MyBatis自定义映射关系和关联查询实现方法详解

MyBatis自定义映射关系 MyBatis关联查询 2023-05-15 14:05:01 529人浏览独家记忆

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摘要

目录一、使用注解实现自定义映射关系1. 编写注解方法2. 编写测试方法3. 查看运行结果二、使用注解实现一对一关联查询1. 编写注解方法2. 编写测试方法3. 查看运行结果三、使用注

一、使用注解实现自定义映射关系

当POJO属性名与数据库列名不一致时，需要自定义实体类和结果集的映射关系，在mybatis注解开发中，使用 @Results 定义并使用自定义映射，使用 @ResultMap 使用自定义映射，用法如下：

前戏：为了体验这个效果，我们可以修改一下User实体类代码，如下

package com.example.pojo;
import java.io.Serializable;
public class User implements Serializable {
    private int id;
    private String username1;
    private String sex1;
    private String address1;
    public User(String programmer, String man, String shangHai) {
        this.username1 = programmer;
        this.sex1 = man;
        this.address1 = shangHai;
    }
    public User(int i, String programmer_1, String woman, String shenzhen) {
        this.id = i;
        this.username1 = programmer_1;
        this.sex1 = woman;
        this.address1 = shenzhen;
    }
    public User() {
    }
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getUsername1() {
        return username1;
    }
    public void setUsername1(String username1) {
        this.username1 = username1;
    }
    public String getSex1() {
        return sex1;
    }
    public void setSex1(String sex1) {
        this.sex1 = sex1;
    }
    public String getAddress1() {
        return address1;
    }
    public void setAddress1(String address1) {
        this.address1 = address1;
    }
    @Override
    public String toString() {
        return "User[ " +
                "id=" + id +
                ", username1='" + username1 + '\'' +
                ", sex1='" + sex1 + '\'' +
                ", address1='" + address1 + '\'' +
                " ]";
    }
}

1. 编写注解方法

    // 查询所有用户
    @Results(id="userDiyMapper",value = {
            @Result(id = true,property = "id",column = "id"),
            @Result(property = "username1",column = "username"),
            @Result(property = "sex1",column = "sex"),
            @Result(property = "address1",column = "address")
    })
    @Select("select * from user")
    List<User> findAll1();
    // 根据id查询
    @ResultMap("userDiyMapper")
    @Select("select * from user where id = #{id}")
    User findById(int id);

注意啊：这里property对应的是实体类属性名，column对应的就是数据库表的列名

2. 编写测试方法

    @Test
    public void testFindAll1(){
        List<User> all = userMapper.findAll1();
        all.forEach(System.out::println);
        System.out.println("---------------------");
        System.out.println(userMapper.findById(5));
    }

看看能否查询出所有用户和id为5的用户，并且留意对应的属性名

3. 查看运行结果

OK，看来都是符合我们的预期的。

二、使用注解实现一对一关联查询

在MyBatis的注解开发中对于多表查询只支持分解查询，不支持连接查询。

这里我们采用学生表和班级表做对比，所以我们先新建Student实体类和Classes实体类

Classes实体类

package com.example.pojo;
import java.util.List;
public class Classes {
    private int cid;
    private String className;
    private List<Student> studentList;
    public int getCid() {
        return cid;
    }
    public void setCid(int cid) {
        this.cid = cid;
    }
    public String getClassName() {
        return className;
    }
    public void setClassName(String className) {
        this.className = className;
    }
    public List<Student> getStudentList() {
        return studentList;
    }
    public void setStudentList(List<Student> studentList) {
        this.studentList = studentList;
    }
    @Override
    public String toString() {
        return "Classes[ " +
                "cid=" + cid +
                ", className='" + className + '\'' +
                ", studentList=" + studentList + 
                " ]";
    }
}

Student实体类

package com.example.pojo;
public class Student {
    private int sid;
    private String name;
    private int age;
    private String sex;
    private Classes classes;
    public int getSid() {
        return sid;
    }
    public void setSid(int sid) {
        this.sid = sid;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    public String getSex() {
        return sex;
    }
    public void setSex(String sex) {
        this.sex = sex;
    }
    public Classes getClasses() {
        return classes;
    }
    public void setClasses(Classes classes) {
        this.classes = classes;
    }
    @Override
    public String toString() {
        return "Student[ " +
                "sid=" + sid +
                ", name='" + name + '\'' +
                ", age=" + age +
                ", sex='" + sex + '\'' +
                ", classes=" + classes +
                " ]";
    }
}

1. 编写注解方法

StudentMapper添加查询所有用户

package com.example.mapper;
import com.example.pojo.Student;
import org.apache.ibatis.annotations.One;
import org.apache.ibatis.annotations.Result;
import org.apache.ibatis.annotations.Results;
import org.apache.ibatis.annotations.Select;
import org.apache.ibatis.mapping.FetchType;
import java.util.List;
public interface StudentMapper {
    @Select("select * from student")
    // 自定义映射关系
    @Results(id = "studentMapper",value = {
            @Result(id = true,property = "sid",column = "sid"),
            @Result(property = "name",column = "name"),
            @Result(property = "age",column = "age"),
            @Result(property = "sex",column = "sex"),
            
            @Result(property = "classes",column = "classId",
                one = @One(select = "com.example.mapper.ClassesMapper.findByCid",
                        fetchType = FetchType.EAGER)
                )
    })
    List<Student> findAll();
}

ClassesMapper添加根据id查询班级

package com.example.mapper;
import com.example.pojo.Classes;
import org.apache.ibatis.annotations.Many;
import org.apache.ibatis.annotations.Result;
import org.apache.ibatis.annotations.Results;
import org.apache.ibatis.annotations.Select;
import org.apache.ibatis.mapping.FetchType;
import java.util.List;
public interface ClassesMapper {
    // 根据id查询班级
    @Select("select * from classes where cid = #{cid}")
    Classes findByCid(Integer id);
}

2. 编写测试方法

// 测试一对一查询
    @Test
    public void testFindAllStudent(){
        StudentMapper studentMapper = session.getMapper(StudentMapper.class);
        List<Student> all = studentMapper.findAll();
        all.forEach(System.out::println);
    }

看看能否查询出学生对应的班级，如果可以则查询成功

3. 查看运行结果

OK，看图我们是已经成功查询出每个学生对应的班级的

三、使用注解实现一对多关联查询

在这里我们主要实现查询所有班级的时候把对应的学生列表也查询出来。

1. 编写注解方法

StudentMapper添加根据班级id查询学生

// 根据班级Id查询学生
    @Select("select * from student where ClassId = #{classId}")
    List<Student> findByClassId(int classId);

Classes添加查询所有班级

// 查询所有班级
    @Select("select * from classes")
    @Results(id = "claSSMapper",value = {
            @Result(id = true,property = "cid",column = "cid"),
            @Result(property = "className",column = "className"),
            // many:表示该属性是一个集合
            @Result(property = "studentList",column = "cid",
            many = @Many(select = "com.example.mapper.StudentMapper.findByClassId",
                        fetchType = FetchType.LAZY))
    })
    List<Classes> findAll();

2. 编写测试方法

// 测试一对多查询
    @Test
    public void findAllClasses(){
        ClassesMapper classesMapper = session.getMapper(ClassesMapper.class);
        List<Classes> all = classesMapper.findAll();
        all.forEach(System.out::println);
    }

观察能否查询出班级对应的学生列表