利用 Servlet 將 JSP 頁面輸出轉為靜態 HTML 檔案

透過 Servlet 捕獲 JSP 執行後的輸出流，寫入檔案系統，即可實現動態頁面靜態化。以下為完整實作類別 ToHtml：

package examples;

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class ToHtml extends HttpServlet {
    private static final String CONTENT_TYPE = "text/html; charset=GBK";

    public void init() throws ServletException { }

    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType(CONTENT_TYPE);
        service(request, response);
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        doGet(request, response);
    }

    public void destroy() { }

    public void service(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        ServletContext sc = getServletContext();
        String url = "/index.jsp";
        String name = "index.htm"; // 產生的 HTML 檔名
        String pName = "e:/Tomcat 5.5/webapps/jspTohtml/index.htm"; // 完整儲存路徑

        RequestDispatcher rd = sc.getRequestDispatcher(url);
        final ByteArrayOutputStream os = new ByteArrayOutputStream();
        final ServletOutputStream stream = new ServletOutputStream() {
            public void write(byte[] data, int offset, int length) {
                os.write(data, offset, length);
            }
            public void write(int b) throws IOException {
                os.write(b);
            }
        };
        final PrintWriter pw = new PrintWriter(new OutputStreamWriter(os));

        HttpServletResponse rep = new HttpServletResponseWrapper(response) {
            public ServletOutputStream getOutputStream() { return stream; }
            public PrintWriter getWriter() { return pw; }
        };

        rd.include(request, rep); // 執行 JSP 並捕獲輸出
        pw.flush();

        FileOutputStream fos = new FileOutputStream(pName);
        os.writeTo(fos); // 寫入檔案
        fos.close();

        response.sendRedirect(name); // 導向靜態頁面
    }
}

web.xml 配置

<servlet>
    <servlet-name>Tohtml</servlet-name>
    <servlet-class>examples.ToHtml</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>Tohtml</servlet-name>
    <url-pattern>/Tohtml</url-pattern>
</servlet-mapping>

測試用 index.jsp

<%@ page contentType="text/html; charset=gb2312" %>
<html><head><title>Cache Filter Test</title>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312"></head>
<body>簡單測試:s=<%
    int s=0; // 模擬耗時運算
    for (int i=0;i<1000;i++) for (int j=0;j<1000;j++) s=s+j;
    out.print(s);
%></body></html>

運作原理

1. Servlet 透過 RequestDispatcher.include() 執行目標 JSP。
2. 使用 HttpServletResponseWrapper 包裝回應，將輸出導向 ByteArrayOutputStream 暫存。
3. JSP 執行完成後，將位元組陣列寫入指定路徑的 .htm 檔案。
4. 最後導向產生的靜態頁面。

此技術常用於靜態化高流量頁面、減輕伺服器負載。