调用Mock方法时出现NullPointerException的原因与修复方案

在调用CustomerProfileService的getCustomerProfile方法时，标注位置抛出NullPointerException，但测试中已经Mock了CustomerProfileRepository，请问原因及修复方法？

类定义 #

@Service("customer-profile-service")
public class CustomerProfileService {
    CustomerProfileRepository customerProfileRepository;

    public CustomerProfileService(CustomerProfileRepository customerProfileRepository){
        this.customerProfileRepository = customerProfileRepository;
    }

    public CustomerProfile getCustomerProfile(RestConsumerRequest<MortgPointsFulfillRequest> consumerRequest) {

        RestConnectorResponse<RestConsumerResponse<CustomerProfile>> restConnectorResponseC1 = customerProfileRepository.customerProfileLookup(consumerRequest);        // <- NullPointerException
        ...
    }
}

测试用例 #

@ExtendWith(MockitoExtension.class)
public class CustomerProfileServiceTest {
    @Mock
    CustomerProfile customerProfile;

    @Mock
    RestConsumerRequest restConsumerRequest;

    @Mock
    CustomerProfileRepository customerProfileRepository;

    @Mock
    RestConnectorResponse<RestConsumerResponse<CustomerProfile>> restConnectorResponseC1;

    @Spy
    CustomerProfileService customerProfileService = new CustomerProfileService(customerProfileRepository);

    @Test
    public void getCustomerProfileTest() {
        doReturn(restConnectorResponseC1).when(customerProfileRepository).customerProfileLookup(restConsumerRequest);
        customerProfileService.getCustomerProfile(restConsumerRequest);
        Mockito.verify(customerProfileRepository, times(1)).customerProfileLookup(any());
    }
}

原因分析 #

1. 泛型类型不匹配，Mock Stub未生效
   服务方法的参数是RestConsumerRequest<MortgPointsFulfillRequest>，但测试中Mock的restConsumerRequest是无泛型的原始类型。Mockito的参数匹配会严格校验类型（包括泛型），导致你定义的doReturn(...) stub不触发，customerProfileLookup返回默认的null，进而抛出NPE。

2. Spy初始化时机错误，依赖未正确注入
   直接用@Spy注解手动创建CustomerProfileService实例时，Mockito可能还未完成customerProfileRepository Mock对象的初始化，导致注入的依赖是未初始化的无效对象（甚至是null）。

修复方案 #

1. 修正泛型类型匹配 #

将测试中的RestConsumerRequest Mock改为带正确泛型的版本：

@Mock
RestConsumerRequest<MortgPointsFulfillRequest> restConsumerRequest;

2. 正确初始化Service实例 #

推荐两种方式：

方式一：使用@InjectMocks自动注入依赖

替换手动创建Spy的方式，让Mockito自动将所有@Mock依赖注入到Service中：

@Mock
CustomerProfile customerProfile;

@Mock
RestConsumerRequest<MortgPointsFulfillRequest> restConsumerRequest;

@Mock
CustomerProfileRepository customerProfileRepository;

@Mock
RestConnectorResponse<RestConsumerResponse<CustomerProfile>> restConnectorResponseC1;

@InjectMocks
CustomerProfileService customerProfileService; // Mockito自动完成依赖注入

方式二：手动初始化Spy（保留Spy场景）

如果需要使用Spy，在@BeforeEach方法中初始化，确保Mock对象已创建完成：

@Mock
CustomerProfile customerProfile;

@Mock
RestConsumerRequest<MortgPointsFulfillRequest> restConsumerRequest;

@Mock
CustomerProfileRepository customerProfileRepository;

@Mock
RestConnectorResponse<RestConsumerResponse<CustomerProfile>> restConnectorResponseC1;

CustomerProfileService customerProfileService;

@BeforeEach
void setUp() {
    customerProfileService = Mockito.spy(new CustomerProfileService(customerProfileRepository));
}

3. 可选：优化参数匹配逻辑 #

如果不需要严格匹配特定的restConsumerRequest对象，可以用any()替代，避免参数匹配失败：

doReturn(restConnectorResponseC1).when(customerProfileRepository).customerProfileLookup(any());

内容的提问来源于stack exchange，提问作者techie11