Mocking & Stubbing Basics

Creating Mocks

@ExtendWith(MockitoExtension.class)
class UserServiceTest {

  @Mock
  private UserRepository userRepository;

  @InjectMocks
  private UserService userService;

  @Test
  void returnsUserWhenFound() {
    when(userRepository.findById(1L)).thenReturn(Optional.of(new User(1L, "Ada")));

    User result = userService.getUser(1L);

    assertEquals("Ada", result.getName());
  }
}

Stubbing Returns & Exceptions

when(repo.findById(1L)).thenReturn(Optional.of(user));
when(repo.count()).thenReturn(5L, 6L, 7L);       // sequential calls
when(repo.findById(99L)).thenThrow(new EntityNotFoundException());

// void methods need doThrow/doAnswer instead of when()
doThrow(new RuntimeException("db down")).when(repo).deleteById(1L);

Argument Matchers

when(repo.findById(anyLong())).thenReturn(Optional.of(user));
when(repo.save(any(User.class))).thenReturn(user);

// mixing a matcher with a raw literal in the same call is not allowed —
// once you use any()/eq()/etc, every argument in that call must be a matcher

Mock vs. Spy

A mock() starts fully fake — every method returns a default (null/0/empty) until stubbed. A spy() wraps a real object and calls its actual methods by default, so you only stub the specific behavior you need to override.

List<String> spyList = spy(new ArrayList<>());
spyList.add("real");           // real ArrayList.add() runs
when(spyList.size()).thenReturn(100);  // only size() is faked
assertEquals(100, spyList.size());