测试REST API

I have problem with test for my rest api. Can anybody tell me how can I do that ? full code on github : https://github.com/kenez92/kodilla-library

此表达式返回null。为什么呢

BookDetailsDto bookDetailsDto = bookDetailsS​​ervice.findBookDetails（id）;

@RunWith(MockitoJUnitRunner.class)
@SpringBootTest
public class BookDetailsServiceTestSuite {
    @Mock
    private BookDetailsMapper bookDetailsMapper;
    @Mock
    private BookDetailsRepository bookDetailsRepository;
    @InjectMocks
    private BookDetailsService bookDetailsService;

    private BookDetails createBookDetails() {
        return BookDetails.builder()
                .id(1L)
                .code(1234L)
                .title("New Title")
                .author("New Author")
                .yearOfPublication(1999)
                .build();
    }

    @Test
    public void testGetBookDetailsById() {
        //Given
        long id = 1;
        Mockito.when(bookDetailsRepository.findById(id)).thenReturn(Optional.ofNullable(createBookDetails()));

        //When
        BookDetailsDto bookDetailsDto = bookDetailsService.findBookDetails(id);
        //Then
        Assert.assertEquals(bookDetailsDto.getId(), id);
        Assert.assertEquals(bookDetailsDto.getCode(), 1234L);
        Assert.assertEquals(bookDetailsDto.getTitle(), "New Title");
        Assert.assertEquals(bookDetailsDto.getAuthor(), "New Author");
        Assert.assertEquals(bookDetailsDto.getYearOfPublication(), 1999);
    }
}