SpringIOC的一个小例子 有一处不明白

package com.jike.spring.chapter01;

//接口

public interface IHelloMessage {

	

	public String sayHello();

}

package com.jike.spring.chapter01;



public class HelloChina implements IHelloMessage {



	@Override

	public String sayHello() {

		return "大家好";

	}



}

package com.jike.spring.chapter01;



public class HelloWorld implements IHelloMessage {



	@Override

	public String sayHello() {

		return "Hello World!";

	}



}

package com.jike.spring.chapter01;



public class Person {

	

	//因为Person类需要引用 IHelloMessage 所以要把IHelloMessage写成一个属性

	private IHelloMessage helloMessage;

	

	public IHelloMessage getHelloMessage() {

		return helloMessage;

	}



	public void setHelloMessage(IHelloMessage helloMessage) {

		this.helloMessage = helloMessage;

	}



	public String sayHello(){    

		return this.helloMessage.sayHello();   //调用IHelloMessage这个接口来输出问候信息

	}

}

package com.jike.spring.chapter01;



import org.springframework.beans.factory.BeanFactory;

import org.springframework.beans.factory.xml.XmlBeanFactory;

import org.springframework.core.io.FileSystemResource;

import org.springframework.core.io.Resource;





public class Main {



	

	public static void main(String[] args) {

		//利用FileSystemResource 来读取配置文件

		Resource r = new FileSystemResource("helloMessage.xml");

		//使用XmlBeanFactory来加载配置文件 启动IOC容器

		BeanFactory f = new XmlBeanFactory(r);

		//从IOC容器中获取Person类的实例

		Person lizi = (Person)f.getBean("chaoji");

		//需要用Person实例来输出问候信息

		String s = lizi.sayHello();

		System.out.println(s);

	}



}

<?xml version="1.0" encoding="UTF-8"?>  

<!DOCTYPE beans PUBLIC "-//SPRING//DTD BEAN//EN" 

"http://www.springframework.org/dtd/spring-beans.dtd">  



<beans>

	<bean id = "helloWorld" class = "com.jike.spring.chapter01.HelloWorld">

	</bean>

	

	<bean id = "helloChina" class = "com.jike.spring.chapter01.HelloChina">

	 

	</bean>

	

	<bean id = "chaoji" class = "com.jike.spring.chapter01.Person">

		 <property name = "helloMessage" ref = "helloWorld">

		</property>

	</bean>

</beans>