Mybatis 返回 Map 对象

一、场景介绍

假设有如下一张学生表：

CREATE TABLE `student` (
  `id` int NOT NULL AUTO_INCREMENT COMMENT '主键',
  `name` varchar(100) NOT NULL COMMENT '姓名',
  `gender` varchar(10) NOT NULL COMMENT '性别',
  `grade` int NOT NULL COMMENT '年级',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB COMMENT='学生信息表';

我们通过一组 ids，想获得 id 跟学生对象的映射 Map，可以这样做：

1、先通过 ids 获取对象列表

2、再通过 stream 流式运算得到 id-Student 映射

这样做虽然也没有问题，但是这样 service 层就会散落一堆这样流式运算的代码。不是很美观，应该在DAO层就提供这种能力。Mybatis 提供给我们查询 Map 的方式。

@Service
public class StudentServiceImpl implements StudentService {

    @Autowired
    private StudentMapper mapper;

    @Override
    public Map findMapByIds(List ids) {
        List students = mapper.listByIds(ids);
        Map map = students.stream().collect(Collectors.toMap(Student::getId, Function.identity()));
        return map;
    }
}
---------------------------------------------------------------
@Test
public void test() {
        List list = Arrays.asList(1,2,3);
        Map map = studentService.findMapByIds(list);
        System.out.println(map);
}

输出：
{1=Student(id=1, name=小明, gender=male, grade=1), 2=Student(id=2, name=小红, gender=female, grade=2), 3=Student(id=3, name=小李, gender=male, grade=3)}

二、Mybatis @MapKey 方式

Mybatis 提供了 @MapKey 注解，注解的 value 用来指定 key 的取值字段。

可以看到，效果是一样的。

@Mapper
public interface StudentMapper {

    /**
     * 这种基础的查询，不应该先获取 List 流，然后再转换，
     * 这样会使整个项目散落一地这种代码
     * 应该在DAO层就提供这种能力
     */
    @MapKey("id")
    Map findMapByIds(List ids);
}
-------------------------------------------------------------



    
    
        select *
        from student s
        
            and s.id in
            
                #{id}
            
        
    


-------------------------------------------------------------
@Service
public class StudentServiceImpl implements StudentService {

    @Autowired
    private StudentMapper mapper;

    @Override
    public Map findMapByIds(List ids) {
        /*
        List students = mapper.listByIds(ids);
        Map map = students.stream().collect(Collectors.toMap(Student::getId, Function.identity()));
        return map;
        */
        return mapper.findMapByIds(ids);
    }
}
-------------------------------------------------------------
@Test
public void test() {
        List list = Arrays.asList(1,2,3);
        Map map = studentService.findMapByIds(list);
        System.out.println(map);
}
输出：
{1=Student(id=1, name=小明, gender=male, grade=1), 2=Student(id=2, name=小红, gender=female, grade=2), 3=Student(id=3, name=小李, gender=male, grade=3)}

 但如果 @MapKey 的 value 配置了记录重复的字段会怎么样？

就比如，@MapKey("grade")，grade = 1，2，3 的记录并非唯一。

@Mapper
public interface StudentMapper {

    List listByIds(List ids);

    /**
     * @MapKey 的 value 是 grade 这种非唯一字段
     */
    @MapKey("grade")
    Map findMapByIds(List ids);
}

 这个时候会发现，返回的记录被覆盖了。

id in [1,6] 的 grade=1 的记录有 小明跟小智，

小明先查询出来，

map.put(1,  Student(id=1, name=小明, gender=male, grade=1))

小智后查询出来，

map.put(1,  Student(id=6, name=小智, gender=male, grade=1))

同一个key，小智覆盖了小明。

那如果我们希望返回 Map> 怎么办呢？

@Test
public void test2() {
        List list = Arrays.asList(1,2,3,4,5,6);
        Map map = studentService.findMapByIds(list);
        System.out.println(map);
}
输出：
{1=Student(id=6, name=小智, gender=male, grade=1), 2=Student(id=4, name=小林, gender=male, grade=2), 3=Student(id=5, name=小婷, gender=female, grade=3)}

三、Stream 返回 Map>

 遗憾的是，笔者找了 Mybatis 跟 MybatisPlus 也没找到此类方法，所以这一块，还是只能用流式计算来做。

@Mapper
public interface StudentMapper {

    List listByIds(List ids);
}
-------------------------------------------------------------



    
    
        select *
        from student s
        
            and s.id in
            
                #{id}
            
        
    


-------------------------------------------------------------
@Service
public class StudentServiceImpl implements StudentService {

    @Autowired
    private StudentMapper mapper;

    @Override
    public Map> getMapByIds(List ids) {
        List students = mapper.listByIds(ids);
        return students.stream().collect(Collectors.groupingBy(Student::getGrade));
    }
}
-------------------------------------------------------------
@Test
public void test3() {
        List list = Arrays.asList(1,2,3,4,5,6);
        Map> map = studentService.getMapByIds(list);
        System.out.println(map);
    }
输出：
{1=[Student(id=1, name=小明, gender=male, grade=1), Student(id=6, name=小智, gender=male, grade=1)], 2=[Student(id=2, name=小红, gender=female, grade=2), Student(id=4, name=小林, gender=male, grade=2)], 3=[Student(id=3, name=小李, gender=male, grade=3), Student(id=5, name=小婷, gender=female, grade=3)]}

四、MybatisPlus 返回 List>

如果希望查询返回 List> Map 的 key 是数据库 column name

package com.study.student.mapper;

import com.study.student.entity.Student;
import com.baomidou.mybatisplus.core.mapper.BaseMapper;
import org.apache.ibatis.annotations.*;

@Mapper
public interface StudentMapper extends BaseMapper {
}
------------------------------------------------------------
package com.study.student.service.impl;

import com.study.student.entity.Student;
import com.study.student.mapper.StudentMapper;
import com.study.student.service.StudentService;
import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
import com.baomidou.mybatisplus.extension.service.impl.ServiceImpl;
import org.springframework.stereotype.Service;

import java.util.List;
import java.util.Map;

@Service
public class StudentServiceImpl extends ServiceImpl implements StudentService {

    @Override
    public List> getMapList(List ids) {
        LambdaQueryWrapper wrapper = new LambdaQueryWrapper<>();
        wrapper.in(Student::getId, ids);
        return this.getBaseMapper().selectMaps(wrapper);
    }
}
------------------------------------------------------------
@Test
public void test4() {
        List list = Arrays.asList(1,2,3,4,5,6);
        List> map = studentService.getMapList(list);
        System.out.println(map);
}
输出：
[{gender=male, grade=1, name=小明, id=1, photo_url=url1}, {gender=female, grade=2, name=小红, id=2, photo_url=url2}, {gender=male, grade=3, name=小李, id=3, photo_url=url3}, {gender=male, grade=2, name=小林, id=4, photo_url=url4}, {gender=female, grade=3, name=小婷, id=5, photo_url=url5}, {gender=male, grade=1, name=小智, id=6, photo_url=url6}]

五、JSON 返回  List>

如果希望查询返回 List> Map 的 key 是对象 property。需要借助 JSON 工具类，比如 cn.hutool.core.bean.BeanUtil#beanToMap

package com.study.student.service.impl;

import com.study.student.entity.Student;
import com.study.student.mapper.StudentMapper;
import com.study.student.service.StudentService;
import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
import com.baomidou.mybatisplus.extension.service.impl.ServiceImpl;
import org.springframework.stereotype.Service;

import cn.hutool.core.bean.BeanUtil;

import java.util.List;
import java.util.Map;

@Service
public class StudentServiceImpl extends ServiceImpl implements StudentService {

    @Override
    public List> getMapList(List ids) {
        List students = this.listByIds(ids);
        return students.stream().map(BeanUtil::beanToMap).collect(Collectors.toList());
    }
}
------------------------------------------------------------
@Test
public void test5() {
        List list = Arrays.asList(1,2,3,4,5,6);
        List> map = studentService.getMapList(list);
        System.out.println(map);
}
输出：
[{id=1, name=小明, gender=male, grade=1, photoUrl=url1}, {id=2, name=小红, gender=female, grade=2, photoUrl=url2}, {id=3, name=小李, gender=male, grade=3, photoUrl=url3}, {id=4, name=小林, gender=male, grade=2, photoUrl=url4}, {id=5, name=小婷, gender=female, grade=3, photoUrl=url5}, {id=6, name=小智, gender=male, grade=1, photoUrl=url6}]

六、MybatisPlus 返回 Map

Map 的 key 是数据库 column name

package com.study.student.service.impl;

import com.study.student.entity.Student;
import com.study.student.mapper.StudentMapper;
import com.study.student.service.StudentService;
import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
import com.baomidou.mybatisplus.extension.service.impl.ServiceImpl;
import org.springframework.stereotype.Service;

import java.util.List;
import java.util.Map;

@Service
public class StudentServiceImpl extends ServiceImpl implements StudentService {

    @Override
    public Map getMapById(Integer id) {
        LambdaQueryWrapper wrapper = new LambdaQueryWrapper<>();
        wrapper.eq(Student::getId, id);
        return this.getMap(wrapper);
    }
}
------------------------------------------------------------
@Test
public void test6() {
        Map map = studentService.getMapById(6);
        System.out.println(map);
    }
输出：
{gender=male, grade=1, name=小智, id=6, photo_url=url6}

 七、JSON 返回 Map

Map 的 key 是对象 property

package com.study.student.service.impl;

import com.study.student.entity.Student;
import com.study.student.mapper.StudentMapper;
import com.study.student.service.StudentService;
import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
import com.baomidou.mybatisplus.extension.service.impl.ServiceImpl;
import org.springframework.stereotype.Service;

import cn.hutool.core.bean.BeanUtil;

import java.util.List;
import java.util.Map;

@Service
public class StudentServiceImpl extends ServiceImpl implements StudentService {

    @Override
    public Map getMapById(Integer id) {
        Student student = this.getById(id);
        return BeanUtil.beanToMap(student);
    }
}
------------------------------------------------------------
@Test
public void test7() {
    Map map = studentService.getMapById(6);
    System.out.println(map);
}
输出：
{id=6, name=小智, gender=male, grade=1, photoUrl=url6}