2.IoC基础

UserDao接口

package com.dream.dao;

public interface UserDao {
    void getUser();
}

UserDao实现类

package com.dream.dao;

public class UserDaoImpl implements UserDao{
    @Override
    public void getUser() {
        System.out.println("获取用户数据");
    }
}

UserService接口

package com.dream.service;

public interface UserService {
    void getUser();
}

UserService实现类

package com.dream.service;

import com.dream.dao.UserDao;
import com.dream.dao.UserDaoImpl;

public class UserServiceImpl implements UserService{
   UserDao userDao = new UserDaoImpl();
    @Override
    public void getUser() {
      userDao.getUser();
    }
}

测试类

import com.dream.service.UserServiceImpl;
import org.junit.Test;

public class MyTest {
    @Test
    public void test(){
        UserServiceImpl userService = new UserServiceImpl();
        userService.getUser();
    }
}

如果我们有新的需求

package com.dream.dao;

public class UserDaoMysqlImpl implements UserDao{
    @Override
    public void getUser() {
        System.out.println("MySql获取数据");
    }
}

就需要去UserService修改代码

package com.dream.service;

import com.dream.dao.UserDao;
import com.dream.dao.UserDaoImpl;
import com.dream.dao.UserDaoMysqlImpl;

public class UserServiceImpl implements UserService{
   UserDao userDao = new UserDaoMysqlImpl();
    @Override
    public void getUser() {
      userDao.getUser();
    }
}

同理

package com.dream.dao;

public class UserDaoOracleImpl implements UserDao{
    @Override
    public void getUser() {
        System.out.println("Oracle获取数据");
    }
}

package com.dream.service;

import com.dream.dao.UserDao;
import com.dream.dao.UserDaoImpl;
import com.dream.dao.UserDaoMysqlImpl;
import com.dream.dao.UserDaoOracleImpl;

public class UserServiceImpl implements UserService{
   UserDao userDao = new UserDaoOracleImpl();
    @Override
    public void getUser() {
      userDao.getUser();
    }
}

可如果这种需求很多，每次变动就需要修改大量代码，代码写死了，耦合性太高，这种方式就不适用了

那这种问题该怎么解决呢

重写修改一下UserServiceImpl代码

package com.dream.service;

import com.dream.dao.UserDao;
import com.dream.dao.UserDaoImpl;
import com.dream.dao.UserDaoMysqlImpl;
import com.dream.dao.UserDaoOracleImpl;

public class UserServiceImpl implements UserService{
   private UserDao userDao;

    public void setUserDao(UserDao userDao) {
        this.userDao = userDao;
    }

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

测试

import com.dream.dao.UserDaoImpl;
import com.dream.dao.UserDaoMysqlImpl;
import com.dream.dao.UserDaoOracleImpl;
import com.dream.service.UserServiceImpl;
import org.junit.Test;

public class MyTest {
    @Test
    public void test(){
        UserServiceImpl userService = new UserServiceImpl();
        userService.setUserDao(new UserDaoImpl());
        userService.getUser();
        userService.setUserDao(new UserDaoMysqlImpl());
        userService.getUser();
        userService.setUserDao(new UserDaoOracleImpl());
        userService.getUser();
    }
}

可以看到UserServiceImpl相当于提供了一个接口，面对不同的需求，我们可以自行创建对象传递给他。主动权交给了调用者，程序不用去管怎么创建,怎么实现，只负责提供一个接口， 这也就是IoC的原型

控制反转(IoC)是一种通过描述（XML或注解）并通过第三方去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，其实现方法是依赖注入(DI)

采用XML方式配置Bean的时候，Bean的定义信息是和实现分离的，而采用注解的方式可以把两者合为一体，Bean的定义信息直接以注解的形式定义在实现类中，从而达到了零配置的目的