springMVC中使用spring的form標簽庫，edit.jsp代碼如下：jspFullname:Username:Nospaces,Please.Password:6charactersormore.EmailAddress:Incaseyouforgetsomething.Sendmeemailupdates!控制器的代碼如下：java@RequestMapping(method=RequestMethod.GET,params="new")publicStringcreateSpitterProfile(Modelmodel){model.addAttribute(newSpitter());return"spitters/edit";}@RequestMapping(method=RequestMethod.POST)publicStringaddSpitterFromForm(@Valid@ModelAttribute("spitter")Spitterspitter,BindingResultbindingResult){if(bindingResult.hasErrors()){return"spitters/edit";}spitterService.saveSpitter(spitter);return"redirect:/spitter/"+spitter.getName();}在jsp的語句中，如果enctype=application/x-www-form-urlencoded則運行正常；如果調成multipart/form-data后，提交表單時不管用戶名寫的是什么，轉到的網頁后面顯示的用戶名都是null。請問這是怎么回事？