spring的value，null标签

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns="http://www.springframework.org/schema/beans" xmlns:p="http://www.springframework.org/schema/p"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
    http://www.springframework.org/schema/beans/spring-beans-3.0.xsd">
    <bean id="saxophone" class="com.stono.sprtest.Saxophone"></bean>
    <bean id="duke" class="com.stono.sprtest.Duke" p:name="Elvis">
        <property name="instrument">
            <null />
        </property>
        <property name="age">
            <value>123</value>
        </property>
    </bean>
</beans>