Mybatis關(guān)聯(lián)映射的實(shí)現(xiàn)

更新時(shí)間：2025年12月29日 10:49:49 作者：期待のcode

本文介紹了MyBatis關(guān)聯(lián)映射的實(shí)現(xiàn)方式,直接查詢和分步查詢,文中通過(guò)示例代碼介紹的非常詳細(xì),對(duì)大家的學(xué)習(xí)或者工作具有一定的參考學(xué)習(xí)價(jià)值,需要的朋友們下面隨著小編來(lái)一起學(xué)習(xí)學(xué)習(xí)吧

SET NAMES utf8mb4;
SET FOREIGN_KEY_CHECKS = 0;

-- ----------------------------
-- Table structure for student
-- ----------------------------
DROP TABLE IF EXISTS `student`;
CREATE TABLE `student`  (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `Sname` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci DEFAULT NULL,
  `sex` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci DEFAULT NULL,
  `age` int(11) DEFAULT NULL,
  `t_id` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`) USING BTREE
) ENGINE = InnoDB AUTO_INCREMENT = 11 CHARACTER SET = utf8 COLLATE = utf8_general_ci ROW_FORMAT = Compact;

-- ----------------------------
-- Records of student
-- ----------------------------
INSERT INTO `student` VALUES (1, '張三', '男', 18, 1);
INSERT INTO `student` VALUES (2, '李四', '女', 18, 1);
INSERT INTO `student` VALUES (3, '王五', '男', 18, 1);
INSERT INTO `student` VALUES (4, '小白', '女', 18, 1);
INSERT INTO `student` VALUES (5, '小黑', '男', 18, 1);
INSERT INTO `student` VALUES (6, '小紅', '女', 20, 2);
INSERT INTO `student` VALUES (7, '小李', '男', 20, 2);
INSERT INTO `student` VALUES (8, '小張', '女', 20, 2);
INSERT INTO `student` VALUES (9, '小趙', '男', 20, 2);
INSERT INTO `student` VALUES (10, '小王', '女', 20, 2);

SET FOREIGN_KEY_CHECKS = 1;

teacher表

SET NAMES utf8mb4;
SET FOREIGN_KEY_CHECKS = 0;

-- ----------------------------
-- Table structure for teacher
-- ----------------------------
DROP TABLE IF EXISTS `teacher`;
CREATE TABLE `teacher`  (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `Tname` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci DEFAULT NULL,
  PRIMARY KEY (`id`) USING BTREE
) ENGINE = InnoDB AUTO_INCREMENT = 3 CHARACTER SET = utf8 COLLATE = utf8_general_ci ROW_FORMAT = Compact;

-- ----------------------------
-- Records of teacher
-- ----------------------------
INSERT INTO `teacher` VALUES (1, '張老師');
INSERT INTO `teacher` VALUES (2, '李老師');

SET FOREIGN_KEY_CHECKS = 1;

查詢老師的學(xué)生sql信息語(yǔ)句與查詢結(jié)果如下：

查詢學(xué)生的老師信息sql語(yǔ)句與查詢結(jié)果如下：

直接查詢

查詢出來(lái)的數(shù)據(jù)用TeacherStudents實(shí)體類進(jìn)行封裝

TeacherStudents.java

package com.qcby.entity;

public class TeacherStudents {
    private Integer id;
    private String Tname;
    private String Sname;
    private String sex;

    @Override
    public String toString() {
        return "TeacherStudents{" +"id=" + id +", Tname='" + Tname + '\'' +", Sname='" + Sname + '\'' +", sex='" + sex + '\'' +'}';
    }

    public Integer getId() {return id;}

    public void setId(Integer id) {this.id = id;}

    public String getTname() {return Tname;}

    public void setTname(String tname) {Tname = tname;}

    public String getSname() {return Sname;}

    public void setSname(String sname) {Sname = sname;}

    public String getSex() {return sex;}

    public void setSex(String sex) {this.sex = sex;}
}

TeacherStudentMapper.xml 內(nèi)寫查詢語(yǔ)句

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.qcby.dao.TeacherStudentsDao">
    <select id="findTeacherStudents" resultType="com.qcby.entity.TeacherStudents">
        select teacher.*,student.Sname,student.sex from teacher left join student on student.t_id = teacher.id
    </select>
</mapper>

SqlMapConfig.xml

  <mappers>
        <mapper resource="mapper/TeacherStudentsMapper.xml"></mapper>
    </mappers>

TeacherStudentsDao.java

package com.qcby.dao;
import com.qcby.entity.TeacherStudents;
import java.util.List;

public interface TeacherStudentsDao {
    List<TeacherStudents> findTeacherStudents();
}

TeacherStudentsTest.java

import com.qcby.dao.TeacherStudentsDao;
import com.qcby.entity.TeacherStudents;
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 org.junit.After;
import org.junit.Before;
import org.junit.Test;
import java.io.IOException;
import java.io.InputStream;
import java.util.List;

public class TeacherStudentsTest {
    private InputStream in = null;
    private SqlSession session = null;
    private TeacherStudentsDao teacherStudentsDao = null;

    @Before  //前置通知, 在方法執(zhí)行之前執(zhí)行
    public void init() throws IOException {
        //加載主配置文件，目的是為了構(gòu)建SqlSessionFactory對(duì)象
        in = Resources.getResourceAsStream("SqlMapConfig.xml");
        //創(chuàng)建SqlSessionFactory對(duì)象
        SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(in);
        //通過(guò)SqlSessionFactory工廠對(duì)象創(chuàng)建SqlSesssion對(duì)象
        session = factory.openSession();
        //通過(guò)Session創(chuàng)建UserDao接口代理對(duì)象
        teacherStudentsDao = session.getMapper(TeacherStudentsDao.class);
    }

    @After  //@After: 后置通知, 在方法執(zhí)行之后執(zhí)行 。
    public void destory() throws IOException {
        //釋放資源
        session.close();
        in.close();
    }


    @Test
    public void findAll(){
        List<TeacherStudents> list = teacherStudentsDao.findTeacherStudents();
        System.out.println(list.toString());
    }
}

直接查詢的查詢結(jié)果：

關(guān)聯(lián)映射

student 表和 teacher 表存在著一對(duì)多與多對(duì)一的關(guān)系，我們介紹 MyBatis 中處理關(guān)聯(lián)關(guān)系的兩種關(guān)聯(lián)映射查詢方式，分別是直接查詢與分布查詢。

直接查詢通過(guò) SQL 的JOIN（如LEFT JOIN、INNER JOIN）關(guān)聯(lián)多個(gè)表，進(jìn)行聯(lián)合查詢獲取主表和關(guān)聯(lián)表的所有所需字段，然后在<resultMap>中通過(guò)<association>一對(duì)一或<collection>一對(duì)多標(biāo)簽，定義主表與關(guān)聯(lián)表字段到對(duì)應(yīng)實(shí)體屬性的映射規(guī)則，MyBatis 會(huì)自動(dòng)將查詢結(jié)果分別封裝為主實(shí)體和關(guān)聯(lián)實(shí)體，并將關(guān)聯(lián)實(shí)體注入主實(shí)體的關(guān)聯(lián)屬性中。

分步查詢是分階段完成查詢，先執(zhí)行主查詢獲取主實(shí)體數(shù)據(jù)及關(guān)聯(lián)字段（select * from student），再由 MyBatis 自動(dòng)根據(jù)關(guān)聯(lián)字段調(diào)用對(duì)應(yīng) Mapper 方法，執(zhí)行關(guān)聯(lián)查詢獲取關(guān)聯(lián)實(shí)體數(shù)據(jù)（select * from teacher where id = #{t_id}），最終將關(guān)聯(lián)實(shí)體自動(dòng)封裝到主實(shí)體的關(guān)聯(lián)屬性中。可結(jié)合延遲加載，僅在使用關(guān)聯(lián)數(shù)據(jù)時(shí)觸發(fā)關(guān)聯(lián)查詢。

下面我們來(lái)看詳細(xì)代碼：

SqlMapConfig.xml

  <mappers>
        <mapper resource="mapper/StudentMapper.xml"></mapper>
        <mapper resource="mapper/TeacherMapper.xml"></mapper>
    </mappers>

Student.java

package com.qcby.entity;

public class Student {
    private Integer id;
    private String Sname;
    private String sex;
    private Integer age;
    private Integer t_id;
    private Teacher teacher;

    @Override
    public String toString() {
        return "Student{" +"id=" + id +", Sname='" + Sname + '\'' +", sex='" + sex + '\'' +", age=" + age +", t_id=" + t_id +", teacher=" + teacher +'}';
    }

    public Teacher getTeacher() {return teacher;}

    public void setTeacher(Teacher teacher) {this.teacher = teacher;}

    public Integer getId() {return id;}

    public void setId(Integer id) {this.id = id;}

    public String getSname() {return Sname;}

    public void setSname(String sname) {Sname = sname;}

    public String getSex() {return sex;}

    public void setSex(String sex) {this.sex = sex;}

    public Integer getAge() {return age;}

    public void setAge(Integer age) {this.age = age;}

    public Integer getT_id() {return t_id;}

    public void setT_id(Integer t_id) {this.t_id = t_id;}
}

StudentMapper.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.qcby.dao.StudentDao">
    
    <select id="findStudentTeacher" resultMap="studentTeacher">
        select student.*,teacher.Tname from student left join teacher on student.t_id = teacher.id
    </select>
    <resultMap id="studentTeacher" type="com.qcby.entity.Student">
        <id property="id" column="id"/>
        <result property="Sname" column="Sname"/>
        <result property="sex" column="sex"/>
        <result property="age" column="age"/>
        <result property="t_id" column="t_id"/>
        <association property="teacher" javaType="com.qcby.entity.Teacher">
            <result property="Tname" column="Tname"/>
        </association>
    </resultMap>

    <select id="findStudentTeacher1" resultMap="studentTeacher1">
        select * from student
    </select>
    <resultMap id="studentTeacher1" type="com.qcby.entity.Student">
        <id property="id" column="id"/>
        <result property="Sname" column="Sname"/>
        <result property="sex" column="sex"/>
        <result property="age" column="age"/>
        <result property="t_id" column="t_id"/>
        <association property="teacher" javaType="com.qcby.entity.Teacher"
                     select="com.qcby.dao.TeacherDao.findTeacherByT_id" column="t_id">
        </association>
    </resultMap>
    
    <select id="findStudentByT_id" resultType="com.qcby.entity.Student">
        select * from student where t_id = #{id}
    </select>
    
</mapper>

<select> 標(biāo)簽用于主查詢，resultMap 屬性用于指定映射規(guī)則
<resultMap> 標(biāo)簽用于結(jié)果映射規(guī)則，其中 id 屬性值要與 select 標(biāo)簽中的 resultMap 屬性值相同，是映射規(guī)則的唯一標(biāo)識(shí)，type 指定映射的目標(biāo)實(shí)體類，result 映射普通字段，property=實(shí)體類中的屬性名，column=數(shù)據(jù)庫(kù)當(dāng)中的字段，而 <association> 標(biāo)簽用于處理對(duì)象的關(guān)聯(lián)信息，其中 property=關(guān)聯(lián)對(duì)象的屬性名、javaType=關(guān)聯(lián)對(duì)象的類型、select=對(duì)應(yīng)的查詢方法、column=傳遞給關(guān)聯(lián)查詢的參數(shù)

StudenDao.java

package com.qcby.dao;
import com.qcby.entity.Student;
import java.util.List;

public interface StudentDao {
    List<Student> findStudentTeacher();

    List<Student> findStudentTeacher1();

    Student findStudentByT_id(Integer t_id);
}

StudentTest.java

import com.qcby.dao.StudentDao;
import com.qcby.entity.Student;
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 org.junit.After;
import org.junit.Before;
import org.junit.Test;
import java.io.IOException;
import java.io.InputStream;
import java.util.List;

public class StudentTest {
    private InputStream in = null;
    private SqlSession session = null;
    private StudentDao studentDao = null;

    @Before  //前置通知, 在方法執(zhí)行之前執(zhí)行
    public void init() throws IOException {
        //加載主配置文件，目的是為了構(gòu)建SqlSessionFactory對(duì)象
        in = Resources.getResourceAsStream("SqlMapConfig.xml");
        //創(chuàng)建SqlSessionFactory對(duì)象
        SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(in);
        //通過(guò)SqlSessionFactory工廠對(duì)象創(chuàng)建SqlSesssion對(duì)象
        session = factory.openSession();
        //通過(guò)Session創(chuàng)建UserDao接口代理對(duì)象
        studentDao = session.getMapper(StudentDao.class);
    }

    @After  //@After: 后置通知, 在方法執(zhí)行之后執(zhí)行 。
    public void destory() throws IOException {
        //釋放資源
        session.close();
        in.close();
    }

    @Test
    public void findStudentTeacher(){
        List<Student> students = studentDao.findStudentTeacher();
        System.out.println(students.toString());
    }

    @Test
    public void findStudentTeacher1(){
        List<Student> students = studentDao.findStudentTeacher();
        System.out.println(students.toString());
    }
}

findStudentTeacher() 直接查詢結(jié)果如下：

findStudentTeacher1() 分步查詢結(jié)果如下：

Teacher.java

package com.qcby.entity;
import java.util.List;

public class Teacher {
    private Integer id;
    private String Tname;
    private List<Student> students;

    @Override
    public String toString() {
        return "Teacher{" +"id=" + id +", Tname='" + Tname + '\'' +", students=" + students +'}';
    }

    public List<Student> getStudents() {return students;}

    public void setStudents(List<Student> students) {this.students = students;}

    public Integer getId() {return id;}

    public void setId(Integer id) {this.id = id;}

    public String getTname() {return Tname;}

    public void setTname(String tname) {Tname = tname;}
}

TeacherMapper.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.qcby.dao.TeacherDao">

    <select id="getStudents" resultMap="teacherStudents">
        select teacher.*,student.Sname,student.sex from teacher left join student on student.t_id = teacher.id
    </select>
    <resultMap id="teacherStudents" type="com.qcby.entity.Teacher">
        <id property="id" column="id"/>
        <result property="Tname" column="Tname"/>
        <collection property="students" ofType="com.qcby.entity.Student">
            <result property="Sname" column="Sname"/>
            <result property="sex" column="sex"/>
        </collection>
    </resultMap>

    <select id="findTeacherByT_id" resultType="com.qcby.entity.Teacher">
        select * from teacher where id = #{t_id}
    </select>

    <select id="getStudents1" resultMap="teacherStudents1">
        select * from teacher
    </select>
    <resultMap id="teacherStudents1" type="com.qcby.entity.Teacher">
        <id property="id" column="id"/>
        <result property="Tname" column="Tname"/>
        <collection property="students" ofType="com.qcby.entity.Student"
                    select="com.qcby.dao.StudentDao.findStudentByT_id" column="id" fetchType="lazy"/>
    </resultMap>
    
</mapper>

在 Teacher.java 中 student 屬性的類型是 List 集合，collection 標(biāo)簽用于處理集合映射到 Student.java 中，oftype=集合泛型的類型

TeacherDao.java

package com.qcby.dao;
import com.qcby.entity.Teacher;
import java.util.List;

public interface TeacherDao {
    List<Teacher> getStudents();

    Teacher findTeacherByT_id(Integer t_id);

    List<Teacher> getStudents1();
}

TeacherTest.java

import com.qcby.dao.TeacherDao;
import com.qcby.entity.Teacher;
import com.qcby.entity.TeacherStudents;
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 org.junit.After;
import org.junit.Before;
import org.junit.Test;
import java.io.IOException;
import java.io.InputStream;
import java.util.List;

public class TeacherTest {
    private InputStream in = null;
    private SqlSession session = null;
    private TeacherDao teacherDao = null;

    @Before  //前置通知, 在方法執(zhí)行之前執(zhí)行
    public void init() throws IOException {
        //加載主配置文件，目的是為了構(gòu)建SqlSessionFactory對(duì)象
        in = Resources.getResourceAsStream("SqlMapConfig.xml");
        //創(chuàng)建SqlSessionFactory對(duì)象
        SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(in);
        //通過(guò)SqlSessionFactory工廠對(duì)象創(chuàng)建SqlSesssion對(duì)象
        session = factory.openSession();
        //通過(guò)Session創(chuàng)建UserDao接口代理對(duì)象
        teacherDao = session.getMapper(TeacherDao.class);
    }

    @After  //@After: 后置通知, 在方法執(zhí)行之后執(zhí)行 。
    public void destory() throws IOException {
        //釋放資源
        session.close();
        in.close();
    }

    @Test
    public void getStudents(){
        List<Teacher> teachers = teacherDao.getStudents();
        for (Teacher teacher:teachers){
            System.out.println(teacher);
        }
    }

    @Test
    public void getStudents1(){
        List<Teacher> teachers = teacherDao.getStudents1();
        for (Teacher teacher:teachers){
            System.out.println(teacher);
            System.out.println(teacher.getTname());
            System.out.println(teacher.getStudents());
        }
    }
}

getStudents() 直接查詢結(jié)果如下：

getStudents1() 分步查詢結(jié)果如下：

延遲加載

MyBatis 的延遲加載（Lazy Loading）是一種優(yōu)化數(shù)據(jù)庫(kù)查詢性能的機(jī)制，它允許在需要時(shí)才加載關(guān)聯(lián)對(duì)象的數(shù)據(jù)，而不是在加載主對(duì)象時(shí)就立即加載所有關(guān)聯(lián)數(shù)據(jù)。這種 “按需加載” 的方式可以減少不必要的數(shù)據(jù)庫(kù)查詢，提升系統(tǒng)性能。

延遲加載實(shí)現(xiàn)效果如圖：

只執(zhí)行了一個(gè)sql’語(yǔ)句，當(dāng)查詢?cè)搶?duì)象時(shí)，只加載該對(duì)象本身的屬性，對(duì)于其關(guān)聯(lián)的對(duì)象暫不加載，直到程序真正需要訪問(wèn)這些關(guān)聯(lián)對(duì)象時(shí)，才會(huì)發(fā)起對(duì)應(yīng)的數(shù)據(jù)庫(kù)查詢

如何配置延遲加載

全局配置：SqlMapConfig.xml

        <setting name="lazyLoadingEnabled" value="true"/>
        <setting name="aggressiveLazyLoading" value="false"/>

lazyLoadingEnabled 是延遲加載的全局開(kāi)關(guān)，aggressiveLazyLoading 對(duì) lazyLoadingEnabled 的補(bǔ)充配置，當(dāng)開(kāi)啟延遲加載，aggressiveLazyLoading 再設(shè)置為 true 時(shí)，任何方式調(diào)用都會(huì)加載該對(duì)象的所有屬性，設(shè)置為 false 則該屬性將按需加載，若 lazyLoadingEnabled=false 關(guān)閉延遲加載時(shí)，此配置無(wú)效，所有關(guān)聯(lián)對(duì)象都會(huì)立即加載。

局部配置：在映射文件中通過(guò)<association>或<collection>標(biāo)簽中由 fetchType 屬性指定

	<collection property="students" ofType="com.qcby.entity.Student"
                    select="com.qcby.dao.StudentDao.findStudentByT_id" column="id" fetchType="lazy"/>

fetchType有兩個(gè)值，一個(gè)是"lazy"為懶加載，一個(gè)是"eager"為立即加載

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