Testing Pyramid

Unit Tests         — fast, no Spring context, mock dependencies    (70%)
Slice Tests        — partial Spring context (@WebMvcTest, @DataJpaTest) (20%)
Integration Tests  — full context + real DB via Testcontainers     (10%)

@ExtendWith(MockitoExtension.class)
class OrderServiceTest {

    @Mock private OrderRepository orderRepository;
    @Mock private InventoryService inventoryService;

    @InjectMocks private OrderService orderService;

    @Test
    void createOrder_whenItemsAvailable_shouldSaveAndReturnOrder() {
        // Given
        var request = new CreateOrderRequest("user@example.com", List.of(new OrderItemRequest(UUID.randomUUID(), 2)));
        var savedOrder = Order.create("user@example.com");
        when(orderRepository.save(any(Order.class))).thenReturn(savedOrder);
        doNothing().when(inventoryService).reserve(any());

        // When
        Order result = orderService.createOrder(request);

        // Then
        assertThat(result).isNotNull();
        assertThat(result.getCustomerEmail()).isEqualTo("user@example.com");
        verify(inventoryService).reserve(request.items());
        verify(orderRepository).save(any(Order.class));
    }

    @Test
    void createOrder_whenInventoryUnavailable_shouldThrowException() {
        // Given
        var request = new CreateOrderRequest("user@example.com", List.of());
        doThrow(new InsufficientInventoryException("Out of stock"))
            .when(inventoryService).reserve(any());

        // When / Then
        assertThatThrownBy(() -> orderService.createOrder(request))
            .isInstanceOf(InsufficientInventoryException.class)
            .hasMessage("Out of stock");
    }
}

@WebMvcTest(OrderController.class)
class OrderControllerTest {

    @Autowired MockMvc mockMvc;
    @Autowired ObjectMapper objectMapper;
    @MockitoBean OrderService orderService; // Spring Boot 3.4+: @MockBean is deprecated

    @Test
    @WithMockUser(roles = "USER")
    void createOrder_withValidRequest_shouldReturn201() throws Exception {
        // Given
        var request = new CreateOrderRequest("user@example.com", List.of());
        var order = Order.create("user@example.com");
        when(orderService.createOrder(any())).thenReturn(order);

        // When / Then
        mockMvc.perform(post("/api/v1/orders")
                .contentType(MediaType.APPLICATION_JSON)
                .content(objectMapper.writeValueAsString(request)))
            .andExpect(status().isCreated())
            .andExpect(jsonPath("$.success").value(true))
            .andExpect(jsonPath("$.data.customerEmail").value("user@example.com"));
    }

    @Test
    @WithMockUser
    void createOrder_withInvalidRequest_shouldReturn400() throws Exception {
        mockMvc.perform(post("/api/v1/orders")
                .contentType(MediaType.APPLICATION_JSON)
                .content("{}")) // missing required fields
            .andExpect(status().isBadRequest())
            .andExpect(jsonPath("$.success").value(false))
            .andExpect(jsonPath("$.error.code").value("VALIDATION_FAILED"));
    }
}

@DataJpaTest
@AutoConfigureTestDatabase(replace = Replace.NONE) // use real DB (Testcontainers)
@Import(TestcontainersConfig.class)
class OrderRepositoryTest {

    @Autowired OrderRepository orderRepository;

    @Test
    void findByStatus_shouldReturnMatchingOrders() {
        // Given
        var order1 = orderRepository.save(Order.create("a@example.com"));
        var order2 = orderRepository.save(Order.create("b@example.com"));
        order2.ship(); // change status
        orderRepository.save(order2);

        // When
        List<Order> pending = orderRepository.findByStatus(OrderStatus.PENDING, Pageable.unpaged()).getContent();

        // Then
        assertThat(pending).hasSize(1);
        assertThat(pending.get(0).getCustomerEmail()).isEqualTo("a@example.com");
    }
}

// Shared config — reuse container across tests
@TestConfiguration(proxyBeanMethods = false)
public class TestcontainersConfig {

    @Bean
    @ServiceConnection
    PostgreSQLContainer<?> postgresContainer() {
        return new PostgreSQLContainer<>("postgres:16-alpine");
    }
}

// Full integration test
@SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT)
@Import(TestcontainersConfig.class)
class OrderIntegrationTest {

    @Autowired TestRestTemplate restTemplate;
    @Autowired OrderRepository orderRepository;

    @Test
    void createAndRetrieveOrder_endToEnd() {
        // Create
        var createRequest = new CreateOrderRequest("user@example.com", List.of());
        var createResponse = restTemplate.postForEntity("/api/v1/orders", createRequest, ApiResponse.class);
        assertThat(createResponse.getStatusCode()).isEqualTo(HttpStatus.CREATED);

        // Retrieve
        // ... assert persisted correctly
    }
}

// Method name: methodName_condition_expectedBehavior
createOrder_whenItemsAvailable_shouldSaveOrder()
findById_whenOrderNotFound_shouldThrowNotFoundException()
login_withInvalidCredentials_shouldReturn401()

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-testcontainers</artifactId>
    <scope>test</scope>
</dependency>
<dependency>
    <groupId>org.testcontainers</groupId>
    <artifactId>postgresql</artifactId>
    <scope>test</scope>
</dependency>