DepartmentController中的问题

问题1：定义的是DepartmentService接口，接口如何调用实例化类的方法？

问题2：为什么不是定义DepartmentService的实例化类呢?

public class DepartmentController {
   @Autowired
   private DepartmentService departmentService;
   // /department/list.do     department_list.jsp
   public void list(HttpServletRequest request,HttpServletResponse response) throws ServletException, IOException {
       List<Department> list= departmentService.getAll();
       request.setAttribute("LIST",list);
       request.getRequestDispatcher("../department_list.jsp").forward(request,response);
   }