1.创建db.properties文件

driver=com.mysql.cj.jdbc.Driver
url=jdbc:mysql://localhost:3306/mybatis_test?characterEncoding=UTF-8&serverTimezone=UTC
username=root
password=123456

2.在mybatis-config.xml中通过properties标签引入该文件

<?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">
<!--核心配置文件-->
<configuration>

	<!-- 引入db.properties文件 -->
    <properties resource="db.properties"/>

    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="${driver}"/>
                <property name="url" value="${url}"/>
                <property name="username" value="${username}"/>
                <property name="password" value="${password}"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <mapper resource="com/lixv/dao/UserMapper.xml"/>
    </mappers>
</configuration>

在properties标签中还可以写property标签来补充

<properties resource="db.properties">
    <property name="username" value="111111"/>
</properties>

相当于db.properties中添加一条数据：username=“111111”
注意：当键重复的时候，${键}会优先使用db.properties文件中的值

3.使用${键}获取db.properties文件中的值

<property name="driver" value="${driver}"/>
<property name="url" value="${url}"/>
<property name="username" value="${username}"/>
<property name="password" value="${password}"/>

4.运行测试