注解配置的servlet请求了一次,doget()执行了两次,不管是用form表单input type="submit" 提交,还是,用AJAX $.post("registUserServlet",$("#registerForm").serialize(),function(data){  ,doget()执行了两次,后台servlet写了response。
        response.setContentType("application/json;charset=utf-8");
        response.getWriter().write(json);