Java Socket 網路程式設計入門：從單執行緒到多執行緒伺服器

網路程式設計核心模型：Client-Server

本質上是兩個行程通訊：Server 提供固定位置（IP + Port），Client 主動連線。Java 在 java.net 封裝底層細節，核心類別：ServerSocket、Socket、InetAddress。

基礎建構：單一連線回聲伺服器

伺服器端

ServerSocket server = new ServerSocket(5678); // 埠號 > 1024
Socket client = server.accept(); // 阻塞等待連線
BufferedReader in = new BufferedReader(new InputStreamReader(client.getInputStream()));
PrintWriter out = new PrintWriter(client.getOutputStream(), true); // auto-flush

while (true) {
    String str = in.readLine();
    System.out.println("收到: " + str);
    out.println("has receive....");
    if ("end".equalsIgnoreCase(str)) break;
}
client.close();

客戶端

Socket server = new Socket(InetAddress.getLocalHost(), 5678);
BufferedReader in = new BufferedReader(new InputStreamReader(server.getInputStream()));
PrintWriter out = new PrintWriter(server.getOutputStream(), true);
BufferedReader keyboard = new BufferedReader(new InputStreamReader(System.in));

String line;
while ((line = keyboard.readLine()) != null) {
    out.println(line);
    if ("end".equalsIgnoreCase(line)) break;
    System.out.println("伺服器回應: " + in.readLine());
}
server.close();

進階：支援多客戶端的迴圈伺服器

外層 while(true) 迴圈接受連線，但同一時間只能服務一個客戶（排隊）：

while (true) {
    Socket client = server.accept();
    // 處理單一客戶完整對話...
    client.close();
}

最終形態：多執行緒並發伺服器

每個連線派生獨立執行緒處理，實現真正並發：

public class MultiUserServer {
    public static void main(String[] args) throws IOException {
        ServerSocket server = new ServerSocket(5678);
        System.out.println("伺服器啟動，監聽埠 5678...");
        while (true) {
            Socket client = server.accept();
            new ClientHandler(client).start(); // 啟動執行緒
        }
    }
}

class ClientHandler extends Thread {
    private final Socket client;
    public ClientHandler(Socket socket) { this.client = socket; }
    @Override
    public void run() {
        try (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("[" + client.getInetAddress() + "] " + line);
                out.println("has receive....");
                if ("end".equalsIgnoreCase(line)) break;
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try { client.close(); } catch (IOException ignored) {}
        }
    }
}

關鍵點：執行緒透過建構子傳遞 Socket 參考，各自擁有獨立 I/O 通道。生產環境建議使用執行緒池（ExecutorService）限制並發數量。

I/O 流處理要訣

• 網路傳輸為位元組流，必須透過 InputStreamReader/OutputStreamWriter 轉為字元流。
• BufferedReader.readLine() 以換行符為界，適合文本協議。
• PrintWriter 建構子啟用 autoFlush=true 確保 println 即時發送。