public class Employee {
    private Integer id;
     String lastName;
     String gender;
     String email;
    Department dept;
}

 Department {
     String deptName;
    private List<Employee> employee;
}

interface EmployeeMapperPlus {
    public List<Employee> getEmpsByDeptId(Integer id);
}

    <select id="getEmpsByDeptId" resultType="com.gong.mybatis.bean.Employee">
        select * from tbl_employee where d_id=#{deptId}
    </select>

 DepartmentMapper {    
    public Department getDeptByIdStep(Integer id);
}

    resultMap type="com.gong.mybatis.bean.Department" id="MyDeptStep">
        id column="id" property="id"/>
        result ="dept_name"="deptName"collection property="employee" select="com.gong.mybatis.dao.EmployeeMapperPlus.getEmpsByDeptId"
        column>    
        collection>
    resultMap="getDeptByIdStep" resultMap
        select id,dept_name from tbl_department where id=#{id}
    >

    settingssetting name="lazyLoadingEnabled" value="true"="aggressiveLazyLoading"="false"/>
    >

package com.gong.mybatis.test;

import java.io.IOException;
 java.io.InputStream;
 java.util.List;
 java.util.Map;

 org.apache.ibatis.io.Resources;
 org.apache.ibatis.session.sqlSession;
 org.apache.ibatis.session.sqlSessionFactory;
 org.apache.ibatis.session.sqlSessionFactoryBuilder;
 org.junit.Test;

 com.gong.mybatis.bean.Department;
 com.gong.mybatis.bean.Employee;
 com.gong.mybatis.dao.DepartmentMapper;
 com.gong.mybatis.dao.EmployeeMapperPlus;

 TestMybatis2 {
    
    public sqlSessionFactory getsqlSessionFactory() throws IOException {
        String resource = "mybatis-config.xml";
        InputStream is = Resources.getResourceAsStream(resource);
        return new sqlSessionFactoryBuilder().build(is);
    }

    @Test
    void test2()  IOException {
        sqlSessionFactory sqlSessionFactory = getsqlSessionFactory();
        sqlSession openSession = sqlSessionFactory.openSession();
        try {
            DepartmentMapper mapper = openSession.getMapper(DepartmentMapper.);
            Department department = mapper.getDeptByIdStep(1);
            System.out.println(department.getDeptName());
            openSession.commit();
        } finally {
            openSession.close();
        }
        
    }

}

System.out.println(department.getEmployee());