Mybatis高级结果映射

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();
     }
 }