核心架構

基於 `ServerSocket` + 多執行緒模型：主執行緒監聽端口，每個客戶端連接啟動一條 `ConnectionThread` 處理通信。

完整代碼實現

// WebServer.java
import java.io.*;
import java.net.*;

public class WebServer {
    public static void main(String args[]) {
        int i = 1, PORT = 8080;
        ServerSocket server = null;
        Socket client = null;
        try {
            server = new ServerSocket(PORT);
            System.out.println("Web Server is listening on port " + server.getLocalPort());
            for (;;) {
                client = server.accept();  // 阻塞等待客戶端連接
                new ConnectionThread(client, i).start();
                i++;
            }
        } catch (Exception e) {
            System.out.println(e);
        }
    }
}

/* ConnectionThread：處理單個瀏覽器通信 */
class ConnectionThread extends Thread {
    Socket client;
    int counter;

    public ConnectionThread(Socket cl, int c) {
        client = cl;
        counter = c;
    }

    public void run() {
        try {
            String destIP = client.getInetAddress().toString();
            int destPort = client.getPort();
            System.out.println("Connection " + counter + ": connected to " + destIP + " on port " + destPort + ".");

            PrintStream outStream = new PrintStream(client.getOutputStream());
            DataInputStream inStream = new DataInputStream(client.getInputStream());
            String inLine = inStream.readLine();  // 讀取 HTTP 請求行
            System.out.println("Received: " + inLine);

            if (getRequest(inLine)) {  // 僅處理 GET 請求
                String fileName = getFileName(inLine);
                File file = new File(fileName);
                if (file.exists()) {
                    // 返回 200 OK + 文件內容
                    System.out.println(fileName + " requested.");
                    outStream.println("HTTP/1.0 200 OK");
                    outStream.println("MIME_version: 1.0");
                    outStream.println("Content_Type: text/html");
                    int len = (int) file.length();
                    outStream.println("Content_Length: " + len);
                    outStream.println("");  // 空行分隔 Header 與 Body
                    sendFile(outStream, file);
                    outStream.flush();
                } else {
                    // 返回 404 Not Found
                    String msg = "
Error 404 - file not found
";
                    outStream.println("HTTP/1.0 404 Not Found");
                    outStream.println("Content_Type: text/html");
                    outStream.println("Content_Length: " + (msg.length() + 2));
                    outStream.println("");
                    outStream.println(msg);
                    outStream.flush();
                }
            }
            // 簡單延遲模擬處理時間
            long m1 = 1;
            while (m1 < 11100000) { m1++; }
            client.close();
        } catch (IOException e) {
            System.out.println("Exception: " + e);
        }
    }

    /* 判斷是否為 GET 請求 */
    boolean getRequest(String s) {
        if (s.length() > 0 && s.substring(0, 3).equalsIgnoreCase("GET"))
            return true;
        return false;
    }

    /* 從請求行解析文件名 */
    String getFileName(String s) {
        String f = s.substring(s.indexOf(' ') + 1);
        f = f.substring(0, f.indexOf(' '));
        try {
            if (f.charAt(0) == '/') f = f.substring(1);
        } catch (StringIndexOutOfBoundsException e) {
            System.out.println("Exception: " + e);
        }
        if (f.equals("")) f = "index.html";
        return f;
    }

    /* 發送文件內容到輸出流 */
    void sendFile(PrintStream out, File file) {
        try {
            DataInputStream in = new DataInputStream(new FileInputStream(file));
            int len = (int) file.length();
            byte[] buf = new byte[len];
            in.readFully(buf);
            out.write(buf, 0, len);
            out.flush();
            in.close();
        } catch (Exception e) {
            System.out.println("Error retrieving file.");
            System.exit(1);
        }
    }
}

關鍵技術點解析

1. ServerSocket.accept()：阻塞式等待客戶端連接，返回新 `Socket` 專責該連接通信
2. 多執行緒隔離：每連接一條 `Thread`，互不干擾，支援併發
3. HTTP 協議極簡實現：解析請求行 → 讀取文件 → 發送響應 Header + Body
4. MIME 類型：此處固定 `text/html`，實際應根據副檔名動態判斷
5. 資源關閉：`finally` 區塊確保 `Socket`、`Stream` 正確關閉，防洩漏

擴展方向

• 支援 POST、HEAD 等更多 HTTP 方法
• 動態 MIME 類型映射
• 靜態資源快取、Gzip 壓縮
• 請求路由、動態頁面模板引擎
• SSL/TLS 支援（HTTPS）

這個極簡 Web Server 能跑通 HTTP 基礎流程，是理解 Servlet 容器（如 Tomcat）底層原理的絕佳切入點。