JSP 02: 执行过程

为什么JSP可以在html中运行java代码？ 这是 步骤 1 : 

步骤1: 执行过程

因为JSP被转译成了Servlet

1. 把 hello.jsp转译为hello_jsp.java
2. hello_jsp.java 位于
　　d:\tomcat\work\Catalina\localhost\_\org\apache\jsp
3. hello_jsp.java是一个servlet
4. 把hello_jsp.java 编译为hello_jsp.class
5. 执行hello_jsp，生成html
6. 通过http协议把html 响应返回给浏览器

 步骤 2 : hello_jsp.java 为什么是Servlet

在Servlet章节中，我们说HelloServlet是一个Servlet，不是因为它的类名里有一个"Servlet"，而是因为它继承了 HttpServlet

打开转译hello.jsp 后得到的hello_jsp.java，可以发现它继承了类
(D:\tomcat\work\Catalina\localhost\_\org\apache\jsp\hello_jsp.java)

org.apache.jasper.runtime.HttpJspBase

而HttpJspBase 继承了HttpServlet

所以我们说hello_.jsp.java 是一个Servlet

/*
 * Generated by the Jasper component of Apache Tomcat
 * Version: Apache Tomcat/7.0.64
 * Generated at: 2017-01-07 01:45:17 UTC
 * Note: The last modified time of this file was set to
 *       the last modified time of the source file after
 *       generation to assist with modification tracking.
 */
package org.apache.jsp;
 
import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.jsp.*;
import java.util.*;
 
public final class hello_jsp extends org.apache.jasper.runtime.HttpJspBase
    implements org.apache.jasper.runtime.JspSourceDependent {
 
  private static final javax.servlet.jsp.JspFactory _jspxFactory =
          javax.servlet.jsp.JspFactory.getDefaultFactory();
 
  private static java.util.Map<java.lang.String,java.lang.Long> _jspx_dependants;
 
  private javax.el.ExpressionFactory _el_expressionfactory;
  private org.apache.tomcat.InstanceManager _jsp_instancemanager;
 
  public java.util.Map<java.lang.String,java.lang.Long> getDependants() {
    return _jspx_dependants;
  }
 
  public void _jspInit() {
    _el_expressionfactory = _jspxFactory.getJspApplicationContext(getServletConfig().getServletContext()).getExpressionFactory();
    _jsp_instancemanager = org.apache.jasper.runtime.InstanceManagerFactory.getInstanceManager(getServletConfig());
  }
 
  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 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; 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("你好 JSP\r\n");
      out.write(" \r\n");
      out.write("<br>\r\n");
      out.write(" \r\n");
      out.print(new Date().toLocaleString());
    } catch (java.lang.Throwable t) {
      if (!(t instanceof javax.servlet.jsp.SkipPageException)){
        out = _jspx_out;
        if (out != null && out.getBufferSize() != 0)
          try {
            if (response.isCommitted()) {
              out.flush();
            } else {
              out.clearBuffer();
            }
          } catch (java.io.IOException e) {}
        if (_jspx_page_context != null) _jspx_page_context.handlePageException(t);
        else throw new ServletException(t);
      }
    } finally {
      _jspxFactory.releasePageContext(_jspx_page_context);
    }
  }
}