注入参数详解:null值和级联属性
可以使用专用的<null/>元素标签为Bean的字符串或其他对象类型的属性注入null值。
和Struts,Hiberante等框架一样,Spring支持级联属性的配置。
可以如下设置null值
<?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 http://www.springframework.org/schema/beans/spring-beans.xsd">
<!-- 通过构造方法来配置Bean的属性 -->
<bean id="car" class="logan.spring.study.Car">
<constructor-arg value="Audi" index="0"></constructor-arg>
<constructor-arg value="Shanghai" index="1"></constructor-arg>
<constructor-arg value="3000000" index="2" type="double"></constructor-arg>
</bean>
<bean id="car2" class="logan.spring.study.Car">
<constructor-arg value="Baoma" type="java.lang.String"></constructor-arg>
<constructor-arg type="java.lang.String">
<value><![CDATA[<Shanghai>]]></value>
</constructor-arg>
<constructor-arg type="int">
<value>250</value>
</constructor-arg>
</bean> <bean id="person" class="logan.spring.study.Person">
<property name="name" value="Tom"></property>
<property name="age" value="24"></property>
<property name="car">
<bean class="logan.spring.study.Car">
<constructor-arg value="Ford"></constructor-arg>
<constructor-arg value="Changan"></constructor-arg>
<constructor-arg value="20000000" type="double"></constructor-arg>
</bean>
</property>
</bean> <bean id="person2" class="logan.spring.study.Person">
<constructor-arg value="Jerry"></constructor-arg>
<constructor-arg value="25"></constructor-arg>
<!--
<constructor-arg ref="car"></constructor-arg>
-->
<constructor-arg><null/></constructor-arg>
</bean></beans>
输出结果
五月 19, 2017 8:54:47 下午 org.springframework.context.support.ClassPathXmlApplicationContext prepareRefresh
信息: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@3eb07fd3: startup date [Fri May 19 20:54:47 CST 2017]; root of context hierarchy
五月 19, 2017 8:54:47 下午 org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions
信息: Loading XML bean definitions from class path resource [applicationContext.xml]
HelloWorld's Constructor...
HelloWorld's Constructor...
Car [brand=Audi, corp=Shanghai, price=3000000.0, maxspeed=0]
Car [brand=Baoma, corp=<Shanghai>, price=0.0, maxspeed=250]
Person [name=Jerry, age=25, car=null]
看下面级联属性的赋值
<?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 http://www.springframework.org/schema/beans/spring-beans.xsd">
<!-- 通过构造方法来配置Bean的属性 -->
<bean id="car" class="logan.spring.study.Car">
<constructor-arg value="Audi" index="0"></constructor-arg>
<constructor-arg value="Shanghai" index="1"></constructor-arg>
<constructor-arg value="3000000" index="2" type="double"></constructor-arg>
</bean>
<bean id="car2" class="logan.spring.study.Car">
<constructor-arg value="Baoma" type="java.lang.String"></constructor-arg>
<constructor-arg type="java.lang.String">
<value><![CDATA[<Shanghai>]]></value>
</constructor-arg>
<constructor-arg type="int">
<value>250</value>
</constructor-arg>
</bean> <bean id="person" class="logan.spring.study.Person">
<property name="name" value="Tom"></property>
<property name="age" value="24"></property>
<property name="car">
<bean class="logan.spring.study.Car">
<constructor-arg value="Ford"></constructor-arg>
<constructor-arg value="Changan"></constructor-arg>
<constructor-arg value="20000000" type="double"></constructor-arg>
</bean>
</property>
</bean> <bean id="person2" class="logan.spring.study.Person">
<constructor-arg value="Jerry"></constructor-arg>
<constructor-arg value="25"></constructor-arg>
<constructor-arg ref="car"></constructor-arg>
<property name="car.maxspeed" value="250"></property>
</bean></beans>
下面是输出结果,注意:属性需要先初始化以后才可以为级联属性赋值,否则会有异常,和Struts2不同。
五月 19, 2017 9:17:51 下午 org.springframework.context.support.ClassPathXmlApplicationContext prepareRefresh
信息: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@3eb07fd3: startup date [Fri May 19 21:17:51 CST 2017]; root of context hierarchy
五月 19, 2017 9:17:51 下午 org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions
信息: Loading XML bean definitions from class path resource [applicationContext.xml]
HelloWorld's Constructor...
HelloWorld's Constructor...
Car [brand=Audi, corp=Shanghai, price=3000000.0, maxspeed=250]
Car [brand=Baoma, corp=<Shanghai>, price=0.0, maxspeed=250]
Person [name=Jerry, age=25, car=Car [brand=Audi, corp=Shanghai, price=3000000.0, maxspeed=250]]