验证失败后，为什么我的redirectAttributes.addFlashAttribute和重新填充的数据不起作用？

Question

I have spent the whole day and night trying to find why my form data disappears when validation fails. I also added a redirectAttributes.addFlashAttribute which is suppose to indicate that an error occurred and also survive one redirect. So far none of these are working. I have done my research on stack over and other forums and i seem to be doing the right thing but it is not working for me. I am not getting an error so i can not even debug to find what is wrong.

<div class="form-group" th:if="${exams.size() lt 6}">
  <form method="post" th:object="${newExam}" th:action="@{/exams}" class="inline new-item">
    <div th:classappend="${#fields.hasErrors('indexNumber')}? 'error' : ''">
      <input type="text" th:field="*{indexNumber}" placeholder="Index Number" />
      <div class="error-message" th:if="${#fields.hasErrors('indexNumber')}" th:errors="*{indexNumber}"></div>
    </div>
    <div th:classappend="${#fields.hasErrors('grade')}? 'error' : ''">
      <select th:field="*{grade}" class="form-control input-lg">
                    <option value="">[Select Grade]</option>
                    <option th:each="grade : ${grades}" th:value="${grade.values}" th:text="${grade.name}">Grade
                    </option>
                </select>
      <div class="error-message" th:if="${#fields.hasErrors('grade')}" th:errors="*{grade}"></div>
    </div>
    <div th:classappend="${#fields.hasErrors('courseOffered')}? 'error' : ''">
      <input type="text" th:field="*{courseOffered}" placeholder="CourseOffered" />
      <div class="error-message" th:if="${#fields.hasErrors('courseOffered')}" th:errors="*{courseOffered}"></div>
    </div>
    <div th:classappend="${#fields.hasErrors('examType')}? 'error' : ''">
      <input type="text" th:field="*{examType}" placeholder="ExamType" />
      <div class="error-message" th:if="${#fields.hasErrors('examType')}" th:errors="*{examType}"></div>
    </div>
    <div th:classappend="${#fields.hasErrors('subject')}? 'error' : ''">
      <input type="text" th:field="*{subject}" placeholder="Subject" />
      <div class="error-message" th:if="${#fields.hasErrors('subject')}" th:errors="*{subject}"></div>
    </div>
    <div th:classappend="${#fields.hasErrors('gradeYear')}?'error' : ''">
      <input type="text" th:field="*{gradeYear}" placeholder="ExamYear" />
      <div class="error-message" th:if="${#fields.hasErrors('gradeYear')}" th:errors="*{gradeYear}"></div>
    </div>
    <button type="submit" class="btn btn-primary">Add</button>
  </form>
</div>

Controller

@RequestMapping(value = "/cert_prog", method = RequestMethod.GET) 
public String examsList(Model model){ 
Iterable<Exams> exams = examService.findAll();
 if(!model.containsAttribute("newExam")){
 model.addAttribute("newExam", new Exams()); } 
model.addAttribute("grades", Grade.values()); 
model.addAttribute("regions", Region.values());    model.addAttribute("schools",schools);
  if(!model.containsAttribute("newSchool")){ 
model.addAttribute("newSchool",new School()); } 
model.addAttribute("regions", Region.values()); 
return "cert_prog"; } 

@RequestMapping(value = "/exams", method = RequestMethod.POST) public String addTask(@Valid
  @ModelAttribute("newExam") Exams exams, BindingResult result, RedirectAttributes redirectAttributes, Principal principal){ 
User user = (User)((UsernamePasswordAuthenticationToken)principal).getPrincipal(); exams.setUser(user); 
if(result.hasErrors()){
redirectAttributes.addFlashAttribute("org.springframework.validation.BindingResult.exams", result); 
redirectAttributes.addFlashAttribute("exams",exams); return "redirect:/cert_prog"; } 
examService.save(exams); 
return "redirect:/cert_prog"; }

Model

@Entity
public class Exams {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @NotNull(message = "The above field must not be blank.")
    private String courseOffered;

    @NotNull(message = "The above field must not be blank.")
    private String examType;

    @NotNull(message = "The above field must not be blank.")
    private String subject;

    @NotNull(message = "The above field must not be blank.")
    private String grade;

    @NotNull(message = "The above field must not be blank.")
    private Long indexNumber;

    @NotNull(message = "The above field must not be blank.")
    private Long gradeYear;

    private boolean isComplete;

Answer 1

In your addTask controller method you have

redirectAttributes.addFlashAttribute("exams",exams)

attribute name as "exams" and you are checking as "newExam" in examsList controller method. This might be the problem. try this,

redirectAttributes.addFlashAttribute("newExam",exams)