Testing with JUnit & Mockito

JUnit 5 Basics

class UserServiceTest {

    @Test
    @DisplayName("should create user with valid data")
    void shouldCreateUser() {
        // Given
        var request = new CreateUserRequest("John", "john@example.com");

        // When
        var user = userService.create(request);

        // Then
        assertThat(user.getName()).isEqualTo("John");
        assertThat(user.getEmail()).isEqualTo("john@example.com");
    }

    @ParameterizedTest
    @ValueSource(strings = {"", " ", "invalid"})
    void shouldRejectInvalidEmail(String email) {
        var request = new CreateUserRequest("John", email);

        assertThatThrownBy(() -> userService.create(request))
            .isInstanceOf(ValidationException.class);
    }

    @BeforeEach
    void setUp() {
        // Setup before each test
    }

    @AfterEach
    void tearDown() {
        // Cleanup after each test
    }
}

Mockito

@ExtendWith(MockitoExtension.class)
class OrderServiceTest {

    @Mock
    OrderRepository orderRepository;

    @Mock
    InventoryService inventoryService;

    @InjectMocks
    OrderService orderService;

    @Test
    void shouldCreateOrder() {
        // Given
        var request = new CreateOrderRequest("product-1", 2);
        when(inventoryService.checkStock("product-1")).thenReturn(true);
        when(orderRepository.save(any())).thenAnswer(inv -> inv.getArgument(0));

        // When
        var order = orderService.create(request);

        // Then
        assertThat(order).isNotNull();
        verify(inventoryService).checkStock("product-1");
        verify(orderRepository).save(any(Order.class));
    }
}

Spring Test Slices

// Full context
@SpringBootTest
class ApplicationTests {}

// Web layer only
@WebMvcTest(UserController.class)
class UserControllerTest {
    @Autowired MockMvc mockMvc;
    @MockBean UserService userService;

    @Test
    void shouldReturnUser() throws Exception {
        when(userService.findById(1L)).thenReturn(Optional.of(new User(1L, "John")));

        mockMvc.perform(get("/api/users/1"))
            .andExpect(status().isOk())
            .andExpect(jsonPath("$.name").value("John"));
    }
}

// JPA layer only
@DataJpaTest
class UserRepositoryTest {
    @Autowired UserRepository repository;
    @Autowired TestEntityManager entityManager;

    @Test
    void shouldFindByEmail() {
        entityManager.persist(new User("john@example.com"));
        var user = repository.findByEmail("john@example.com");
        assertThat(user).isPresent();
    }
}

Best Practices

1. Given-When-Then structure for readability
2. One assertion concept per test
3. Use test slices to minimize context
4. AssertJ for fluent assertions
5. Parameterized tests for multiple inputs

References

• Test Slices - Available slices, customization
• Mockito Patterns - Stubbing, verification, argument captors