id一直是Null

@RequestMapping(value = "/obj", produces = "application/json;charset=UTF-8")//7
    @ResponseBody //8
    public String passObj(DemoObj obj, HttpServletRequest request) {
        
         return "url:" + request.getRequestURL() 
                     + " can access, obj id: " + obj.getId()+" obj name:" + obj.getName();

    }