Incorrect documentation for RETURNS_MOCKS

[sergey-morenets]

Hi

I checked the latest (5.11.0) documentation for Mockito class (https://javadoc.io/static/org.mockito/mockito-core/5.11.0/org/mockito/Mockito.html#23) and it states:
ReturnsMocks first tries to return ordinary values (zeros, empty collections, empty string, etc.) then it tries to return mocks. If the return type cannot be mocked (e.g. is final) then plain null is returned.

I have an enumeration Department with abstract methods (Mockito is unable to mock it). Let's say it's used in OrderRepository:

public interface OrderRepository {
	
	Department getDepartment();
}

Then I inject mocked version of OrderRepository:

@ExtendWith(MockitoExtension.class)
public class OrderServiceTest {

	@Mock(answer = Answers.RETURNS_MOCKS)
	OrderRepository orderRepository;

and when I try to invoke getDepartment():
Department department = orderRepository.getDepartment();

I suppose to receive null value according to the documentation but get an error:

org.mockito.exceptions.base.MockitoException: 
Mockito cannot mock this class: class Department.
Sealed abstract enums can't be mocked. Since Java 15 abstract enums are declared sealed, which prevents mocking.
You can still return an existing enum literal from a stubbed method call.

[TimvdLippe]

I am assuming here that Department is an enum here, hence the error message. We delegate this check to MockUtil:

mockito/src/main/java/org/mockito/internal/stubbing/defaultanswers/RetrieveGenericsForDefaultAnswers.java

Line 40 in d0a919c

if (!MockUtil.typeMockabilityOf(type, mockSettings.getMockMaker()).mockable()) {

If I understand correctly, we delegate this check to ByteBuddy:

mockito/src/main/java/org/mockito/internal/creation/bytebuddy/InlineDelegateByteBuddyMockMaker.java

Line 556 in d0a919c

return INSTRUMENTATION.isModifiableClass(type) && !EXCLUDES.contains(type);

Hence I am not sure why ByteBuddy thinks we can mock this type. Can you check whether we indeed hit this check in ByteBuddy and what ByteBuddy returns? That should help us understand why we think we can mock a sealed class, even though we can't.

[sergey-morenets]

Hi @TimvdLippe

I just checked that code in InlineDelegateByteBuddyMockMaker:

            @Override
            public boolean mockable() {
                return INSTRUMENTATION.isModifiableClass(type) && !EXCLUDES.contains(type);
            }

And INSTRUMENTATION.isModifiableClass(type) returns true for Department.

[sergey-morenets]

Here's Department:

	enum Department {
		QA {
			@Override
			String getName() {
				return "Quality Assurance";
			}
		},
		DEV {
			@Override
			String getName() {
				return "Development";
			}
		};

		abstract String getName();
	}