相当于 走父类 临时走了一趟
HttpServletRequest ->ServletRequets -> HttpServeltRequest
/* */ public void service(ServletRequest req, ServletResponse res) /* */ throws ServletException, IOException /* */ { /* */ HttpServletRequest request; /* */ /* */ /* */ /* */ /* */ /* */ HttpServletResponse response; /* */ /* */ /* */ /* */ /* */ /* */ try /* */ { /* 663 */ request = (HttpServletRequest)req; /* 664 */ response = (HttpServletResponse)res; /* */ } catch (ClassCastException e) { /* 666 */ throw new ServletException("non-HTTP request or response"); /* */ } /* 668 */ service(request, response); /* */ } /* */ }