        System.out.println("[송신] " + message);

        socket.close();
    }
}