那么在注入pencilCase的list的数据中为什么是<value>字符串</value> 为什么不是Pen的实体店对象呢？

package com.imooc.spring.ioc.entity;

import java.util.List;

public class SchoolBag {
    private String color;
    private List<Pen> pencilCase;

    public SchoolBag() {
    }

    public SchoolBag(String color, List<Pen> pencilCase) {
        this.color = color;
        this.pencilCase = pencilCase;
    }

    public List<Pen> getPencilCase() {
        return pencilCase;
    }

    public void setPencilCase(List<Pen> pencilCase) {
        this.pencilCase = pencilCase;
    }

    public String getColor() {
        return color;
    }

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

    @Override
    public String toString() {
        return "SchoolBag{" +
                "color='" + color + '\'' +
                ", pencilCase=" + pencilCase +
                '}';
    }
}

<?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="pen1" class="com.imooc.spring.ioc.entity.Pen">
        <property name="name" value="钢笔"></property>
    </bean>

    <bean id="shoolBag" class="com.imooc.spring.ioc.entity.SchoolBag">
        <property name="color" value="blue"></property>
        <property name="pencilCase">
            <list>
                <value>钢笔</value>
                <value>铅笔</value>
            </list>
        </property>
    </bean>
</beans>

如果SchoolBag类的属性pengcilCase 的类型是List<pen>，

那么在注入pencilCase的list的数据中为什么是<value>字符串</value>
为什么不是Pen的实体店对象呢？