如何实现多层Mock对象注入？解决UserSshKeyService空指针异常

场景代码 #

UserSshKeyService实现 #

@Service
public class UserSshKeyService {

    private final RecordEventService eventService;
    private final UserSshKeyRepository repository;

    public UserSshKeyService(RecordEventService eventService,
                          UserSshKeyRepository repository){}

}

RecordEventService实现 #

@Service
public class RecordEventService {

    private final UserSshKeyEventLogRepository repository;

    // 注意：原始代码此处构造方法类名写错，需修正为RecordEventService
    public RecordEventService(UserSshKeyEventLogRepository repository){}

}

尝试的测试写法 #

写法一 #

@InjectMocks
private UserSshKeyService userSshKeyService;
@InjectMocks
private RecordEventService eventService;
@Mock
UserSshKeyRepository userSshKeyRepository;
@Mock
UserSshKeyEventLogRepository userSshKeyEventLogRepository;

写法二 #

@InjectMocks
private UserSshKeyService userSshKeyService;
@Spy
@InjectMocks
private RecordEventService eventService;
@Mock
UserSshKeyRepository userSshKeyRepository;
@Mock
UserSshKeyEventLogRepository userSshKeyEventLogRepository;

遇到的问题 #

执行测试时，UserSshKeyService中的eventService抛出空指针异常，无法完成多层对象的Mock注入。

解决方法 #

核心问题说明 #

@InjectMocks的逻辑很明确：只把测试类中标记为@Mock/@Spy的对象，注入到被@InjectMocks标记的目标对象里。你之前的写法里，userSshKeyService和eventService都是被注入的目标，Mockito不会把一个注入目标当成另一个的依赖源，所以userSshKeyService里的eventService自然是空的。

正确实现方式 #

方案一：用@Mock完全模拟RecordEventService

如果不需要RecordEventService的真实逻辑，直接把它标记为@Mock，让Mockito自动注入到上层的UserSshKeyService中：

// 测试类需加注解：JUnit4用@RunWith(MockitoJUnitRunner.class)，JUnit5用@ExtendWith(MockitoExtension.class)
@InjectMocks
private UserSshKeyService userSshKeyService;

@Mock
private RecordEventService eventService;
@Mock
private UserSshKeyRepository userSshKeyRepository;
@Mock
private UserSshKeyEventLogRepository userSshKeyEventLogRepository;

方案二：用@Spy保留RecordEventService的真实逻辑

如果需要RecordEventService执行真实代码，就用@Spy标记，同时确保它的依赖被正确注入：

@InjectMocks
private UserSshKeyService userSshKeyService;

@Spy
private RecordEventService eventService;

@Mock
private UserSshKeyRepository userSshKeyRepository;
@Mock
private UserSshKeyEventLogRepository userSshKeyEventLogRepository;

@Before
public void setUp() {
    // 初始化Mockito注解，确保依赖注入生效
    MockitoAnnotations.openMocks(this);
    // 也可以手动构造Spy对象，避免依赖自动注入可能的问题
    // eventService = new RecordEventService(userSshKeyEventLogRepository);
}

额外注意事项 #

• 修正RecordEventService的构造方法：原始代码里构造方法的类名写错成了UserSshKeyService，这个错误会导致Mockito无法正常实例化对象，也是空指针的潜在诱因。
• 必须给测试类添加Mockito的运行器/扩展注解，否则@Mock、@InjectMocks等注解不会生效。

内容的提问来源于stack exchange，提问作者seunggyu lee