Spring boot addFlashAttribute not displaying message in HTML

Question

I've been searching here and online how to use addFlashAttribute in the best way and it's not working for me no matter what I do. I have a Student model which I'm trying to update and flash message of success or fail. The database and all the queries do work and I get redirected to the showStudentDetail page, it's just the message that doesn't show. Here is the code:

StudentService:

public String updateStudent(Student student) {
    if(studentDAO.update(student))
        return "update student was successful!";
    return "error: failed to update student, please try again";
}

public Student getStudentInfo(String username) {
    Student student = studentDAO.get(username);
    return student;
}

StudentController:

@PostMapping("/updateStudent")
public String updateStudent(@ModelAttribute("student") Student student, RedirectAttributes redirectAttrs) {
    String res = studentService.updateStudent(student);
    redirectAttrs.addAttribute("username", student.getUsername()).addFlashAttribute("message", res);
    return "redirect:/students/{username}/details";
}

@GetMapping("/students/{username}/details")
public String showDetailPage(@PathVariable String username, Model model) {
    Student student = studentService.getStudentInfo(username);
    model.addAttribute("student", student);
    return "student_detail";
}

student_detail (div not showing)

<div>
    <div th:if="${param.username}" class="alert alert-info" role="alert" th:text="${param.message}"></div>
</div>

Answer 1

I finally solved this problem by changing to message instead of param.message. I really don't know why now it's working but that's the solution to my problem.

only change the html to this:

<div th:if="${message}" class="alert alert-info" role="alert" th:text="${message}"></div>