主对象

@Data
public class AllocationInfo2 {

    @ApiModelProperty(value = "项目编号")
    private String projectId;

    @ApiModelProperty(value = "项目类别")
    private String category;

    @ApiModelProperty(value = "项目负责人")
    private String leader;

    @ApiModelProperty(value = "项目说明")
    private String instruction;

    @ApiModelProperty(value = "项目分配的成员信息")
    private List<Teacher> teachers;

}

子对象

@Data
public class Teacher {

    /**
     * 用户名
     */
    private String username;

    /**
     * 用户ID
     */
    private String userId;

    @Override
    public String toString() {
        return "教师名称:"+username+" 教师工号"+userId+"\n";
    }
}

xml中的resultMap(核心)

注意子对象的前缀, columnPrefix="t_"

<resultMap id="Allocation" type="group.uchain.project_management_system.vo.AllocationInfo2">
        <id column="id" property="projectId"/>
        <result column="category" property="category"/>
        <result column="leader" property="leader"/>
        <result column="instruction" property="instruction"/>
        <collection property="teachers" ofType="group.uchain.project_management_system.vo.Teacher"
        javaType="java.util.ArrayList" columnPrefix="t_">
            <id column="user_id" property="userId" jdbcType="BIGINT"/>
            <id column="username" property="username" jdbcType="VARCHAR"/>
        </collection>
    </resultMap>

xml中的SQL

<select id="getAllAllocationInfo" resultMap="Allocation" resultType="group.uchain.project_management_system.vo.AllocationInfo2">
        select p.id,p.category,p.leader,p.instruction,a.user_id as t_user_id,u.username as t_username
          from project_info p,allocation_info a
          inner join user u
          on a.user_id = u.user_id
        where p.id = a.project_id 
    </select>

测试结果

2019-08-18 11:50:05.824 INFO 32141 --- [ main] com.alibaba.druid.pool.DruidDataSource : {dataSource-1} inited
[AllocationInfo2(projectId=V28, category=专业认证, leader=王五, instruction=计算机公共基础协助2018年石工和化工的专业认证, teachers=[教师名称:小明 教师工号123456789101
, 教师名称:李四 教师工号123456789102
])]