幾周前我開始使用 Spring Boot，但我仍然無法理解“AbstractController”的概念。public abstract class AbstractController {    protected static final String CUSTOMER_SESSION_ID = "auth";    protected static final String SHOOPY_CART_ID = "SHOOPY_CART";    protected static final String REFERER_HEADER = "Referer";    @Autowired    protected CatalogService catalogService;    @Autowired    protected TotalAmountCalculator amountCalculator;    @Autowired    protected CustomerRepository customerRepo;    @Autowired    protected CartRepository cartRepository;    //Insert necessary data to the header tag    public void populateHeaderData(Model model, Cart activeCart) {        HeaderDTOBuilder.HeaderDTO headerDTO = new HeaderDTOBuilder().                withAmountCalculator(amountCalculator)                .withActiveCart(activeCart)                .withCategories(catalogService.getCategories()).build();        model.addAttribute("headerDTO", headerDTO);    }    public void populateHeaderData(Model model, HttpServletRequest request) {        Cart activeCart = getCartFrom(request);        if(activeCart == null){            activeCart = new Cart();        }        populateHeaderData(model, activeCart);    }}我很想了解 Controller 和 AbstractController 之間的區別。