IoC（Inversion of Control，控制反转）容器是Spring框架中非常 重要的核心组件，可以说它是伴随Spring的诞生和成长的组件。Spring通过IoC容器来管理所有的Java对象（也称bean）及其相互间的依赖关系。

1 依赖注入和控制反转

依赖注入(Dependency Injection)是对“控制反转”的不同说法，本质是一回事。

2 IoC容器和bean

Spring通过IoC容器来管理所有Java对象及其相互之间的依赖关系。
IoC容器在创建bean的时候，会注入其依赖项。
IoC的应用有以下两种设计模式：

• 反射：在运行状态中，根据提供的类的路径或类名，通过反射来动态获取该类的所有属性和方法。
• 工厂模式：把IoC容器当做一个工厂，在配置文件或注解中给出定义，然后利用反射技术，根据给出的类名生成相应的对象。对象生成的代码及对象之间的依赖关系在配置文件中定义，实现了解耦。

Spring IoC容器的核心基础包：

• org.springframework.beans
• org.springframework.context

3 配置和使用

配置方式有:

• xml形式

<bean id = "..." class="...">
   <!-- 放置这个bean的协作者和配置 -->
</bean>

• 注解形式

@Configuration
public class AppConfig{
    @Bean
    public MyService myService(){
          return new MyServiceImpl();
   }
}

4 注入方式

• 构造器注入
• setter注入

5 实战：依赖注入的例子

• 消息服务接口和实现类

public interface MessageService {
   String getMessage();
}

public class MessageServiceImpl implements MessageService {
    private String username;
    private int age;

    public MessageServiceImpl(String username, int age) {
        this.username = username;
        this.age = age;
    }

    @Override
    public String getMessage() {
        return "Hello World!" + "\nusername is " + username + ",age is " + age;
    }
}

• 打印器类

public class MessagePrinter {
    final private MessageService messageService;

    public MessagePrinter(MessageService messageService) {
        this.messageService = messageService;
    }

    public void printMessage() {
        System.out.println(this.messageService.getMessage());
    }
}

• 配置文件

 <!--定义bean,并使用构造器注入-->
    <bean id="messageServiceImpl" class="com.spring.quickstart.MessageServiceImpl">
        <constructor-arg name="username" value="Tom"/>
        <constructor-arg name="age" value="20"/>
    </bean>

    <bean id="messagePrinter" class="com.spring.quickstart.MessagePrinter">
        <constructor-arg name="messageService" ref="messageServiceImpl"/>
    </bean>

• 应用主类

public class MessageApp {
    public static void main(String[] args) {
        ApplicationContext context = new ClassPathXmlApplicationContext("/applicationContext.xml");
        MessagePrinter messagePrinter = context.getBean(MessagePrinter.class);
        messagePrinter.printMessage();
    }
}

• 运行结果

6 依赖注入详细配置

• 直接赋值
• 引用其他bean
• 内部bean
• 集合
• Null及空字符的值
• xml短域名空间
• 复合属性名称

7 IoC综合练习：Boss、Car、Meeting

• Car：属性有brand、color、parameter

• Boss：属性有name、company、car、hobbies

• Meeting：属性theme、bosses
  综合采用所学内容，设计并完成以上综合练习

• Car类

public class Car {
   private String brand;
   private String color;
   private String parameter;

   public Car(String brand, String color, String parameter) {
       this.brand = brand;
       this.color = color;
       this.parameter = parameter;
   }

   public Car() {
   }

   public String getBrand() {
       return brand;
   }

   public void setBrand(String brand) {
       this.brand = brand;
   }

   public String getColor() {
       return color;
   }

   public void setColor(String color) {
       this.color = color;
   }

   public String getParameter() {
       return parameter;
   }

   public void setParameter(String parameter) {
       this.parameter = parameter;
   }

   @Override
   public String toString() {
       return "Car{" +
               "brand='" + brand + '\'' +
               ", color='" + color + '\'' +
               ", parameter='" + parameter + '\'' +
               '}';
   }
}

• Boss类

public class Boss {
   private String name;
   private String Company;
   private Car car;
   private List<String> hobbies;

   public Boss(String name, String company, Car car, List<String> hobbies) {
       this.name = name;
       Company = company;
       this.car = car;
       this.hobbies = hobbies;
   }

   public Boss() {
   }

   public String getName() {
       return name;
   }

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

   public String getCompany() {
       return Company;
   }

   public void setCompany(String company) {
       Company = company;
   }

   public Car getCar() {
       return car;
   }

   public void setCar(Car car) {
       this.car = car;
   }

   public List<String> getHobbies() {
       return hobbies;
   }

   public void setHobbies(List<String> hobbies) {
       this.hobbies = hobbies;
   }

   @Override
   public String toString() {
       return "Boss{" +
               "name='" + name + '\'' +
               ", Company='" + Company + '\'' +
               ", car=" + car +
               ", hobbies=" + hobbies +
               '}';
   }
}

• Meeting类

public class Meeting {
   private String theme;
   private List<Boss> bosses;

   public Meeting(String theme, List<Boss> bosses) {
       this.theme = theme;
       this.bosses = bosses;
   }

   public Meeting() {
   }

   public String getTheme() {
       return theme;
   }

   public void setTheme(String theme) {
       this.theme = theme;
   }

   public List<Boss> getBosses() {
       return bosses;
   }

   public void setBosses(List<Boss> bosses) {
       this.bosses = bosses;
   }

   @Override
   public String toString() {
       return "Meeting{" +
               "theme='" + theme + '\'' +
               ", bosses=" + bosses +
               '}';
   }
}

• 配置文件

   <!--配置三个Car的bean-->
   <bean id="car1" class="com.spring.ioc.Car">
       <constructor-arg name="brand" value="奔驰"/>
       <constructor-arg name="color" value="黑色"/>
       <constructor-arg name="parameter" value="4.0T"/>
   </bean>

   <bean id="car2" class="com.spring.ioc.Car">
       <constructor-arg name="brand" value="宝马X6"/>
       <constructor-arg name="color" value="白色"/>
       <constructor-arg name="parameter" value="3.8T"/>
   </bean>

   <bean id="car3" class="com.spring.ioc.Car">
       <property name="brand" value="劳斯莱斯"/>
       <property name="color" value="金色"/>
       <property name="parameter" value="4.5T"/>
   </bean>

   <!--配置三个Boss的bean-->
   <bean id="boss1" class="com.spring.ioc.Boss">
       <constructor-arg name="name" value="马云"/>
       <constructor-arg name="company" value="阿里巴巴"/>
       <constructor-arg name="car" ref="car1"/>
       <constructor-arg name="hobbies">
           <list>
               <value>演讲</value>
               <value>写作</value>
           </list>
       </constructor-arg>
   </bean>

   <bean id="boss2" class="com.spring.ioc.Boss">
       <property name="name" value="马化腾"/>
       <property name="company" value="腾讯"/>
       <property name="car" ref="car2"/>
       <property name="hobbies">
           <list>
               <value>打游戏</value>
               <value>看电影</value>
           </list>
       </property>
   </bean>

   <bean id="boss3" class="com.spring.ioc.Boss">
       <property name="name" value="李彦宏"/>
       <property name="company" value="百度"/>
       <property name="car" ref="car3"/>
       <property name="hobbies">
           <list>
               <value>阅读</value>
               <value>运动</value>
           </list>
       </property>
   </bean>


   <!--配置一个Meeting的bean-->
   <bean id="meeting" class="com.spring.ioc.Meeting">
       <property name="theme" value="2019互联网高层峰会"/>
       <property name="bosses">
           <list>
               <ref bean="boss1"/>
               <ref bean="boss2"/>
               <ref bean="boss3"/>
           </list>
       </property>
   </bean>

• MeetingApp主类

public class MeetingApp {
   public static void main(String[] args) {
       ApplicationContext context = new ClassPathXmlApplicationContext("/applicationContext.xml");
       Meeting meeting = context.getBean(Meeting.class);
       System.out.println(meeting);
   }
}

• 运行结果

Meeting{theme='2019互联网高层峰会', bosses=[Boss{name='马云', Company='阿里巴巴', car=Car{brand='奔驰', color='黑色', parameter='4.0T'}, hobbies=[演讲, 写作]}, Boss{name='马化腾', Company='腾讯', car=Car{brand='宝马X6', color='白色', parameter='3.8T'}, hobbies=[打游戏, 看电影]}, Boss{name='李彦宏', Company='百度', car=Car{brand='劳斯莱斯', color='金色', parameter='4.5T'}, hobbies=[阅读, 运动]}]}