spring整合mybaits时Connections could not be acquired from the underlying database!异常解决

<?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:mvc="http://www.springframework.org/schema/mvc"   xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.1.xsdhttp://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.1.xsd http://www.springframework.org/schema/mvc http://www.springframework.org/schema/mvc/spring-mvc-3.1.xsd"><!-- 使用注解自动创建bean，设置bean所在的包 --><context:component-scan base-package="com.leo.dao,com.leo.service" /><!-- @Autowired自动装配 <bean class="org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor"/>--><!-- 配置数据源 --><!-- <context:property-placeholder location="classpath:db.properties"/> --><bean id = "propertyConfigurer"  class = "org.springframework.beans.factory.config.PropertyPlaceholderConfigurer" >    <property  name ="location"  value ="com/leo/resources/db.properties"/>        </bean><bean id="dataSource" class="com.mchange.v2.c3p0.ComboPooledDataSource"><property name="driverClass" value="${driverClass}"></property><property name="jdbcUrl" value="${url}"></property><property name="user" value="${username}"></property><property name="password" value="${password}"></property></bean><!-- 创建SqlSessionFactory--><bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean"><!-- 指定数据源 --><property name="dataSource" ref="dataSource" /><property name="typeAliasesPackage" value="com.leo.entity"></property><property name="mapperLocations" value="classpath*:com/leo/dao/*Mapper.xml" /></bean><!-- 配置事务 --><bean id="transactionManager" class="org.springframework.jdbc.datasource.DataSourceTransactionManager"><property name="dataSource" ref="dataSource" /></bean><!-- 映射接口 --><bean class="org.mybatis.spring.mapper.MapperScannerConfigurer"><property name="basePackage" value="com.leo.dao" /><property name="sqlSessionFactoryBeanName" value="sqlSessionFactory" /></bean></beans>

当我使用这句读取数据源文件时，就总是找不到

<context:property-placeholder location="classpath:db.properties"/>

换成

<bean id = "propertyConfigurer"  class = "org.springframework.beans.factory.config.PropertyPlaceholderConfigurer" >    <property  name ="location"  value ="com/leo/resources/db.properties"/></bean>

就可以，我也不知道为什么？

希望可以获得帮助，知道的人可以帮忙解答一下，