What is the mybatis-config.xml file used for?

The mybatis-config.xml file is MyBatis's top-level, global configuration file, holding settings that apply across the entire application — environment/data source configuration, type aliases, plugin registrations, and the list of mapper files or interfaces to load — distinct from the individual mapper XML files that define specific SQL statements.

<configuration>
  <environments default="development">
    <environment id="development">
      <transactionManager type="JDBC"/>
      <dataSource type="POOLED">
        <property name="driver" value="com.mysql.cj.jdbc.Driver"/>
        <property name="url" value="jdbc:mysql://localhost:3306/mydb"/>
        <property name="username" value="root"/>
        <property name="password" value="password"/>
      </dataSource>
    </environment>
  </environments>
  <mappers>
    <mapper resource="UserMapper.xml"/>
  </mappers>
</configuration>

This file is what a SqlSessionFactoryBuilder reads when constructing a SqlSessionFactory in a plain, non-Spring MyBatis application; in a Spring Boot application using MyBatis-Spring-Boot-Starter, this XML configuration is often replaced or supplemented by Spring Boot's own application.properties/application.yml settings and Java-based configuration, since auto-configuration handles much of what mybatis-config.xml would otherwise define manually.

Common elements configured here beyond the data source include typeAliases (shorthand names for fully-qualified Java class names used in mapper XML), plugins (registering custom interceptors), and global settings like whether lazy loading or the second-level cache is enabled by default.