Spring 中如何使用外部属性文件

<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"
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-4.0.xsd"> 
<!-- 配置c3p0数据源  这种配置不方便维护
<bean id="dataSource" class="com.mchange.v2.c3p0.ComboPooledDataSource">
<property name="user" value="root"></property>
<property name="password" value="chuck"></property>
<property name="driverClass" value="com.mysql.jdbc.Driver"></property>
<property name="jdbcUrl" value="jdbc:mysql:///test"></property>
</bean>
-->

<!--~~~~~~~~~~~~~~~~~配置c3p0数据源 ~~~~~~~~~~~~~~~~~~~~~~ -->

<!-- 导入属性文件 -->
<context:property-placeholder location="classpath:db.properties" />

<bean id="dataSource" class="com.mchange.v2.c3p0.ComboPooledDataSource">
<!--使用外部属性文件的属性 -->
<property name="user" value="${user}"></property>
<property name="password" value="${password}"></property>
<property name="driverClass" value="${driverClass}"></property>
<property name="jdbcUrl" value="${jdbcUrl}"></property>
</bean>

</beans>

package com.baidu.c3p0;

import java.sql.SQLException;
import javax.sql.DataSource;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class TestSpringC3P0 {

public static void main(String[] args) throws SQLException {

ApplicationContext ac = new ClassPathXmlApplicationContext("beans_c3p0.xml");

DataSource dataSource = (DataSource) ac.getBean("dataSource");
System.out.println(dataSource.getConnection());

}

}

com.mchange.v2.c3p0.impl.NewProxyConnection@f76eb40