初始模式:

dao层接口:
    public interface UserDao {
        void getUser();
    }
dao层实现类:
    public class UserDaoImpl implements UserDao {
        public void getUser() {
            System.out.println("默认获取用户的数据");
        }
    }
service层接口:
    public interface UserService {
        void getUser();
    }
service层实现类:
    public class UserServiceImpl implements UserService {
        //程序猿手动创建dao层对象,用来调用dao层方法
        private UserDao userDao = new UserDaoImpl();
        public void getUser() {
            userDao.getUser();
        }
    }
servlet调用方法时:
    UserService userService = new UserServiceImpl();
    userServiceImpl.getUser();
需要扩展功能时:
    public class UserDaoMysqlImpl implements UserDao{
        @Override
        public void getUser() {
            System.out.println("Mysql获取用户数据！");
        }
    }
    sevice层需要修改代码:
    private UserDao userDao = new UserDaoImpl();-->private UserDao userDao = new UserDaoMysqlImpl();

此种方式若在dao层增加功能时,需要程序员修改service层代码,以实现对不同dao层对象的调用,此时,程序主动创建对象,控制权掌握在程序员手上！这样不利于后期的更新维护,违反了开闭性原则.

修改之后:

service层
    public class UserServiceImpl implements UserService {
        private UserDao userDao;
        //利用set进行动态实现值的注入！
        public void setUserDao(UserDao userDao){
            this.userDao = userDao;
        }
        public void getUser() {
            userDao.getUser();
        }
    }
servlet调用方法时:
    UserServiceImpl service = new UserServiceImpl();
    service.setUserDao( new UserDaoMySqlImpl() );
    service.getUser();

==这样添加了set方法之后,巧妙的利用了多态,实现了对控制权的反转,原先由程序来创建的userDao,现在交给了调用者决定创建谁,使得系统的耦合性降低.==
现在当你需要横向扩展你的dao层代码时,不会再影响到service层,这不正是==面向切面编程==的思想吗,spring框架在很多地方都巧妙地实现了控制反转.
本文为学习狂神sping框架有感,附上链接:

https://www.cnblogs.com/hellokuangshen/p/11249253.html