Could not load driverClass ${jdbc.driver}

今天在整合ssm做spring-mvc的Junit测试时，出现这样的错误如下：

Could not load driverClass ${jdbc.driver}
java.lang.ClassNotFoundException: ${jdbc.driver}
	at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
	at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:335)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
	at java.lang.Class.forName0(Native Method)
	at java.lang.Class.forName(Class.java:264)
	at com.mchange.v2.c3p0.DriverManagerDataSource.ensureDriverLoaded(DriverManagerDataSource.java:100)
	at com.mchange.v2.c3p0.DriverManagerDataSource.getConnection(DriverManagerDataSource.java:132)
	at com.mchange.v2.c3p0.WrapperConnectionPoolDataSource.getPooledConnection(WrapperConnectionPoolDataSource.java:182)
	at com.mchange.v2.c3p0.WrapperConnectionPoolDataSource.getPooledConnection(WrapperConnectionPoolDataSource.java:171)
	at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool$1PooledConnectionResourcePoolManager.acquireResource(C3P0PooledConnectionPool.java:137)
	at com.mchange.v2.resourcepool.BasicResourcePool.doAcquire(BasicResourcePool.java:1014)
	at com.mchange.v2.resourcepool.BasicResourcePool.access$800(BasicResourcePool.java:32)
	at com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask.run(BasicResourcePool.java:1810)
	at com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread.run(ThreadPoolAsynchronousRunner.java:547)

spring-dao.xml配置如下：

<!-- 配置整合mybatis过程 -->
    <!-- 1.配置数据库相关参数的properties的属性 -->
 	<context:property-placeholder location="classpath:jdbc.properties"/>
 	
 	
 	
 	<!-- 配置数据库连接池 -->
 	<bean id="dataSource" class="com.mchange.v2.c3p0.ComboPooledDataSource">
		<!-- 配置连接池属性 -->
		<property name="driverClass" value="${jdbc.driver}" />
		<property name="jdbcUrl" value="${jdbc.url}" />
		<property name="user" value="${jdbc.username}" />
		<property name="password" value="${jdbc.password}" />

		<!-- c3p0连接池的私有属性 -->
		<property name="maxPoolSize" value="30" />
		<property name="minPoolSize" value="10" />
		<!-- 关闭连接后不自动commit -->
		<property name="autoCommitOnClose" value="false" />
		<!-- 获取连接超时时间 -->
		<property name="checkoutTimeout" value="10000" />
		<!-- 当获取连接失败重试次数 -->
		<property name="acquireRetryAttempts" value="2" />
	</bean>
 	
 	<!-- 3.配置SqlSessionFactory -->
 	<bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
 		<!-- 注入数据库连接池 -->
 		<property name="dataSource" ref="dataSource"/>
 		<!-- 配置mybatis全局配置文件：mybatis-config.xml -->
 		<property name="configLocation" value="classpath:mybatis-config.xml"></property>
 		<!-- 扫描entity -->
 		<property name="typeAliasesPackage" value="com.itwx.o2o.entity"></property>
 		<!-- 扫描SQL配置文件mapper需要的xml文件 -->
 		<property name="mapperLocations" value="classpath:mapper/*.xml"></property>
 	</bean>
 	
 	<!-- 4.配置扫描Dao接口包，动态实现Dao接口，注入到Spring容器中 -->
 	<bean class="org.mybatis.spring.mapper.MapperScannerConfigurer">
 		<!-- 注入sqlSessionFactoy -->
 		<property name="sqlSessionFactory" ref="sqlSessionFactory" />
 		<!-- 扫描的dao层接口包 -->
 		<property name="basePackage" value="com.itwx.o2o.dao"></property>
 	</bean>
 </beans>

重点错误是在配置扫描接口的时候出现错误，加红处配置错误；

如果将 ${jdbc.driverClassName} 改成具体的值就不会报错！
解决方案一：
将
　　<property name="sqlSessionFactory" ref="sqlSessionFactory"/>
改成
　　<property name="sqlSessionFactoryBeanName" value="sqlSessionFactory"/>
记住ref要改成value 
原因分析：是因为配置了class="org.mybatis.spring.mapper.MapperScannerConfigurer"，而这句代码会在数据源加载前就执行了，之后就把表达式${jdbc.driverClassName}当成字符串执行了。