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Question (in short) - how do I mock a spring dependency that is not directly referenced in my JUnit.

Below is the setup that am working with.
ServiceA is an interface and its implementation requires a handful of autowired dependencies for its many logic to work.

public interface ServiceA
 void invoke();
 ...10 more methods


@Service
public class ServiceAImpl implements ServiceA
 @Autowired
 private ServiceB serviceB; 

 @Autowired
 private ServiceC serviceC;

 @Autowired
 private DAOA daoA;

 @Autowired
 private DAOB daoB;

 @Override
 public void invoke(arg1, arg2)
 // use above dependencies to do something
 serviceB.sendEmail(args);
 

ServiceB sends an email by invoking yet another util dependency.

public interface ServiceB
 void sendEmail();
 ...10 more methods


@Service
public class ServiceBImpl implements ServiceB{
 @Autowired
 private EmailUtil emailUtil;

 @Override
 public void sendEmail(args)
 emailUtil.sendEmail(args);

Its worth noting that the sendEmail method returns void.

@Component
publIt'sclass EmailUtil
 @Autowired
 private JavaMailSenderImpl javaMailSenderImpl;

 public void sendEmail(args)
 // send email using spring & other APIs
 

My integration test JUnit looks like this.

@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(locations = "/test-context.xml" )
public class ServiceIT
 @Autowired
 private ServiceA serviceA;

 @Test
 public void testA()
 // prepare data mock in H2 DB
 serviceA.invoke(arg1, arg2);
 // assertions
 

Question (long) - Am using H2 as mock DB so my DAO calls can (need to) function. I want to mock other 3rd party integrations, such as emailUtil.sendEmail call in serviceB so that email is not sent. Since in my JUnit am using serviceA, my understanding is that, I need to create @Mock of EmailUtil and @InjectMocks it into ServiceB which is @InjectMocks into ServiceA. When I do this, Mockito throws errors saying ServiceB is an interface. Creating mock objects of ServiceA, ServiceB may not be a great option, because then I might have to stub/mock too many behaviors. I want the actual logic to be executed in these classes, the mocking should be done only for sendEmail method.

1. Is this a common problem?


2. How can I address this?


3. Is there a problem with the design of this code (i.e. using
   interfaces and autowiring everything), which makes it difficult to
   test/mock. If yes, what's a better way to do this?

Spring - 4.0.3.RELEASE, JUnit - 4.12, mockito-core - 1.10.19

In the test class ServiceIT mock EmailUtil using @MockBean

Annotation that can be used to add mocks to a Spring ApplicationContext. Can be used as a class level annotation or on fields in either @Configuration classes, or test classes that are @RunWith the SpringRunner.

@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(locations = "/test-context.xml" )
public class ServiceIT
@Autowired
private ServiceA serviceA;

@MockBean
private EmailUtil emailUtil;

@Test
public void testA()
// prepare data mock in H2 DB

//given(this.emailUtil.sendEmail(ArgumentMatchers.any()).willReturn( custom object);
serviceA.invoke(arg1, arg2);
// assertions