JUnit 5 - Quick Reference

Deep Knowledge: Use mcp__documentation__fetch_docs with technology: junit for comprehensive documentation.

When NOT to Use This Skill

Integration Tests with Containers - Use testcontainers for Docker-based tests
REST API Testing - Use rest-assured for HTTP/REST testing
E2E Web Testing - Use Selenium or Playwright
JavaScript/TypeScript - Use vitest or jest for JS/TS
Database Integration Tests - Combine with spring-boot-integration skill

Essential Patterns

Basic Test

@Test
void shouldAddNumbers() {
    assertEquals(5, calculator.add(2, 3));
    assertThrows(ArithmeticException.class, () -> calculator.divide(10, 0));
}

Mockito + Service Test

@ExtendWith(MockitoExtension.class)
class UserServiceTest {

    @Mock
    private UserRepository userRepository;

    @InjectMocks
    private UserServiceImpl userService;

    @Test
    void shouldCreateUser() {
        when(userRepository.save(any())).thenReturn(user);

        UserResponse result = userService.create(request);

        assertNotNull(result);
        verify(userRepository, times(1)).save(any());
    }
}

Spring Boot Test

@SpringBootTest
@ActiveProfiles("test")
class UserServiceIntegrationTest {

    @Autowired
    private UserService userService;

    @MockBean
    private UserRepository userRepository;

    @Test
    void contextLoads() {
        assertNotNull(userService);
    }
}

Controller Test

@WebMvcTest(UserController.class)
class UserControllerTest {

    @Autowired
    private MockMvc mockMvc;

    @MockBean
    private UserService userService;

    @Test
    void shouldReturnUsers() throws Exception {
        when(userService.findAll()).thenReturn(users);

        mockMvc.perform(get("/api/v1/users"))
            .andExpect(status().isOk())
            .andExpect(jsonPath("$[0].name").value("John"));
    }
}

Repository Test

@DataJpaTest
class UserRepositoryTest {

    @Autowired
    private TestEntityManager entityManager;

    @Autowired
    private UserRepository userRepository;

    @Test
    void shouldFindByEmail() {
        entityManager.persist(user);
        Optional<User> found = userRepository.findByEmail("john@email.com");
        assertTrue(found.isPresent());
    }
}

Common Annotations

| Annotation | Usage | |------------|-------| | @Test | Test method | | @BeforeEach | Setup before each test | | @Mock | Creates mock | | @InjectMocks | Injects mocks | | @SpringBootTest | Integration test | | @WebMvcTest | Controller test | | @DataJpaTest | Repository test |

Anti-Patterns

| Anti-Pattern | Why It's Bad | Solution | |--------------|--------------|----------| | Using @SpringBootTest for unit tests | Extremely slow | Use @ExtendWith(MockitoExtension.class) | | Testing private methods | Coupled to implementation | Test through public API | | No mock cleanup | Tests affect each other | Use @BeforeEach, Mockito.reset() | | Hardcoded test data | Hard to maintain | Use test data builders or factories | | Not verifying mock interactions | Silent failures | Use verify() to ensure methods called | | Too many assertions per test | Hard to debug | One logical assertion per test | | Ignoring @Disabled tests | Technical debt accumulates | Fix or remove disabled tests |

Quick Troubleshooting

| Problem | Likely Cause | Solution | |---------|--------------|----------| | "NullPointerException in test" | Mock not injected | Check @Mock and @InjectMocks annotations | | "Wanted but not invoked" | Method not called or wrong args | Verify method call, check argument matchers | | Test takes too long | Using @SpringBootTest unnecessarily | Use Mockito for unit tests | | "UnnecessaryStubbingException" | Mock setup but not used | Remove unused when() statements | | Flaky test | Shared state or timing | Isolate setup, avoid Thread.sleep | | "No tests found" | Wrong naming convention | Use test* prefix or @Test annotation |