@Controller
@RequestMapping("/passwordaenderung")
public class Passwortaenderung {

@Autowired
private UserRepository userRepo;

@Autowired
private PasswordEncoder passwordEncoder;

@Autowired
private updateOldPasswordRepository passwordRepo;

@GetMapping
public String showPasswordChangeForm(Model model) {
    UserCreationDto userForm = new UserCreationDto();
    User user;

    List<Benutzer> users = userRepo.findAll();

    for(int i = 0; i<users.size(); i++) {
        user = new User();
        user.setUsername(users.get(i).getBenutzername());
        user.setPassword(users.get(i).getPasswort());
        userForm.addUser(user);
    }
    model.addAttribute("UserCreationDto", userForm);
    return "passwordaenderung";
}

@PostMapping
public String updateOldPassword(@ModelAttribute UserCreationDto userTableSettings,
                                @RequestParam ("radiobutton") String radiobutton,
                                BindingResult result,
                                Model model,
                                Errors errors) {
    if (errors.hasErrors()) {
        return "error";
    }

    List<User> users = userTableSettings.getUsers();

    for(int i = 0; i<users.size(); i++) {
        if(users.get(i).getUsername().equals(radiobutton)) {
            String username = users.get(i).getUsername();
            String newPassword = passwordEncoder.encode(users.get(i).getPassword());
            passwordRepo.updatePassword(username, newPassword);
        }
    }
    return "redirect:/passwordaenderung?success";
}

<form action="#" th:action="@{/passwordaenderung}" th:object="${UserCreationDto}" method="post" onsubmit="return checkPassword()">
    <fieldset>
        <table>
            <thead>
                <tr>
                    <th>Username</th>
                    <th>Password</th>
                </tr>
            </thead>
            <tbody>
                <tr th:each="user, itemStat : *{users}">
                    <td><input th:field="*{users[__${itemStat.index}__].username}" /></td>
                    <td><input type="password" name="passwordField" th:field="*{users[__${itemStat.index}__].password}" id="pwd" /></td>
                    <!-- <td><input type="checkbox" name="checkboxName" id="myCheck" /></td> -->
                    <td><input type="radio" name="radiobutton" th:value="*{users[__${itemStat.index}__].username}" id="raidoButtonCheck" /></td>
                </tr>
            </tbody>
        </table>
        <input type="submit" id="submitButton" th:value="Speichern">
    </fieldset>
</form>

<div th:if="${param.success}">
    <div class="alert alert-info">
        The user password was successfully changed.
    </div>
</div>