mybatis中配置中引入properties文件

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
  PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
  "http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
 <!--引入外部properties文件 必须放在最前面,否则会报错 -->
 <properties resource="db.properties"></properties>
<!--和Spring整合后 environments 配置将废除 -->
  <environments default="development">
    <environment id="development">
    <!--使用JDBC事务管理  -->
      <transactionManager type="JDBC"/>
      <!--数据库连接池  -->
      <dataSource type="POOLED">
        <property name="driver" value="${jdbc.driverClass}"/>
        <property name="url" value="${jdbc.url}"/>
        <property name="username" value="${jdbc.username}"/>
        <property name="password" value="${jdbc.password}"/>
      </dataSource>
    </environment>
  </environments>
  <!--加载xml文件  -->
  <mappers>
    <mapper resource="mapper/UserMapper.xml"/>
  </mappers>
</configuration>