spring-mybatis数据库配置文件读取不了

Question

 <?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:tx="http://www.springframework.org/schema/tx"
    xmlns:p="http://www.springframework.org/schema/p"
    xmlns:aop="http://www.springframework.org/schema/aop"
    xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-4.2.xsd
        http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-4.2.xsd
        http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-4.1.xsd
        http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-4.1.xsd">

    <context:property-placeholder location="classpath:config.properties" />
    <bean id="dataSource" class="org.springframework.jdbc.datasource.DriverManagerDataSource" >
<!--        <property name="driverClassName" value="com.mysql.jdbc.Driver" /> -->
<!--        <property name="url" value="jdbc:mysql://localhost:3306/test?useUnicode=true&amp;characterEncoding=UTF-8" /> -->
<!--        <property name="username" value="root" /> -->
<!--        <property name="password" value="root" /> -->
        <property name="driverClassName" value="${driverClassName}" />
        <property name="url" value="${url}" />
        <property name="username" value="${username}" />
        <property name="password" value="${password}" />
    </bean>
    <bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">  
         <property name="dataSource" ref="dataSource" />  
         <property name="mapperLocations" value="org/mydemo/mapper/*.xml"></property>
    </bean>  
    <bean id="sqlSessionTemplate" class="org.mybatis.spring.SqlSessionTemplate">
        <constructor-arg ref="sqlSessionFactory" />
    </bean>
</beans>

把注释解掉他就能正常，
好多人说是因为sqlSessionFactory在context:property-placeholder之前已经加载了
所以driverClassName是空，请问代码该怎么改呢？

Answer 1

求大神解答！！!!!!!!!!!!!!!!!!!!!!!!!!