Can't pass param when redirect - spring-boot

Question

In my spring boot app I want to pass param errorMessage when user is admin.
snippet:
#Controller
public class UsersController {
#GetMapping("/users")
public String getAllUsers(Model model, RedirectAttributes redirectAttributes) {
logger.info("getAllUsers: model = " + model + ", redirectAttributes= " + redirectAttributes);
Set<Role> roleSet = new HashSet<>();
roleSet.add(Role.ADMIN);
adminsList = userRepository.findAllByRolesIn(roleSet);
model.addAttribute("usersList", userRepository.findAll());
model.addAttribute("adminsList", adminsList);
model.addAttribute("appName", appName);
return "users";
}
#RequestMapping("user/delete/{id}")
public String deleteUser(#PathVariable("id") int id, RedirectAttributes redirectAttributes) {
logger.info("user_id_to_delete = " + id);
Optional<User> userAdmin = adminsList.stream()
.filter(user -> user.getId() == id)
.findFirst();
if (userAdmin.isPresent() && adminsList.size() == 1) {
String errorMessage = "In system must have at least one administrator";
logger.error(errorMessage);
redirectAttributes.addAttribute("errorMessage", errorMessage);
} else {
userRepository.deleteById(id);
}
return "redirect:/users";
}
after try to delete user (method deleteUser) success call
redirectAttributes.addAttribute("errorMessage", errorMessage);
And after that success call method getAllUsers. But param redirectAttributes is empty.

Answer 1

Based on the api documentation
A RedirectAttributes model is empty when the method is called and is
never used unless the method returns a redirect view name or a
RedirectView.
After the redirect, attributes are automatically added to the model of
the controller that serves the target URL.
The errorMessage will be available in the model of getAllUsers() after the redirect .