JSP脚本中的9个内置对象：
application: javax.servlet.ServletContext
config: javax.servlet.ServletConfig
exception: java.lang.Throwable
out: javax.servlet.jsp.JspWriter
page: javax.servlet.Servlet中的this
pageContext: javax.servlet.jsp.PageContext
request: javax.servlet.http.HttpServletRequest
response: javax.servlet.http.HttpServletResponse
session: javax.servlet.http.HttpSession