JSP入门和JSP运行原理

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


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