I know about these two options:

1. Use @SpyBean annotation from spring-boot-test as the only annotation

@Autowired
@InjectMocks
private ProductController productController;

@SpyBean
private ProductService productServiceSpy;

2. Use Java reflection to “autowire” the spy object, e.g. ReflectionTestUtils

@Autowired
private ProductController productController;

@Autowired
private ProductService productService;

@Before
public void setUp() {
    ProductService productServiceSpy = Mockito.spy(productService);
    ReflectionTestUtils.setField(productController, "productService", productServiceSpy);
}