1. Ioc

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一. 传统方法的一般实现

Dao层：

# 基类
public interface UserDao {
    String getUser();
}

# 一般的实现
public class UserDaoImp implements UserDao{
    public String getUser() {
        // do something
        String str = "The data of User";
       return str;
    }
}

# Mysql的实现
public class UserDaoMySqImp implements UserDao{

    public String getUser() {
        return "the Data of Mysql";
    }
}

# Oracle的实现
public class UserDaoOracleImp implements UserDao{
    public String getUser() {
        return "The data of Oracle";
    }
}

Service层：

# 基类
public interface UserService {
    String getUser();
}

# 代码耦合性太高
public class UserServiceImp implements UserService{

    private UserDao userDao = new UserDaoImp();
    # MySql的实现
    private UserDao userDao = new UserDaoMySqImp();
    #Oracle的实现
    private UserDao userDao = new UserDaoOracleImp();
    
    public String getUser() {
        return userDao.getUser();
    }
}

public static void main(String[] args) {
     UserService userService = new UserServiceImp();
     String user = userService.getUser();
     System.out.println(user);
}

############### 改进 1.0 
public class UserServiceImp implements UserService{

    private UserDao userDao;
    
    # 这里是多态实现的，不涉及到具体的，接耦
    public void setUserDao(UserDao userDao) {
        this.userDao = userDao;
    }

    public String getUser() {
        return userDao.getUser();
    }
}

public static void main(String[] args) {
     UserService userService = new UserServiceImp();
     # 对MySql的实现
     ((UserServiceImp) userService).setUserDao(new UserDaoMySqImp());
     # 对 ((UserServiceImp) userService).setUserDao(new UserDaoOracleImp());
    ((UserServiceImp) userService).setUserDao(new UserDaoMySqImp());

     String user = userService.getUser();
     System.out.println(user);
     String user = userService.getUser();
     System.out.println(user);
}


#### 改进2.0 工厂模式

#### 改进3.0 工程方法模式

二. Spring 依赖倒置实现（DI）(HelloSpring)

1.0 创建 maven 项目
2.0 在 pom.xml 中导入spring 相关的依赖:

<dependencies>
    <dependency>
        <groupId>org.springframework</groupId>
        <artifactId>spring-webmvc</artifactId>
        <version>5.3.10</version>
    </dependency>
</dependencies>

'''

3. 在 com.pojo 实体中创建

public class Hello {
    private String name;
    
    ### 一定要设置 setter 和 getter，spring 通过它来实现
    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "Hello{" +
                "name='" + name + '\'' +
                '}';
    }
}

4.0 在Resouce 下面创建 beans.xml : ( 到 Spring的文档官可以找到)

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
        https://www.springframework.org/schema/beans/spring-beans.xsd">

   <bean id="hello" class="com.pojo.Hello">
        <property name="name" value="Spring-name">

        </property>
   </bean>

</beans>

在Test中

public class MyTest {
    public static void main(String[] args) {
        //获取Spring的上下文对象， 可以去Spring 文档中找到
        ApplicationContext context = new ClassPathXmlApplicationContext("beans.xml");

        Hello hello = (Hello) context.getBean("hello");

        System.out.println(hello.toString() );
    }
}

• 其中 Hello 是谁创建的 ？
  Spring 创建的
• Hello的属性怎么设置
  通过Spring 容易来设置的

5.0 用这个来实现一种的东西
首先，在beans中配置：

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
        https://www.springframework.org/schema/beans/spring-beans.xsd">

    <bean id="mySqlImp" class="com.dao.UserDaoMySqImp" />
    <bean id="OracleImp" class="com.dao.UserDaoOracleImp" />

    <bean id="userServiceImp" class="com.service.UserServiceImp">
        <!-- ref  : 创建文件中创建好的对象，
             value: 具体的值
        -->
        <property name="userDao" ref="mySqlImp" />
    </bean>

</beans>

在Main 就可以不用new 来实现

import com.dao.UserDaoMySqImp;
import com.dao.UserDaoOracleImp;
import com.service.UserService;
import com.service.UserServiceImp;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class MyTesy {
    public static void main(String[] args) {
        ApplicationContext contex = new ClassPathXmlApplicationContext("beans.xml");

        UserServiceImp userService = (UserServiceImp) contex.getBean("userServiceImp");
        System.out.println( userService.getUser());
    }
}

如需要改用，只需要在配置文件 pom.xml 中进行修改即可：
ref = "mySqlImp" 换成 ref = "OracleImp" 即可

 <bean id="userServiceImp" class="com.service.UserServiceImp">
        <property name="userDao" ref="mySqlImp" />
    </bean>