servlet http异常
response.sendError可以自定义当前页面http错误状态，HttpServletResponse定义了常规的HTTP错误状态码。例如给浏览器返回一个403错误：
public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
response.sendError(HttpServletResponse.SC_FORBIDDEN, "禁止使用GET请求.");
}
HTTP Status 403 - 禁止使用GET请求.
type Status report
message 禁止使用GET请求.
description Access to the specified resource (禁止使用GET请求.) has been forbidden.