一：直接使用context命名空间

<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:websocket="http://www.springframework.org/schema/websocket"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/websocket
http://www.springframework.org/schema/websocket/spring-websocket.xsd
http://www.springframework.org/schema/context
http://www.springframework.org/schema/context/spring-context.xsd">

<context:property-placeholder location="classpath:log4j.properties" />  

二：在配置文件中配置PropertyPlaceholderConfigurer

<bean class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
<property name="locations" value="http://www.mamicode.com/classpath:jdbc.properties" />
</bean>

实例：

<!-- property-placeholder是一个属性遍历器，定位一个属性文件，属性文件存放的是jdbc一些连接数据 -->
    <context:property-placeholder location="classpath:jdbc.properties" />
    
    <!-- 配置数据源 -->
    <bean id="dataSource" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close">
        <property name="driverClassName" value="${driverClassName}" />
        <property name="url" value="${url}" />
        <property name="username" value="${username}" />
        <property name="password" value="${password}" />
        <property name="initialSize" value="${initialSize}" />
        <property name="maxActive" value="${maxActive}" />
        <property name="maxIdle" value="${maxIdle}" />
        <property name="minIdle" value="${minIdle}" />
    </bean>

spring 读取properties的两种方法