Spring多个Xml配置文件中PropertySourcesPlaceholderConfigurer的加载问题？

在Spring框架写Junit测试类时，引用了两个Spring配置文件：spring-mail.xml和spring-mybatis.xml，两个配置文件中分别引用了Properties：QQMail.properties和estore.properties，在debug Junit时发生了占位符无法识别的错误。
spring-mail相关配置信息如下：

spring-mybatis相关配置信息如下：

测试日志如下：

2015-11-9 14:10:19 org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions
信息: Loading XML bean definitions from class path resource [config/spring-mybatis.xml] 2015-11-9 14:10:19 org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions
信息: Loading XML bean definitions from class path resource [config/spring-mail.xml] 2015-11-9 14:10:19 org.springframework.beans.factory.support.DefaultListableBeanFactory registerBeanDefinition
信息: Overriding bean definition for bean 'estorePropConfigurer': replacing [Generic bean: class [org.springframework.context.support.PropertySourcesPlaceholderConfigurer]; scope=; abstract=false; lazyInit=false; autowireMode=0; dependencyCheck=0; autowireCandidate=true; primary=false; factoryBeanName=null; factoryMethodName=null; initMethodName=null; destroyMethodName=null; defined in class path resource [config/spring-mybatis.xml]] with [Generic bean: class [org.springframework.context.support.PropertySourcesPlaceholderConfigurer]; scope=; abstract=false; lazyInit=false; autowireMode=0; dependencyCheck=0; autowireCandidate=true; primary=false; factoryBeanName=null; factoryMethodName=null; initMethodName=null; destroyMethodName=null; defined in class path resource [config/spring-mail.xml]] 2015-11-9 14:10:19 org.springframework.context.support.AbstractApplicationContext prepareRefresh
信息: Refreshing org.springframework.context.support.GenericApplicationContext@a0b1cd0: startup date [Mon Nov 09 14:10:19 CST 2015]; root of context hierarchy 2015-11-9 14:10:20 org.springframework.core.io.support.PropertiesLoaderSupport loadProperties
信息: Loading properties file from class path resource [properties/estore.properties] 2015-11-9 14:10:20 org.springframework.test.context.TestContextManager prepareTestInstance
....
Caused by: org.springframework.beans.factory.BeanDefinitionStoreException: Invalid bean definition with name 'QQMailAuthenticator' defined in class path resource [config/spring-mail.xml]: Could not resolve placeholder 'Mail.username' in string value "${Mail.username}" ....
日志上在加载estore.properties配置文件后（所属于spring-mybatis.xml），就开始注入spring-mail.xml的bean类，此时QQMail.properties文件（所属于spring-mail.xml）还未加载，理所当然找不到${Mail.username}占位符。
想知道如何控制properties的加载？