Testing the Service Layer

Direct Answer

Use unit tests for service rules that can be proved with a small object graph, and integration tests for behavior that depends on Spring proxies, transactions, persistence mappings, queries, configuration, or other real boundaries. A service often needs both. Mocks are useful at a boundary, but a fake or real lightweight collaborator can be clearer; speed differences must be measured rather than assumed.

Code Example

@ExtendWith(MockitoExtension.class)
class OrderServiceTest {
    @Mock private OrderRepository orderRepository;
    @InjectMocks private OrderService orderService;

    @Test
    void shouldCalculateDiscountForLargeOrder() {
        var order = new Order(1L, "Laptop", 1500.0);
        when(orderRepository.findById(1L)).thenReturn(Optional.of(order));

        var discountedPrice = orderService.applyDiscount(1L); // logic under test

        assertEquals(1350.0, discountedPrice); // 10% discount
        verify(orderRepository).findById(1L);  // interaction verified
    }
}

Common Mistakes & Pitfalls

• Using @SpringBootTest for local calculations: A broad context can hide the simple behavior under test.
• Mocking the persistence contract everywhere: A mock cannot prove query semantics, mapping, constraints, or transaction behavior.
• Assuming annotations work in a unit test: Calling a service created with new does not apply proxy-based @Transactional, security, or caching behavior.