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

@Transactional
public 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().

@Test
void canUpdateUser() throws UpdateUserException {
    Long id = 1L;
    String newEmail = "[email protected]";
    String newPassword = "12345678";
    String newUsername = "NewUsername";

    // given
    UserEntity user = new UserEntity(
            id,
            "[email protected]",
            "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);
}