spring中的PropertyPlaceholderConfigurer作用，用法
Spring的框架中为您提供了一个 BeanFactoryPostProcessor 的类： org.springframework.beans.factory.config.PropertyPlaceholderConfigurer。通过这个类，您可以将一些组态设定，移出至.properties文件中，而.properties文件可以作为客户根据需求，自定义一些相关的参数。
来看一个Bean定义档的实际例子：
<?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:p="http://www.springframework.org/schema/p" xmlns:context="http://www.springframework.org/schema/context" xmlns:aop="http://www.springframework.org/schema/aop" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.0.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.0.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-3.0.xsd"><bean id="propertyConfigurer" class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer"><property name="fileEncoding" value="GBK" /> <property name="locations"><list><value>classpath:mailsender.properties</value></list></property></bean><bean id="javaMailSender" class="org.springframework.mail.javamail.JavaMailSenderImpl" ><property name="host"><value>${mail.host}</value></property><property name="port"><value>${mail.port}</value></property><property name="javaMailProperties"> <props> <prop key="mail.smtp.auth">false</prop> <prop key="mail.smtp.timeout">25000</prop> </props> </property><property name="username"><value>${mail.username}</value></property><property name="password"><value>${mail.password}</value></property></bean></beans>