###### tags: `Servlet&JSP` # JSP 最後還是會被Tomcat9轉譯為 Servlet 原始碼 ## 希望是觀念? 基本上 Servlet 作的到的功能，使用 JSP 也都作的到。 JSP 最後還是會被容器(web容器 Tomcat9)轉譯為 Servlet 原始碼、自動編譯為 .class 檔案、載入 .class 檔案然後生成 Servlet 物件。 JSP 網頁範例： ``` <!DOCTYPE html> <html> <head> <title>Hello! World!</title> </head> <body> <h1>Hello! World!</h1> </body> </html> ``` JSP 網頁最後還是成爲 Servlet，在第一次請求 JSP 時，容器會進行轉譯、編譯與載入的動作（所以第一次請求JSP頁面回應較慢） 由Tomcat9容器轉譯後的 Servlet 類別如下: ``` package org.apache.jsp; import javax.servlet.*; import javax.servlet.http.*; import javax.servlet.jsp.*; public final class helloworld_jsp extends org.apache.jasper.runtime.HttpJspBase implements org.apache.jasper.runtime.JspSourceDependent, org.apache.jasper.runtime.JspSourceImports { ...略 public void _jspInit() { // 略... } public void _jspDestroy() { } public void _jspService(final javax.servlet.http.HttpServletRequest request, final javax.servlet.http.HttpServletResponse response) throws java.io.IOException, javax.servlet.ServletException { final java.lang.String _jspx_method = request.getMethod(); if (!"GET".equals(_jspx_method) && !"POST".equals(_jspx_method) && !"HEAD".equals(_jspx_method) && !javax.servlet.DispatcherType.ERROR.equals(request.getDispatcherType())) { response.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, "JSPs only permit GET POST or HEAD"); return; } final javax.servlet.jsp.PageContext pageContext; javax.servlet.http.HttpSession session = null; final javax.servlet.ServletContext application; final javax.servlet.ServletConfig config; javax.servlet.jsp.JspWriter out = null; final java.lang.Object page = this; javax.servlet.jsp.JspWriter _jspx_out = null; javax.servlet.jsp.PageContext _jspx_page_context = null; try { response.setContentType("text/html"); pageContext = _jspxFactory.getPageContext(this, request, response, null, true, 8192, true); _jspx_page_context = pageContext; application = pageContext.getServletContext(); config = pageContext.getServletConfig(); session = pageContext.getSession(); out = pageContext.getOut(); _jspx_out = out; out.write("<!DOCTYPE html>\r\n"); out.write("<html>\r\n"); out.write(" <head>\r\n"); out.write(" <title>Hello! World!</title>\r\n"); out.write(" </head>\r\n"); out.write(" <body>\r\n"); out.write(" <h1>Hello! World!</h1>\r\n"); out.write(" </body>\r\n"); out.write("</html>"); } catch (java.lang.Throwable t) { ...略 } finally { ...略 } } } ``` 現在聚焦於 _jspInit()、 _jspDestroy() 與 _jspService() 這三個方法。