[Java知识库]Spring Why

1. 先看一个案例

Pojo 实体类

package com.lmy.pojo;

public class User {
    public int id;
    public String name;
    public User(String name) {
        this.name = name;
    }
}

Dao 接口

package com.lmy.dao;

import com.lmy.pojo.User;

public interface UserDao {
    User selectUser();
}

Dao 实现

package com.lmy.dao;

import com.lmy.pojo.User;

public class UserDaoImpl implements UserDao{
    public User selectUser() {
        System.out.println("从默认数据库查询");
        return new User("独行者");
    }
}

package com.lmy.dao;

import com.lmy.pojo.User;

public class UserDaoMysqlImpl implements UserDao{
    public User selectUser() {
        System.out.println("从Mysql数据库查询");
        return new User("独行者");
    }
}

package com.lmy.dao;

import com.lmy.pojo.User;

public class UserDaoOracleImpl implements UserDao{
    public User selectUser() {
        System.out.println("从Oracle数据库查询");
        return new User("独行者");
    }
}

package com.lmy.dao;

import com.lmy.pojo.User;

public class UserDaoExcelImpl implements UserDao{
    public User selectUser() {
        System.out.println("从Excel数据库查询");
        return new User("独行者");
    }
}

Service 业务接口

package com.lmy.service;

import com.lmy.pojo.User;

public interface UserService {
    User selectUser();
}

Service 业务实现

package com.lmy.service;

import com.lmy.dao.*;
import com.lmy.pojo.User;

public class UserServiceImpl implements UserService {
    private UserDao userDao;

    public UserServiceImpl() {
        userDao = new UserDaoImpl();
        userDao = new UserDaoExcelImpl();
        userDao = new UserDaoOracleImpl();
        userDao = new UserDaoMysqlImpl();
    }

    public User selectUser() {
        return userDao.selectUser();
    }
}

测试

package com.lmy.test;

import com.lmy.service.UserService;
import com.lmy.service.UserServiceImpl;

public class MyTest {
    public static void main(String[] args) {
        UserService userService = new UserServiceImpl();
        System.out.println(userService.selectUser().name);
    }
}

用户需求
用户的需求经常变更
第一天他说MySQL比较好，想用MySQL查询数据
第二天他说Oracle比较牛，想用Oracle查询数据
第三天他说Excel比较简单，想用Excel查询数据
第四天他说MySQL比较好，又想用MySQL查询数据
用户需要每变更一次，在需要重新设置userDao的实现类

public UserServiceImpl() {
    userDao = new UserDaoImpl();
    userDao = new UserDaoExcelImpl();
    userDao = new UserDaoOracleImpl();
    userDao = new UserDaoMysqlImpl();
}

只有一个查询业务还好说，如果成百上千个Service(业务 登录业务 用户业务 产品业务)
也就是有成百上千个地方在new userDao
每一个Service都要改一下userDao的实现(重新new)
那就是一个灾难

2. 解决问题

如何解决上述问题，首先想到的是让用户自己去选择去使用哪个数据库
Service 业务层修改

package com.lmy.service;

import com.lmy.dao.UserDao;
import com.lmy.pojo.User;

public interface UserService {
    void setUserDao(UserDao userDao); //修改: 设置Dao层接口
    User selectUser();
}

package com.lmy.service;

import com.lmy.dao.*;
import com.lmy.pojo.User;

public class UserServiceImpl implements UserService {
    private UserDao userDao;

    public UserServiceImpl() {
        userDao = new UserDaoImpl();
    }

    //修改: 设置Dao层接口
    public void setUserDao(UserDao userDao) {
        this.userDao = userDao;
    }

    public User selectUser() {
        return userDao.selectUser();
    }
}

再次测试，这里相当于用户调用，用户来选择使用哪个数据库

public class MyTest {
    public static void main(String[] args) {
        UserService userService = new UserServiceImpl();
        userService.setUserDao(new UserDaoImpl());
        userService.setUserDao(new UserDaoMysqlImpl());
        userService.setUserDao(new UserDaoOracleImpl());
        userService.setUserDao(new UserDaoExcelImpl());
        System.out.println(userService.selectUser().name);
    }
}

这个就相当于控制反转IOC的原型
之前代码主动权在业务层 Service
现在代码主动权在用户的 Test

3. 总结

控制反转是一种通过描述(XML或注解)并通过第三方生产或获取特定对象的方式
在Spring中实现控制反转的是IOC容器，实现方法是依赖注入DI