QueryRunner查询结果中内部类为空：如何利用MyBatis关联解决？

<mapper namespace="com.example.mapper.customermapper">
  <resultmap id="customerresultmap" type="com.example.model.customer">
    <result property="id" column="id" />
    <result property="name" column="name" />
    <association property="region" column="region_id" javatype="com.example.model.region" />
  </resultmap>

  <select id="selectallcustomers" resultmap="customerresultmap">
    select * from customer
  </select>
</mapper>

public class customer {
  private int id;
  private string name;
  private region region;
  // getter and setter methods
}

public class region {
  private int id;
  private string name;
  // getter and setter methods
}

SqlSession session = sqlSessionFactory.openSession();
try {
  CustomerMapper customerMapper = session.getMapper(CustomerMapper.class);
  List<Customer> customers = customerMapper.selectAllCustomers();
  for (Customer customer : customers) {
    System.out.println(customer.getName() + " is located in " + customer.getRegion().getName());
  }
} finally {
  session.close();
}