连接池用${}取不到值

Question

如果是手写驱动类、URL、用户名这些，不用${}取jdbc.properties文件里的值的话，就可以，怎么回事啊<?xml version="1.0" encoding="UTF-8"?><beans xmlns="http://www.springframework.org/schema/beans"     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"     xmlns:context="http://www.springframework.org/schema/context"     xmlns:aop="http://www.springframework.org/schema/aop"     xmlns:tx="http://www.springframework.org/schema/tx"     xsi:schemaLocation="http://www.springframework.org/schema/beans     http://www.springframework.org/schema/beans/spring-beans.xsd     http://www.springframework.org/schema/context     http://www.springframework.org/schema/context/spring-context.xsd     http://www.springframework.org/schema/aop     http://www.springframework.org/schema/aop/spring-aop.xsd     http://www.springframework.org/schema/tx     http://www.springframework.org/schema/tx/spring-tx.xsd">          <context:property-placeholder location="classpath:jdbc.properties"/>          <bean id="dataSource" class="com.mchange.v2.c3p0.ComboPooledDataSource">      <property name="driverClass" value="${driver}"/>        <property name="jdbcUrl" value="${url}"/>        <property name="user" value="${username}"/>        <property name="password" value="${password}"/>     </bean>