demo

demo.java

@RequestMapping(value = "/user/create", method = RequestMethod.POST)
public String create(@Validated @ModelAttribute UserRequest userRequest, BindingResult result, Model model) {
    if (result.hasErrors()) {
        List<String> accountNameErrors = new ArrayList<>();
        List<String> loginIdErrors = new ArrayList<>();
        List<String> passwordErrors = new ArrayList<>();

        for (ObjectError error : result.getAllErrors()) {
            String defaultMessage = Objects.requireNonNull(error.getDefaultMessage());
            String errorMessage = messageSource.getMessage(defaultMessage, null, defaultMessage, LocaleContextHolder.getLocale());

            if (defaultMessage.contains("アカウント名")) {
                accountNameErrors.add(errorMessage);
            } else if (defaultMessage.contains("ログインID")) {
                loginIdErrors.add(errorMessage);
            } else if (defaultMessage.contains("パスワード")) {
                passwordErrors.add(errorMessage);
            }
        }

        model.addAttribute("accountNameErrors", accountNameErrors);
        model.addAttribute("loginIdErrors", loginIdErrors);
        model.addAttribute("passwordErrors", passwordErrors);

        return "user/add";
    }

    userService.create(userRequest);
    return "redirect:/user/list";
}

title.html

<div th:if="${accountNameErrors}">
    <div th:each="error : ${accountNameErrors}">
        <p th:text="${error}"></p>
    </div>
</div>

<div th:if="${loginIdErrors}">
    <div th:each="error : ${loginIdErrors}">
        <p th:text="${error}"></p>
    </div>
</div>

<div th:if="${passwordErrors}">
    <div th:each="error : ${passwordErrors}">
        <p th:text="${error}"></p>
    </div>
</div>