How should I test an UPDATE method

I have a Spring Boot application and an UPDATE method for user entity:

@Transactionalpublic void updateUser(@Min(1) Long userId,                       @Email String email,                       @NotBlank @Length(min = 8, max = 20) String password,                       @NotBlank @Length(max = 30) String username) throws UpdateUserException {    UserEntity user = userRepository.findById(userId).orElseThrow(            () -> new UpdateUserException("User with ID " + userId +" does not exist")    );    if (isUserExists(email, username)) {        throw new UpdateUserException("User with email " + email +" or username " + username +" already exists");    }    user.setEmail(email);    user.setPassword(password);    user.setUsername(username);}private boolean isUserExists(String email, String username) {    return userRepository.findByEmail(email).isPresent() ||            userRepository.findByUsername(username).isPresent();}

Now I want to write a Unit test for it.

I tried to write such test, but it does not work because Spring automatically updates user in DB without invoking userRepository.save().

@Testvoid canUpdateUser() throws UpdateUserException {    Long id = 1L;    String newEmail = "newmail@gmail.com";    String newPassword = "12345678";    String newUsername = "NewUsername";    // given    UserEntity user = new UserEntity(            id,"oldmail@gmail.com","00000000","OldUsername"    );    given(userRepository.findById(anyLong())).willReturn(Optional.of(user));    // when    underTest.updateUser(id, newEmail, newPassword, newUsername);    // then    ArgumentCaptor<UserEntity> userEntityArgumentCaptor=ArgumentCaptor.forClass(UserEntity.class);    verify(userRepository).save(userEntityArgumentCaptor.capture());    UserEntity capturedUser = userEntityArgumentCaptor.getValue();    assertThat(capturedUser.getId()).isEqualTo(id);    assertThat(capturedUser.getEmail()).isEqualTo(newEmail);    assertThat(capturedUser.getPassword()).isEqualTo(newPassword);    assertThat(capturedUser.getUsername()).isEqualTo(newUsername);}