@One

package com.flying.entity;

public class Mentor {
    private Integer mentorId;
    private String mentorName;
    private String researchArea;

    public Integer getMentorId() { return mentorId; }
    public void setMentorId(Integer mentorId) { this.mentorId = mentorId; }
    public String getMentorName() { return mentorName; }
    public void setMentorName(String mentorName) { this.mentorName = mentorName; }
    public String getResearchArea() { return researchArea; }
    public void setResearchArea(String researchArea) { this.researchArea = researchArea; }

    @Override
    public String toString() {
        return "Mentor{mentorId=" + mentorId + ", mentorName='" + mentorName + "', researchArea='" + researchArea + "'}";
    }
}

package com.flying.entity;

public class Student {
    private Integer stuId;
    private String stuName;
    private Integer stuAge;
    private String stuMajor;
    private Double stuScore;
    private Mentor mentor;

    public Integer getStuId() { return stuId; }
    public void setStuId(Integer stuId) { this.stuId = stuId; }
    public String getStuName() { return stuName; }
    public void setStuName(String stuName) { this.stuName = stuName; }
    public Integer getStuAge() { return stuAge; }
    public void setStuAge(Integer stuAge) { this.stuAge = stuAge; }
    public String getStuMajor() { return stuMajor; }
    public void setStuMajor(String stuMajor) { this.stuMajor = stuMajor; }
    public Double getStuScore() { return stuScore; }
    public void setStuScore(Double stuScore) { this.stuScore = stuScore; }
    public Mentor getMentor() { return mentor; }
    public void setMentor(Mentor mentor) { this.mentor = mentor; }

    @Override
    public String toString() {
        return "Student{stuId=" + stuId + ", stuName='" + stuName + "', stuAge=" + stuAge +
               ", stuMajor='" + stuMajor + "', stuScore=" + stuScore + ", mentor=" + mentor + "}";
    }
}

package com.flying.mapper;

import com.flying.entity.Mentor;
import org.apache.ibatis.annotations.Select;

public interface MentorMapper {

    /**
     * 根据导师 ID 查询导师信息
     * 该方法将被 StudentMapper 中的 @One 引用
     */
    @Select("SELECT mentor_id, mentor_name, research_area FROM mentor WHERE mentor_id = #{mentorId}")
    Mentor selectById(Integer mentorId);
}

package com.flying.mapper;

import com.flying.entity.Student;
import org.apache.ibatis.annotations.Result;
import org.apache.ibatis.annotations.Results;
import org.apache.ibatis.annotations.Select;
import org.apache.ibatis.annotations.One;
import java.util.List;

public interface StudentMapper {

    @Select("SELECT stu_id, stu_name, stu_age, stu_major, stu_score, mentor_id FROM student")
    @Results(id = "studentWithMentorMap", value = {
            @Result(column = "stu_id", property = "stuId", id = true),
            @Result(column = "stu_name", property = "stuName"),
            @Result(column = "stu_age", property = "stuAge"),
            @Result(column = "stu_major", property = "stuMajor"),
            @Result(column = "stu_score", property = "stuScore"),
            @Result(column = "mentor_id", property = "mentor",
                    one = @One(select = "com.flying.mapper.MentorMapper.selectById", fetchType = FetchType.EAGER))
    })
    List<Student> selectAllWithMentor();
}

package com.flying.test;

import com.flying.entity.Student;
import com.flying.mapper.StudentMapper;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;

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

public class OneTest {
    public static void main(String[] args) throws Exception {
        InputStream is = Resources.getResourceAsStream("mybatis-config.xml");
        SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(is);
        SqlSession session = factory.openSession();

        StudentMapper mapper = session.getMapper(StudentMapper.class);

        System.out.println("=== 查询所有学生（含导师信息）===");
        List<Student> all = mapper.selectAllWithMentor();
        all.forEach(System.out::println);

        session.close();
    }
}