spring学习（1）IOC理论推导

1.创建项目

2.往pom.xml(spring-study)中导包

    
        
        
            org.springframework
            spring-webmvc
            5.2.0.RELEASE
        
    

3.创建接口类UserDao和对应实现类UserDaoImpl

package com.kuang.dao;

public interface UserDao {
    void getUser();
}

Impl要实现其接口

package com.kuang.dao;
//Impl要实现其接口
public class UserDaoImpl implements UserDao{
    public void main(String[] args) {
        getUser();
    }

    public void getUser() {
        System.out.println("默认获取用户的数据");
    }
}

4.创建实现类UserService和UserServiceImpl

package com.kuang.service;

public interface UserService {
    void getUser();
}

业务层调Dao层

package com.kuang.service;

import com.kuang.dao.UserDao;
import com.kuang.dao.UserDaoImpl;

public class UserServiceImpl implements UserService{
    private UserDao userDao = new UserDaoImpl();

    public void getUser(){
        userDao.getUser();
    }
}

5.在java包下创建测试类MyTest

import com.kuang.dao.UserDaoMysqlImpl;
import com.kuang.service.UserService;
import com.kuang.service.UserServiceImpl;

public class MyTest {
    public static void main(String[] args) {
        //用户实际调用的是业务层，dao层他们不需要接触！
        UserService userService = new UserServiceImpl();
        userService.getUser();
    }
}

问题：如果Dao层中加了一个UserDaoMysqlImpl或UserDaoOracleImpl并需要调用

就要修改UserServiceImpl中的

private UserDao userDao = new UserDaoImpl();

将UserDaoImpl改为UserDaoMysqlImpl或UserDaoOracleImpl，违背开闭原则

解决：更改UserServiceImpl为

package com.kuang.service;
import com.kuang.dao.UserDao;
import com.kuang.dao.UserDaoImpl;
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();
    }
}

再修改测试类 

import com.kuang.dao.UserDaoMysqlImpl;
import com.kuang.service.UserService;
import com.kuang.service.UserServiceImpl;
public class MyTest {
    public static void main(String[] args) {
        //用户实际调用的是业务层，dao层他们不需要接触！
        UserService userService = new UserServiceImpl();
        ((UserServiceImpl) userService).setUserDao(new UserDaoMysqlImpl());
        userService.getUser();
    }
}

这样就可以只修改里面的 UserDaoMysqlImpl为用户需要的方法即可（选择权交给用户，而基本实现留在自己）

((UserServiceImpl) userService).setUserDao(new UserDaoMysqlImpl());