Myabtis中的ResultMap

2021/4/14 10:55:15

编程Tag： import id name com public resultMap xiaoli Myabtis

本文主要是介绍Myabtis中的ResultMap，对大家解决编程问题具有一定的参考价值，需要的程序猿们随着小编来一起学习吧！

结果集映射ResultMap

Mybtis中的结果集映射是一个强大的工具。在Mybatis官网有描述 “MyBatis 的真正强大在于它的语句映射，这是它的魔力所在。由于它的异常强大，映射器的 XML 文件就显得相对简单。如果拿它跟具有相同功能的 JDBC 代码进行对比，你会立即发现省掉了将近 95% 的代码。MyBatis 致力于减少使用成本，让用户能更专注于 SQL 代码。”

1.数据库表与实体集不匹配

  • 对于数据库中的表与实体集（POJO）的对象关系映射不一致是，ResultMap就有着巨大的作用。

image-20210413110912235

  • 在实体集中，我们发现实体集中的属性与数据库中的属性不匹配，在这忠情况下，我们需要使用实体集的映射功能。

  • 使用步骤：

    1. 配置mapper接口和xml文件，将sql语句的xml文件注册到mybatis文件到映射器中。

      备注：

        <mappers>
        <mapper resource="com/xiaoli/dao/UserMapper.xml"/>
        <mapper resource="com/xiaoli/dao/StudentMapper.xml"/>
    </mappers>

      image-20210413112132847

    2. 需要注意的是，在类路径下配置的资源文件和xml文件可能会出现导出不了的问题，我们需要在pom.xml文件中添加配置文件。

      <!--导出文件失效问题解决-->
    <build>
        <resources>
            <resource>
                <directory>src/main/resources</directory>
                <includes>
                    <include>**/*.properties</include>
                    <include>**/*.xml</include>
                </includes>
                <filtering>true</filtering>
            </resource>
            <resource>
                <directory>src/main/java</directory>
                <includes>
                    <include>**/*.properties</include>
                    <include>**/*.xml</include>
                </includes>
                <filtering>true</filtering>
            </resource>
        </resources>
    </build>

    3. 我们配置对象关系映射,创建Mapper接口，创建mapper.xml配置文件

      StudentMapper接口

      package com.xiaoli.dao;

import com.xiaoli.pojo.Student;
import com.xiaoli.pojo.User;

import java.util.List;

public interface StudentMapper {
    //select查询语句
    public List<Student> getStudentList();

    //select条件查询
    public Student getStudentById(int 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">

<!--绑定Mapper借口-->
<mapper namespace="com.xiaoli.dao.StudentMapper">

    <!--
        结果集映射配置:
        1.resultMap:xml映射器
        2.id=""：进行映射的类的别名，可以随意配置，但是需要注意与sql语句中的resultMap保持一致
        3.type：映射的数据类型(在该sql语句中，需要映射的是Student类）
    -->
    <resultMap id="getStudent" type="Student">
        <!--
        result:进行映射的字段
        property:实体集的属性
        column：数据库的字段名
         -->
        <result property="id" column="stu_id"/>
        <result property="name" column="stu_name"/>
        <result property="password" column="stu_password"/>
    </resultMap>
    <!--select查询语句-->
    <select id="getStudentList" resultMap="getStudent">
        select * from student
    </select>

    <!--条件查询-->
    <select id="getStudentById" parameterType="_int" resultMap="getStudent">
        select * from student where stu_id = #{id}
    </select>
</mapper>

      4.测试，编写测试代码

      测试代码：

      package com.xiaoli.dao;

import com.xiaoli.pojo.Student;
import com.xiaoli.pojo.User;
import com.xiaoli.util.MybatisUtil;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;

import java.util.List;

public class testStudentMapper {

    //select查询
    @Test
    public void testGetStudentList(){
        //获取SqlSession对象
        SqlSession sqlSession = MybatisUtil.getSqlSession();

        //  获取映射器
        StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);

        List<Student> studentList = mapper.getStudentList();

        //遍历
        for (Student student : studentList) {
            System.out.println(student);
        }
        //关闭连接
        sqlSession.close();

    }

    //select条件查询
    @Test
    public void testGetStudentById(){
        //获取SqlSession对象
        SqlSession sqlSession = MybatisUtil.getSqlSession();

        //  获取映射器
        StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);

        Student studentById = mapper.getStudentById(1);

        //遍历
        System.out.println(studentById);

        //关闭连接
        sqlSession.close();

    }
}

      运行结果：

      testGetStudentList

      image-20210413161007696

      testGetStudentById

      image-20210413162135141

2.数据库表与数据库表的对应关系

2.1 多对一的关系

2.1.1 案例分析：

现在我们需要对多对一进行分析，案例：

数据库中有两个实体：

stduent实体：

CREATE TABLE `student` (
  `id` int NOT NULL,
  `name` varchar(30) DEFAULT NULL,
  `tid` int DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `fktid` (`tid`),
  CONSTRAINT `fktid` FOREIGN KEY (`tid`) REFERENCES `teacher` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

INSERT INTO teacher(`id`, `name`) VALUES (1, '秦老师');

teacher实体：

CREATE TABLE `student` (
  `id` INT(10) NOT NULL,
  `name` VARCHAR(30) DEFAULT NULL,
  `tid` INT(10) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `fktid` (`tid`),
  CONSTRAINT `fktid` FOREIGN KEY (`tid`) REFERENCES `teacher` (`id`)
) ENGINE=INNODB DEFAULT CHARSET=utf8

INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('1', '小明', '1'); 
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('2', '小红', '1'); 
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('3', '小张', '1'); 
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('4', '小李', '1'); 
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('5', '小王', '1');

通过创建的两个实体集：student与teacher，我们不难发现，在表中存在两种对应关系：

  • 多对一的关系

    image-20210413164546767

    ​ 对于学生而言，使用多对一的关系

  • 一对多的关系

    image-20210413164558725

    ​ 对于老师而言，是一对多的关系

2.1.2 编码

  1. 创建项目，创建两个项目的实体集（POPJO）

    Teacher实体集

    package com.xiaoli.pojo;

public class Teacher {
    private int id;
    private String name;

    public Teacher() {
    }

    public Teacher(int id, String name) {
        this.id = id;
        this.name = name;
    }

    public int getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

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

    Stduent实体集：
    备注：我们需要注意多对一的关系，学生中包含了老师的对象

    package com.xiaoli.pojo;

public class Student {
    private int id;
    private String name;
    private Teacher teacher;//代表学生中含有一个老师，对应与student字段tid外键

    public Student() {
    }

    public Student(int id, String name, Teacher teacher) {
        this.id = id;
        this.name = name;
        this.teacher = teacher;
    }

    public int getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Teacher getTeacher() {
        return teacher;
    }

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

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

  2. 编写mapper接口与mapper.xml配置文件(无论实体的mapper映射有没有用，都创建，以防万一)

    • TeacherMapper接口

      package com.xiaoli.dao;

public interface TeacherMapper {
    
}

    • TeacherMapper.xml配置文件

      <?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">

<mapper namespace="com.xiaoli.dao.TeacherMapper">

</mapper>

    • Studentmapper接口

      package com.xiaoli.dao;


import com.xiaoli.pojo.Student;

import java.util.List;

public interface StudentMapper {
}

    • StudentMapper.xml配置文件

      <?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">

<mapper namespace="com.xiaoli.dao.StudentMapper">
</mapper>

  3. 在config.xml配置文件中配置mapper映射器和为实体集添加别名（alias），方便以后使用

    <?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
    <properties resource="db.properties"/>

    <typeAliases>
        <typeAlias type="com.xiaoli.pojo.Student" alias="Student"/>
        <typeAlias type="com.xiaoli.pojo.Teacher" alias="Teacher"/>
    </typeAliases>

    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="${driver}"/>
                <property name="url" value="${url}"/>
                <property name="username" value="${username}"/>
                <property name="password" value="${password}"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <mapper resource="com/xiaoli/dao/StudentMapper.xml"/>
        <mapper resource="com/xiaoli/dao/TeacherMapper.xml"/>
    </mappers>
</configuration>

  4. 编写代码，测试目前会出现的问题

    StudentMapper

    package com.xiaoli.dao;


import com.xiaoli.pojo.Student;

import java.util.List;

public interface StudentMapper {
    //查询所有学生以及学生的老师
    public List<Student> getStudentList();
}

    StudentMapper.xml

    <?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">

<mapper namespace="com.xiaoli.dao.StudentMapper">
    <select id="getStudentList" resultType="Student">
        select * from student
    </select>
</mapper>

    运行结果：我们发现teacher字段为查询出结果，我们需要考虑多对一的关系。

    image-20210413172406411

  5. 处理字段为空情况

    • 按结果嵌套处理

      <?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">

<mapper namespace="com.xiaoli.dao.StudentMapper">

<!--
    按结果嵌套处理：
    1.编写sql语句
    2.使用结果集映射：resultMap
-->
    <resultMap id="getStudent" type="Student">
        <!--
            property：对应实体集的属性
            column：对应数据库sql语句的字段名
        -->
        <result property="id" column="sid"/>
        <result property="name" column="sname"/>
        <!--
            association:表示该属性是对象
            collection:表示该属性是集合
        -->
        <association property="teacher" javaType="Teacher">
            <result property="id" column="tid"/>
            <result property="name" column="tname"/>
        </association>
    </resultMap>
    <select id="getStudentList" resultMap="getStudent">
        select s.id sid,s.name sname,t.id tid,t.name tname
        from student s, teacher t
    </select>
</mapper>

    • 按查询嵌套处理

      该方法不推荐，不是那么好懂

2.2 一对多的关系

2.2.1 案例分析

我们通过上面的案例分析，知道，在老师看来，老师与学生的关系是一对多的关系，即一个老师可以拥有多个学生。

2.2.2 编码

  1. 创建项目，创建两个实体对象（POJOP）

    Teacher实体集

    注意：我们需要注意一对多的问题，一个老师包含多个学生

    package com.xiaoli.pojo1;

import java.util.List;

public class Teacher {
    private int id;
    private String name;

    private List<Student> studentList;//一对多的关系，一个老师包含多个学生

    public Teacher() {
    }

    public Teacher(int id, String name, List<Student> studentList) {
        this.id = id;
        this.name = name;
        this.studentList = studentList;
    }

    public int getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public List<Student> getStudentList() {
        return studentList;
    }

    public void setStudentList(List<Student> studentList) {
        this.studentList = studentList;
    }

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

    Student实体

    package com.xiaoli.pojo1;

public class Student {
    private int id;
    private String name;
    private int tid;

    public Student() {
    }

    public Student(int id, String name, int tid) {
        this.id = id;
        this.name = name;
        this.tid = tid;
    }

    public int getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getTid() {
        return tid;
    }

    public void setTid(int tid) {
        this.tid = tid;
    }

    @Override
    public String toString() {
        return "Student{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", tid=" + tid +
                '}';
    }
}

  2. 编写mapper接口与mapper.xml配置文件(无论实体的mapper映射有没有用，都创建，以防万一)

    TeacherMapper接口

    package com.xiaoli.dao1;

public class TeacherMapper {
}

    TeacherMapper.xml配置文件

    <?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">

<mapper namespace="com.xiaoli.dao1.TeacherMapper">

</mapper>

    StudentMapper接口

    package com.xiaoli.dao1;

public class StudentMapper {

}

    StudentMapper.xml配置文件

    <?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">

<mapper namespace="com.xiaoli.dao1.StudentMapper">

</mapper>

  3. 在config.xml配置文件中配置mapper映射器和为实体集添加别名（alias），方便以后使用

    <typeAliases>
        <typeAlias type="com.xiaoli.pojo1.Student" alias="Student1"/>
        <typeAlias type="com.xiaoli.pojo1.Teacher" alias="Teacher1"/>
    </typeAliases>

<mappers>
    <mapper resource="com/xiaoli/dao1/StudentMapper.xml"/>
    <mapper resource="com/xiaoli/dao1/TeacherMapper.xml"/>
</mappers>

  4. 编写测试代码，查看我们目前出现的问题

    代码：

    StudentMapper接口

    package com.xiaoli.dao1;

public interface StudentMapper {

}

    StudentMapper.xml配置文件

    <?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">

<mapper namespace="com.xiaoli.dao1.StudentMapper">

</mapper>

    TeacherMapper接口

    package com.xiaoli.dao1;

import com.xiaoli.pojo1.Teacher;

import java.util.List;

public interface TeacherMapper {

    List<Teacher> getTeacherList();
}

    TeacherMapper.xml配置文件

    <?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">

<mapper namespace="com.xiaoli.dao1.TeacherMapper">

    <!--
        步骤：
        1.编写sql语句
        2.进行结果集映射
    -->
    <resultMap id="getTeacher" type="Teacher1">
        <result property="id" column="tid"/>
        <result property="name" column="tname"/>
        <!--
           collection：代表集合   ofType:该属性的返回类型，在collection中使用
           association：代表对象  javaType:该属性的返回类型，在association中使用
        -->
        <collection property="studentList" ofType="Student1">
            <result property="id" column="sid"/>
            <result property="name" column="sname"/>
        </collection>
    </resultMap>
    <select id="getTeacherList" resultMap="getTeacher">
        select t.id tid,t.name tname,s.id sid,s.name sname from student s,teacher t
    </select>
</mapper>

  5. 测试代码与测试结果

    package com.xioali.dao;

import com.xiaoli.dao.StudentMapper;
import com.xiaoli.dao1.TeacherMapper;
import com.xiaoli.pojo.Student;
import com.xiaoli.pojo1.Teacher;
import com.xiaoli.util.MybatisUtil;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;

import java.util.List;

public class testStudentMapper {

    //一对多的关系
    @Test
    public void testGetTeacherList(){
        SqlSession sqlSession = MybatisUtil.getSqlSession();

        TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);

        List<Teacher> teacherList = mapper.getTeacherList();

        for (Teacher teacher : teacherList) {
            System.out.println(teacher);
        }

        sqlSession.close();
    }
}

    运行结果：

    image-20210414102259043



这篇关于Myabtis中的ResultMap的文章就介绍到这儿，希望我们推荐的文章对大家有所帮助，也希望大家多多支持为之网！


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