有时侯，我们用SQL取得的结果需要映射到类似Map<key, Bean>这样的数据结构中或是映射到多个实体类中时，我们就需要使用到resultMap。下面用3个例子说明Mybatis高级结果映射的用法。

<环境准备>

请参照上一篇blog<Mybatis Guide>

<数据准备>

创建班级--学生表的一对多的数据结构

 --创建班级表

 CREATE TABLE tempdb..test_class

 (

 class_id int NOT NULL PRIMARY KEY,

 class_name varchar(255),

 class_admin_name varchar(255)

 );

 --创建学生表

 CREATE TABLE tempdb..test_student

 (

 student_id int NOT NULL PRIMARY KEY,

 class_id int,

 student_name varchar(255),

 student_age int,

 student_address varchar(255)

 );

 --班级表中插入数据

 insert tempdb..test_class values (101, 'Class 1, Grade 1', 'Mr. Zhang');

 insert tempdb..test_class values (102, 'Class 2, Grade 1', 'Mr. Li');

 insert tempdb..test_class values (103, 'Class 3, Grade 1', 'Mr. Wang');

 insert tempdb..test_class values (201, 'Class 1, Grade 2', 'Mr. Zhao');

 insert tempdb..test_class values (202, 'Class 2, Grade 2', 'Mr. Liu');

 --学生表中插入数据

 insert tempdb..test_student values (101001, 101, 'Name 1', 20, 'Address 1');

 insert tempdb..test_student values (101002, 101, 'Name 2', 21, 'Address 2');

 insert tempdb..test_student values (101003, 101, 'Name 3', 20, 'Address 3');

 insert tempdb..test_student values (102001, 102, 'Name 4', 22, 'Address 4');

 insert tempdb..test_student values (102002, 102, 'Name 5', 21, 'Address 5');

 insert tempdb..test_student values (201001, 201, 'Name 6', 23, 'Address 6');

 insert tempdb..test_student values (201002, 201, 'Name 7', 22, 'Address 7');

 insert tempdb..test_student values (202001, 202, 'Name 8', 22, 'Address 8');

<创建实体类Entity>

 package mybatistest;

 import java.io.Serializable;

 public class Clazz implements Serializable {

     private static final long serialVersionUID = 1L;

     private Integer classId;

     private String className;

     private String classAdminName;

     public Integer getClassId() {

         return classId;

     }

     public void setClassId(Integer classId) {

         this.classId = classId;

     }

     public String getClassName() {

         return className;

     }

     public void setClassName(String className) {

         this.className = className;

     }

     public String getClassAdminName() {

         return classAdminName;

     }

     public void setClassAdminName(String classAdminName) {

         this.classAdminName = classAdminName;

     }

     @Override

     public String toString() {

         return "Clazz [classId=" + classId + ", className=" + className + ", classAdminName=" + classAdminName + "]";

     }

 }

Class

 package mybatistest;

 import java.io.Serializable;

 public class Student implements Serializable {

     private static final long serialVersionUID = 1L;

     private Integer studentId;

     private Integer classId;

     private String studentName;

     private Integer studentAge;

     private String studentAddress;

     public Integer getStudentId() {

         return studentId;

     }

     public void setStudentId(Integer studentId) {

         this.studentId = studentId;

     }

     public String getStudentName() {

         return studentName;

     }

     public Integer getClassId() {

         return classId;

     }

     public void setClassId(Integer classId) {

         this.classId = classId;

     }

     public void setStudentName(String studentName) {

         this.studentName = studentName;

     }

     public Integer getStudentAge() {

         return studentAge;

     }

     public void setStudentAge(Integer studentAge) {

         this.studentAge = studentAge;

     }

     public String getStudentAddress() {

         return studentAddress;

     }

     public void setStudentAddress(String studentAddress) {

         this.studentAddress = studentAddress;

     }

     @Override

     public String toString() {

         return "Student [classId=" + classId + ", studentId=" + studentId + ", studentName=" + studentName

                 + ", studentAge=" + studentAge + ", studentAddress=" + studentAddress + "]";

     }

 }

Student

<创建StudentsPerClass(OutputBean)>

创建一个Class的OutBean，其中包含classId，Class实体类，及Student实体类List以对应班级--学生的1对多关系。

 package mybatistest;

 import java.io.Serializable;

 import java.util.List;

 public class StudentsPerClass implements Serializable {

     private static final long serialVersionUID = 1L;

     private Integer classId;

     private Clazz clazz;

     private List<Student> students;

     public Integer getClassId() {

         return classId;

     }

     public void setClassId(Integer classId) {

         this.classId = classId;

     }

     public Clazz getClazz() {

         return clazz;

     }

     public void setClazz(Clazz clazz) {

         this.clazz = clazz;

     }

     public List<Student> getStudents() {

         return students;

     }

     public void setStudents(List<Student> students) {

         this.students = students;

     }

     @Override

     public String toString() {

         StringBuilder sb = new StringBuilder(

                 "StudentsPerClass [classId=" + classId + ", clazz=" + clazz + ", students=" + System.lineSeparator());

         students.forEach(p -> {

             sb.append("--").append(p).append(System.lineSeparator());

         });

         return sb.append("]").toString();

     }

 }

StudentsPerClass

<创建三个SQL实例>

(1) 用Map<学号, 学生实体类>映射一个学生集合

(2) 用List<StudentsPerClass>映射班级--学生的1对多关系

(3) 用Map<班级号, StudentsPerClass>映射班级--学生集合

ClassStudentRepository.java

 package mybatistest;

 import java.util.List;

 import java.util.Map;

 import org.apache.ibatis.annotations.MapKey;

 public interface ClassStudentRepository {

     // 获取一个班级的所有学生[Map<[Student Id], [Student Info]>]

     @MapKey("studentId")

     Map<Integer, Student> getStudentsFromOneClass(Integer classId);

     // 获取一个年级的所有学生

     List<StudentsPerClass> getStudentsFromOneGrade(Integer gradeId);

     // 获取所有学生

     @MapKey("classId")

     Map<Integer, StudentsPerClass> getAllStudents();

 }

ClassStudentRepository.xml

注意黄色部分非常重要，只有设置了1对多关系中的主键class_id，Mybatis才能自动映射这种1对多的嵌套关系。

 <?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="mybatistest.ClassStudentRepository">

     <!-- // 获取一个班级的所有学生[Map<[Student Id], [Student Info]>] -->

     <select id="getStudentsFromOneClass" resultMap="studentsFromOneClassMap">

         SELECT

             t2.student_id,

             t2.class_id,

             t2.student_name,

             t2.student_age,

             t2.student_address

         FROM

             tempdb..test_class t1

         INNER JOIN

             tempdb..test_student t2

         ON

             t1.class_id = t2.class_id

         WHERE

             t1.class_id = #{classId}

     </select>

     <resultMap id="studentsFromOneClassMap" type="mybatistest.Student">

         <id property="studentId" column="student_id" />

         <result property="classId" column="class_id" />

         <result property="studentName" column="student_name" />

         <result property="studentAge" column="student_age" />

         <result property="studentAddress" column="student_address" />

     </resultMap>

     <!-- 获取一个年级的所有学生 -->

     <select id="getStudentsFromOneGrade" resultMap="studentsFromOneGradeMap">

         SELECT

             t1.class_id,

             t1.class_name,

             t1.class_admin_name,

             t2.student_id,

             t2.student_name,

             t2.student_age,

             t2.student_address

         FROM

             tempdb..test_class t1

         INNER JOIN

             tempdb..test_student t2

         ON

             t1.class_id = t2.class_id

         WHERE

             t1.class_id/100 = #{gradeId}

     </select>

     <resultMap id="studentsFromOneGradeMap" type="mybatistest.StudentsPerClass">

         <id property="classId" column="class_id"/>

         <association property="clazz" javaType="mybatistest.Clazz" >

             <id property="classId" column="class_id"/>

             <result property="className" column="class_name"/>

             <result property="classAdminName" column="class_admin_name"/>

         </association>

         <collection property="students" ofType="mybatistest.Student">

             <id property="studentId" column="student_id" />

             <result property="classId" column="class_id"/>

             <result property="studentName" column="student_name" />

             <result property="studentAge" column="student_age" />

             <result property="studentAddress" column="student_address" />

         </collection>

     </resultMap>

     <!-- 获取所有学生 -->

     <select id="getAllStudents" resultMap="allStudentsMap">

         SELECT

             t1.class_id,

             t1.class_name,

             t1.class_admin_name,

             t2.student_id,

             t2.student_name,

             t2.student_age,

             t2.student_address

         FROM

             tempdb..test_class t1

         INNER JOIN

             tempdb..test_student t2

         ON

             t1.class_id = t2.class_id

     </select>

     <resultMap id="allStudentsMap" type="mybatistest.StudentsPerClass">

         <id property="classId" column="class_id"/>

         <association property="clazz" javaType="mybatistest.Clazz" >

             <id property="classId" column="class_id"/>

             <result property="className" column="class_name"/>

             <result property="classAdminName" column="class_admin_name"/>

         </association>

         <collection property="students" ofType="mybatistest.Student">

             <id property="studentId" column="student_id" />

             <result property="classId" column="class_id"/>

             <result property="studentName" column="student_name" />

             <result property="studentAge" column="student_age" />

             <result property="studentAddress" column="student_address" />

         </collection>

     </resultMap>

 </mapper>

mybatis-config.xml中添加映射关系

     <mappers>

         <mapper resource="mybatistest/ClassStudentRepository.xml" />

     </mappers>

<测试结果>

 package mybatistest;

 import java.util.List;

 import java.util.Map;

 import org.apache.ibatis.session.SqlSession;

 import org.apache.ibatis.session.SqlSessionFactory;

 public class MainTest {

     public static void main(String[] args) {

         SqlSessionFactory sqlSessionFactory = MybatisTestSessionFactory.getSqlSessionFactoryInstance();

         SqlSession session = sqlSessionFactory.openSession();

         ClassStudentRepository classStudentRepository = session.getMapper(ClassStudentRepository.class);

         Map<Integer, Student> res1 = classStudentRepository.getStudentsFromOneClass(102);

         res1.forEach((p, q) -> {

             System.out.println("Student ID: " + p + "; Student info: " + q);

         });

         // [Output]

         // Student ID: 102001; Student info: Student [classId=102, studentId=102001, studentName=Name 4, studentAge=22, studentAddress=Address 4]

         // Student ID: 102002; Student info: Student [classId=102, studentId=102002, studentName=Name 5, studentAge=21, studentAddress=Address 5]

         List<StudentsPerClass> res2 = classStudentRepository.getStudentsFromOneGrade(1);

         res2.forEach((p) -> {

             System.out.println(p);

         });

         // [Output]

         // StudentsPerClass [classId=101, clazz=Clazz [classId=101, className=Class 1, Grade 1, classAdminName=Mr. Zhang], students=

         // --Student [classId=101, studentId=101001, studentName=Name 1, studentAge=20, studentAddress=Address 1]

         // --Student [classId=101, studentId=101002, studentName=Name 2, studentAge=21, studentAddress=Address 2]

         // --Student [classId=101, studentId=101003, studentName=Name 3, studentAge=20, studentAddress=Address 3]

         // ]

         // StudentsPerClass [classId=102, clazz=Clazz [classId=102, className=Class 2, Grade 1, classAdminName=Mr. Li], students=

         // --Student [classId=102, studentId=102001, studentName=Name 4, studentAge=22, studentAddress=Address 4]

         // --Student [classId=102, studentId=102002, studentName=Name 5, studentAge=21, studentAddress=Address 5]

         // ]

         Map<Integer, StudentsPerClass> res3 = classStudentRepository.getAllStudents();

         res3.forEach((p, q) -> {

             System.out.println("Class ID: " + p + System.lineSeparator() + q);

         });

         // [Output]

         // Class ID: 101

         // StudentsPerClass [classId=101, clazz=Clazz [classId=101, className=Class 1, Grade 1, classAdminName=Mr. Zhang], students=

         // --Student [classId=101, studentId=101001, studentName=Name 1, studentAge=20, studentAddress=Address 1]

         // --Student [classId=101, studentId=101002, studentName=Name 2, studentAge=21, studentAddress=Address 2]

         // --Student [classId=101, studentId=101003, studentName=Name 3, studentAge=20, studentAddress=Address 3]

         // ]

         // Class ID: 102

         // StudentsPerClass [classId=102, clazz=Clazz [classId=102, className=Class 2, Grade 1, classAdminName=Mr. Li], students=

         // --Student [classId=102, studentId=102001, studentName=Name 4, studentAge=22, studentAddress=Address 4]

         // --Student [classId=102, studentId=102002, studentName=Name 5, studentAge=21, studentAddress=Address 5]

         // ]

         // Class ID: 201

         // StudentsPerClass [classId=201, clazz=Clazz [classId=201, className=Class 1, Grade 2, classAdminName=Mr. Zhao], students=

         // --Student [classId=201, studentId=201001, studentName=Name 6, studentAge=23, studentAddress=Address 6]

         // --Student [classId=201, studentId=201002, studentName=Name 7, studentAge=22, studentAddress=Address 7]

         // ]

         // Class ID: 202

         // StudentsPerClass [classId=202, clazz=Clazz [classId=202, className=Class 2, Grade 2, classAdminName=Mr. Liu], students=

         // --Student [classId=202, studentId=202001, studentName=Name 8, studentAge=22, studentAddress=Address 8]

         // ]

         session.close();

     }

 }

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