我想模拟并测试这段Java代码：

public class BusterClient {

    public BusterClient() {
    }

    @Autowired
    public BusterClient(PromoCodeService promoCodeService) {
        this.promoCodeService = promoCodeService;
    }

    private Optional<PromoCode> getPromoCode(CustomerRegistrationEvent event) {
        Optional<Long> campaignId = getCampaignIdAsLong(event);
        if (!event.hasPromoCode() || !campaignId.isPresent()) {
            return Optional.empty();
        }

        return promoCodeService.findByCampaignPromoCodeIds(campaignId.get(), event.getPromoCode());
    }
}

我创建了以下测试代码：

@InjectMocks
private BusterClient app = new BusterClient();

@Test
public void testTrackedReferralNotMatchingPromoCode() throws Exception {

    PromoCodeService o = mock(PromoCodeService.class);
    when(o.findByCampaignPromoCodeIds(Long.valueOf(12), "test")).thenReturn(Optional.of(PromoCode.builder().id(Long.valueOf(1)).build()));

    try {

        Method method = BusterClient.class.getDeclaredMethod("getPromoCode", CustomerRegistrationEvent.class);
        method.setAccessible(true);
        method.invoke(app, customerRegistrationEvent);

    } catch (InvocationTargetException e) {
        e.getCause().printStackTrace();
    } catch (Exception e) {
        e.printStackTrace();
    }
}

但我得到错误：promoCodeService.findByCampaignPromoCodeIds(java.lang.Long, String)" because "this.promoCodeService" is null
你知道什么是正确的方式来模仿PromoCodeService服务吗？