Java 網路程式設計實戰：TCP/UDP Socket 核心差異、代碼範例與多執行緒擴展指南

Socket 是網路程式設計核心抽象。Java 提供 java.net.Socket (TCP) 與 DatagramSocket (UDP) 兩套 API。本文對比兩大協定特性，提供完整客戶端/伺服端代碼範例，並指導多執行緒併發擴展與 GUI 整合方向。

TCP vs UDP 核心差異

TCP 範例：回聲伺服器與客戶端

伺服端

public class TcpEchoServer {
    public static void main(String[] args) throws IOException {
        try (ServerSocket server = new ServerSocket(2000)) {
            System.out.println("TCP Server listening on port 2000...");
            while (true) {
                try (Socket client = server.accept();
                     BufferedReader in = new BufferedReader(new InputStreamReader(client.getInputStream()));
                     PrintWriter out = new PrintWriter(client.getOutputStream(), true)) {
                    
                    String line;
                    while ((line = in.readLine()) != null) {
                        System.out.println("Received: " + line);
                        out.println("Echo: " + line); // 回傳
                    }
                } catch (IOException e) {
                    System.err.println("Client error: " + e.getMessage());
                }
            }
        }
    }
}

客戶端

public class TcpEchoClient {
    public static void main(String[] args) throws IOException {
        try (Socket socket = new Socket("localhost", 2000);
             PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
             BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
             BufferedReader stdIn = new BufferedReader(new InputStreamReader(System.in))) {
            
            String userInput;
            while ((userInput = stdIn.readLine()) != null) {
                out.println(userInput);
                System.out.println("Server: " + in.readLine());
            }
        }
    }
}

UDP 範例：資料報發送與接收

發送端

public class UdpSender {
    public static void main(String[] args) throws IOException {
        if (args.length < 3) {
            System.err.println("Usage: java UdpSender    [-f file]");
            return;
        }
        
        InetAddress address = InetAddress.getByName(args[0]);
        int port = Integer.parseInt(args[1]);
        byte[] data;
        
        if ("-f".equals(args[2])) {
            // 讀取檔案
            Path path = Paths.get(args[3]);
            data = Files.readAllBytes(path);
        } else {
            // 合併剩餘參數為訊息
            String msg = String.join(" ", Arrays.copyOfRange(args, 2, args.length));
            data = msg.getBytes(StandardCharsets.UTF_8);
        }
        
        try (DatagramSocket socket = new DatagramSocket()) {
            DatagramPacket packet = new DatagramPacket(data, data.length, address, port);
            socket.send(packet);
            System.out.println("Sent " + data.length + " bytes to " + address + ":" + port);
        }
    }
}

接收端

public class UdpReceiver {
    public static void main(String[] args) throws IOException {
        if (args.length != 1) {
            System.err.println("Usage: java UdpReceiver ");
            return;
        }
        
        int port = Integer.parseInt(args[0]);
        byte[] buffer = new byte[2048];
        
        try (DatagramSocket socket = new DatagramSocket(port)) {
            System.out.println("UDP Receiver listening on port " + port);
            while (true) {
                DatagramPacket packet = new DatagramPacket(buffer, buffer.length);
                socket.receive(packet); // 阻塞等待
                
                String msg = new String(packet.getData(), 0, packet.getLength(), StandardCharsets.UTF_8);
                System.out.println(packet.getAddress().getHostAddress() + ": " + msg);
                
                // 重設長度供下次複用 (Java 1.1+ 必須)
                packet.setLength(buffer.length);
            }
        }
    }
}

關鍵技術細節

1. UDP 緩衝區大小：buffer.length 限制單次接收上限，超截資料將丟失。大檔案需分片或改用 TCP
2. DatagramPacket.setLength()：複用 Packet 前必須重設長度，否則後續接收會被截斷
3. TCP 訊息邊界：readLine() 依賴換行符；二進位協議需自定義長度前綴或分隔符
4. 資源管理：try-with-resources 確保 Socket/Stream 正確關閉

多執行緒併發擴展：TCP 伺服端模板

public class ConcurrentTcpServer {
    private static final ExecutorService pool = Executors.newCachedThreadPool();
    
    public static void main(String[] args) throws IOException {
        try (ServerSocket server = new ServerSocket(2000)) {
            System.out.println("Concurrent TCP Server started...");
            while (true) {
                Socket client = server.accept();
                pool.submit(() -> handleClient(client));
            }
        } finally {
            pool.shutdown();
        }
    }
    
    private static void handleClient(Socket client) {
        try (BufferedReader in = new BufferedReader(new InputStreamReader(client.getInputStream()));
             PrintWriter out = new PrintWriter(client.getOutputStream(), true)) {
            
            String line;
            while ((line = in.readLine()) != null) {
                out.println("Thread-" + Thread.currentThread().getId() + ": " + line);
            }
        } catch (IOException e) {
            System.err.println("Client handler error: " + e.getMessage());
        }
    }
}

GUI 整合方向：Swing/JavaFX 聊天室原型

• 客戶端：JTextArea 顯示訊息、JTextField 輸入、背景執行緒接收伺服端推送
• 伺服端：維護 Set<PrintWriter> 廣播訊息給所有連線客戶端
• 關鍵點：Swing 單執行緒規則 → SwingUtilities.invokeLater() 更新 UI

生產級增強清單

1. 連線/閒置超時設定：socket.setSoTimeout()
2. SSL/TLS 加密：SSLSocketFactory / SSLServerSocketFactory
3. 訊息協議框架：Netty、Mina、gRPC 替代裸 Socket
4. 服務發現與負載均衡：Nacos、Consul、Kubernetes Service
5. 監控指標：連線數、QPS、延遲分位、錯誤率 (Micrometer + Prometheus)
6. 優雅關閉：訊號處理、執行緒池關閉、進行中請求完成

掌握 TCP/UDP Socket 基礎，是理解 HTTP、RPC、微服務通訊底層原理的關鍵。從裸 Socket 起步，逐步引入成熟框架，能建立扎實的網路程式設計功力。