spring 配置文件beans.xml使用占位符报异常java.lang.ClassNotFoundException: com.mysql.jdbc.Driver

1.不使用占位符，beans.xml配置如下：

<?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" 
    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">
        
<bean id="userDao" class="dao.impl.UserDaoImpl">
<property name="dataSource" ref="dataSource"></property>
</bean>

<bean id="userInterceptor" class="aop.UserInterceptor"></bean>
<bean id="userService" class="service.UserService">
<property name="userDao" ref="userDao"></property>
</bean>

<bean id="dataSource" class="org.apache.commons.dbcp.BasicDataSource"> 
    <property name="driverClassName" value="com.mysql.jdbc.Driver"/> 
    <property name="url" value="jdbc:mysql://localhost:3306/spring"/> 
    <property name="username" value="root"/> 
    <property name="password" value="****"/> 
  </bean> 

<aop:config>
<aop:aspect id="userInterceptor" ref="userInterceptor">
<aop:pointcut expression="execution(public * dao..*.*(..))" id="myMethod"/>
<!-- aop pointcut也可以声明在config内，aspect外，这样可以让别的aspect使用 -->

<aop:before method="before" pointcut-ref="myMethod"/>
<!-- 也可以直接写<aop:before method="before" pointcut="execution(public * dao..*.*(..))"/> -->

<aop:after method="after" pointcut-ref="myMethod"/>
</aop:aspect>
</aop:config>

</beans>
2.测试时没有问题，而使用占位符，beans.xml如下：

<?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" 
    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">
        
        <context:property-placeholder location="classpath:jdbc.properties" /> 
     
<bean id="dataSource" class="org.apache.commons.dbcp.BasicDataSource"> 
    <property name="driverClassName" value="${jdbc.driverClassName}"/> 
    <property name="url" value="${jdbc.url}"/> 
    <property name="username" value="${jdbc.username}"/> 
    <property name="password" value="${jdbc.password}"/>
  </bean> 
<bean id="userDao" class="dao.impl.UserDaoImpl">
<property name="dataSource" ref="dataSource"></property>
</bean>

<bean id="userInterceptor" class="aop.UserInterceptor"></bean>
<bean id="userService" class="service.UserService">
<property name="userDao" ref="userDao"></property>
</bean>



</beans>
3.在src目录下新建jdbc.properties，内容如下：

jdbc.driverClassName=com.mysql.jdbc.Driver 
jdbc.url=jdbc:mysql://localhost:3306/spring 
jdbc.username=root 
jdbc.password=****

4.运行时报如下异常：

org.apache.commons.dbcp.SQLNestedException: Cannot load JDBC driver class 'com.mysql.jdbc.Driver '

Caused by: java.lang.ClassNotFoundException: com.mysql.jdbc.Driver 

请各位看官解析一下，不胜感激！