JSP入门和JSP运行原理-优快云博客

本文解释了JSP是如何被服务器转换为Servlet的，并通过一个显示当前时间的例子详细说明了这一过程。展示了JSP如何利用Servlet的功能，如直接访问out、application和session等资源。

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你访问JSP的时候，实际上浏览器访问的并不是JSP，而是由服务器把这个JSP页面翻译成的对应的Servlet，这个Servlet对于Tomcat来说，存放在Work目录你的Web项目的下面，他是继承自HttpJspBase这个抽象类，同时这个抽象类继承了Servlet，所以JSP说白了还是一个Servlet.

下面举一个例子，使用JSP页面显示当前时间

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%> <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <html> <head> <title>显示当前时间</title> </head> <body> 当前时间是: <% Date date = new Date(); String time = date.toLocaleString(); out.write(time); %> </body> </html>
然后你可以在服务器的目录下面找到这个JSP被翻译成的Servlet

package org.apache.jsp; import javax.servlet.*; import javax.servlet.http.*; import javax.servlet.jsp.*; import java.util.*; public final class _1_jsp extends org.apache.jasper.runtime.HttpJspBase implements org.apache.jasper.runtime.JspSourceDependent { private static final JspFactory _jspxFactory = JspFactory.getDefaultFactory(); private static java.util.List _jspx_dependants; private javax.el.ExpressionFactory _el_expressionfactory; private org.apache.AnnotationProcessor _jsp_annotationprocessor; public Object getDependants() { return _jspx_dependants; } public void _jspInit() { _el_expressionfactory = _jspxFactory.getJspApplicationContext(getServletConfig().getServletContext()).getExpressionFactory(); _jsp_annotationprocessor = (org.apache.AnnotationProcessor) getServletConfig().getServletContext().getAttribute(org.apache.AnnotationProcessor.class.getName()); } public void _jspDestroy() { } public void _jspService(HttpServletRequest request, HttpServletResponse response) throws java.io.IOException, ServletException { PageContext pageContext = null; HttpSession session = null; ServletContext application = null; ServletConfig config = null; JspWriter out = null; Object page = this; JspWriter _jspx_out = null; PageContext _jspx_page_context = null; try { response.setContentType("text/html;charset=UTF-8"); 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("\r\n"); out.write("\r\n"); out.write("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">\r\n"); out.write("<html>\r\n"); out.write(" <head>\r\n"); out.write(" <title>显示当前时间</title>\r\n"); out.write(" \r\n"); out.write(" </head>\r\n"); out.write(" \r\n"); out.write(" <body>\r\n"); out.write(" 当前时间是:\r\n"); out.write(" "); Date date = new Date(); String time = date.toLocaleString(); out.write(time); out.write("\r\n"); out.write(" </body>\r\n"); out.write("</html>\r\n"); } catch (Throwable t) { if (!(t instanceof SkipPageException)){ out = _jspx_out; if (out != null && out.getBufferSize() != 0) try { out.clearBuffer(); } catch (java.io.IOException e) {} if (_jspx_page_context != null) _jspx_page_context.handlePageException(t); } } finally { _jspxFactory.releasePageContext(_jspx_page_context); } } }
注意这段

public void _jspService(HttpServletRequest request, HttpServletResponse response) throws java.io.IOException, ServletException { PageContext pageContext = null; HttpSession session = null; ServletContext application = null; ServletConfig config = null; JspWriter out = null; Object page = this; JspWriter _jspx_out = null; PageContext _jspx_page_context = null;这样就可以理解，为什么JSP里面可以直接使用out,application和session等资源而不需要获取和创建了，因为JSP的底层Servlet已经帮你实现好了，所以你根本不需要什么了，呵呵，感觉很不错哦。亲