Testing - JUnit 5 and Mockito

Why this step matters

Reliable software requires tests that are fast, deterministic, and meaningful. Unit tests are your first safety net for refactoring.

Unit test scope

A unit test validates one class behavior in isolation. External dependencies are replaced by test doubles.

JUnit 5 basics

class PriceServiceTest {

    @Test
    void should_apply_discount_for_premium_user() {
        PriceService service = new PriceService();
        int result = service.applyDiscount(100, true);
        assertEquals(80, result);
    }
}

Good test names describe expected behavior.

Mockito: mocks and stubs

@ExtendWith(MockitoExtension.class)
class UserServiceTest {

    @Mock
    UserRepository userRepository;

    @InjectMocks
    UserService userService;

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

        UserDto dto = userService.findById(1L);

        assertEquals("briac", dto.username());
    }
}

• stubbing: when(...).thenReturn(...)
• verification: verify(...)

Test structure suggestion

Use Arrange / Act / Assert:

1. prepare inputs and stubs
2. execute target method
3. assert result and interactions

Common mistakes

• over-mocking everything
• asserting implementation details instead of behavior
• brittle tests coupled to internal refactoring
• naming tests too vaguely

Takeaway

1. Keep unit tests fast and isolated
2. Use Mockito only for real external dependencies
3. Name tests by behavior
4. Prefer clear Arrange/Act/Assert structure