JSP HttpServletRequest 請求物件完整指南

HttpServletRequest 封裝客戶端 HTTP 請求。JSP 隱含物件 request 即為其實例，提供取得參數、標頭、Cookie、Session、屬性、輸入流、客戶端資訊等功能。

HTTP 請求結構

GET /search?q=Java&cat=programming HTTP/1.1
Host: example.com
User-Agent: Mozilla/5.0 ...
Accept: text/html,application/xhtml+xml
Accept-Language: zh-TW,zh;q=0.9,en;q=0.8
Cookie: JSESSIONID=abc123; lang=zh-TW
Connection: keep-alive

• 請求行：方法、URL、HTTP 版本
• 請求標頭：Key-Value
• 空行
• 請求主體 (POST/PUT)：表單資料、JSON、檔案

核心 API 分類速查

1. 參數取得 (表單、查詢字串)

2. 標頭資訊

3. Cookie 讀取

4. Session 管理

5. 請求屬性 (伺服器端暫存資料)

6. 客戶端與連線資訊

7. URL 路徑相關

8. 輸入流 (讀取原始 Body)

實作範例

1. 印出所有請求標頭 (除錯用)

<%@ page contentType="text/html;charset=UTF-8" import="java.util.Enumeration" %>
<!DOCTYPE html>
<html><head><meta charset="UTF-8"><title>Request Headers</title></head>
<body>
  <h2>請求標頭</h2>
  <table border="1"><tr><th>名稱</th><th>值</th></tr>
  <%
    Enumeration names = request.getHeaderNames();
    while (names.hasMoreElements()) {
      String name = names.nextElement();
      out.println("<tr><td>" + name + "</td><td>" + request.getHeader(name) + "</td></tr>");
    }
  %>
  </table>
</body></html>

2. 取得真實客戶端 IP (反向代理/負載平衡器後)

public String getClientIp(HttpServletRequest request) {
  String ip = request.getHeader("X-Forwarded-For");
  if (ip == null || ip.isEmpty() || "unknown".equalsIgnoreCase(ip)) {
    ip = request.getHeader("Proxy-Client-IP");
  }
  if (ip == null || ip.isEmpty() || "unknown".equalsIgnoreCase(ip)) {
    ip = request.getHeader("WL-Proxy-Client-IP");
  }
  if (ip == null || ip.isEmpty() || "unknown".equalsIgnoreCase(ip)) {
    ip = request.getRemoteAddr();
  }
  // X-Forwarded-For 可能包含多個 IP，取第一個
  if (ip != null && ip.contains(",")) {
    ip = ip.split(",")[0].trim();
  }
  return ip;
}

3. 讀取 JSON 請求主體 (REST API)

<%@ page import="java.io.*, jakarta.servlet.*" %>
<%
  StringBuilder sb = new StringBuilder();
  try (BufferedReader br = request.getReader()) {
    String line;
    while ((line = br.readLine()) != null) sb.append(line);
  }
  String json = sb.toString(); // {"action":"login","username":"admin"}
  // 使用 Jackson/Gson 解析...
%>

4. Request 範圍傳遞資料 (Forward 時)

<%-- Controller Servlet --%>
request.setAttribute("userList", userService.findAll());
request.getRequestDispatcher("/user/list.jsp").forward(request, response);

<%-- list.jsp --%>
<c:forEach items="${userList}" var="u">
  <tr><td>${u.name}</td><td>${u.email}</td></tr>
</c:forEach>

中文編碼問題總整理

常見方法對照表 (JSP 隱含物件 vs Servlet API)