spring 注入的问题，请大佬解惑

代码是这样的：
   第一段：public interface UserDao {
                public int addUser(User user);
                public int updateUser(User user);
           }
  第二段：public class User {
          }
  第三段：public class UserDaoImpl implements UserDao {

            public int addUser(User user) {
            System.out.println( "UserDaoImpl.addUser()" );
            return 0;
            }

            public int updateUser(User user) {
                System.out.println( "UserDaoImpl.updateUser()" );
                    return 0;
            }

            }

  第四段：public interface UserService {
             public boolean addUser(User user);
             public boolean updateUser(User user);
 
          }

  第五段：public class UserServiceImpl implements UserService {

                public boolean addUser(User user) {
                    System.out.println( "UserServiceImpl.updateUser()" );
                     return false;
                }

                public boolean updateUser(User user) {
                    System.out.println( "UserServiceImpl.updateUser()" );
                    return false;
                }

            }

问题是怎么看出来UserServiceImpl 依赖了 userDao?
这里面似乎并没有私有属性啊