SpringのFlashAttributeはRedirectViewしか返せない

@RequestMapping(value="addcustomer", method=RequestMethod.POST)
public String addCustomer(@ModelAttribute("customer") Customer customer,
        final RedirectAttributes redirectAttributes) {

    redirectAttributes.addFlashAttribute("customer", customer);
    redirectAttributes.addFlashAttribute("message","Added successfully.");

    return "redirect:showcustomer.html";    
}

"redirect:"からはじまるStringはRedirectViewに変換されるのでFlashAttributeが使える
たとえば返り値が@ResponseBodyだとFlashAttributeは使えない。

org.springframework.web.servlet.mvc.support.RedirectAttributes to specify the exact set of attributes to use in case of a redirect and also to add flash attributes (attributes stored temporarily on the server-side to make them available to the request after the redirect). RedirectAttributes is used instead of the implicit model if the method returns a "redirect:" prefixed view name or RedirectView.