Spring bean reference injection definition by external configuration.

applicationContext.xml

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans" 
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:context="http://www.springframework.org/schema/context"
    xsi:schemaLocation="
        http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.0.xsd
        http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.0.xsd"
    default-autowire="byType">

    <context:property-placeholder location="file:bin/leoliang/spring/test1/bean.properties"/>

    <bean id="fooA" class="leoliang.spring.test1.FooImplA" />
    <bean id="fooB" class="leoliang.spring.test1.FooImplB" />
    
    <bean id="bar" class="leoliang.spring.test1.Bar">
        <property name="foo" ref="${foo_bean_id}"/>
    </bean>
</beans>

bean.properties

# We have two beans implementing FooInterface: fooA, fooB
foo_bean_id = fooB