16.3 Service-Layer Slice Testing with Mockito

Mockito lets you cut dependencies between layers in unit tests — no real DB or external API connections required.

1. @Mock and @InjectMocks

@ExtendWith(MockitoExtension.class)
class ProductServiceTest {

    @Mock
    private ProductRepository productRepository;

    @Mock
    private CacheManager cacheManager;

    @InjectMocks
    private ProductService productService;

    @Test
    @DisplayName("When no cache hit, product is fetched from DB.")
    void fetch_product_from_db() {
        // Given
        Long productId = 1L;
        Product mockProduct = new Product(productId, "Spring Book", 35000);
        given(productRepository.findById(productId)).willReturn(Optional.of(mockProduct));

        // When
        ProductDto result = productService.getProduct(productId);

        // Then
        assertThat(result.getName()).isEqualTo("Spring Book");
        then(productRepository).should(times(1)).findById(productId);
    }

    @Test
    @DisplayName("Fetching a non-existent product throws an exception.")
    void non_existent_product_throws() {
        given(productRepository.findById(anyLong())).willReturn(Optional.empty());
        assertThatThrownBy(() -> productService.getProduct(999L))
            .isInstanceOf(ProductNotFoundException.class);
    }
}

2. ArgumentCaptor — Capturing Method Arguments

Verify exactly what arguments a mocked method was called with.

@Test
@DisplayName("Order creation passes correct data to the event publisher.")
void order_creation_event_verification() {
    ArgumentCaptor<OrderCreatedEvent> captor = ArgumentCaptor.forClass(OrderCreatedEvent.class);

    orderService.createOrder(new CreateOrderRequest(userId, productId, quantity));

    then(eventPublisher).should().publishEvent(captor.capture());
    assertThat(captor.getValue().getUserId()).isEqualTo(userId);
    assertThat(captor.getValue().getQuantity()).isEqualTo(quantity);
}