Spring 读取Properties 配置文件

<bean class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">  
         <property name="systemPropertiesModeName" value="SYSTEM_PROPERTIES_MODE_OVERRIDE" />  
         <property name="ignoreResourceNotFound" value="true" />  
         <property name="locations">  
             <list>  
                 <value>classpath:application.properties</value>  
             </list>  
         </property>  
     </bean>  

    <bean id="dataSource"
        class="org.springframework.jdbc.datasource.DriverManagerDataSource">
        <property name="driverClassName"
            value="com.mysql.jdbc.Driver">
        </property>
        <property name="url">
            <value>${db.url}</value>
        </property>
        <property name="username" value=""></property>
        <property name="password" value=""></property>
    </bean>