为什么mapper返回null？

问题：为什么mapper返回null？

谁能解释我为什么我的mapper（mapstruct）返回null？当我实现自己的映射器时，就可以了。

@RunWith(MockitoJUnitRunner.class)
@SpringBootTest
class CouponServiceTestSuite {
    @InjectMocks
    private CouponService couponService;
    @Mock
    private CouponRepository couponRepository;
    @Spy
    private CouponDto couponDto;
    @Spy
    private CouponMapper couponMapper;


    @Test
    public void testMapper() {
        Coupon coupon = createCoupon();
        CouponDto couponDto = couponMapper.mapToCouponDto(coupon);

        System.out.println(couponDto); // RETURN NULL
    }

@Component
@Mapper
public interface CouponMapper {

    Coupon mapToCoupon(CouponDto couponDto);

    CouponDto mapToCouponDto(Coupon coupon);

    default List<CouponDto> mapToCouponDtoList(List<Coupon> couponList) {
        if (couponList == null) {
            return new ArrayList<>();
        }
        return new ArrayList<>(couponList).stream()
                .map(this::mapToCouponDto)
                .collect(Collectors.toList());
    }
}

https://github.com/kenez92/BetWinner2/blob/CouponServiceTestSuite/src/test/java/com/kenez92/betwinner/service/CouponServiceTestSuite.java

谢谢 ：）