@PostMapping("/addStudent")
String addStudent(@ModelAttribute("student") Students student, Model model) {
  studentsRepository.save(student);

  return "index";
}

这段代码没有返回到索引页，而是给出了一个错误：

This application has no explicit mapping for /error, so you are seeing this as a fallback.

Tue Oct 18 20:32:54 IST 2022
There was an unexpected error (type=Internal Server Error, status=500).