Mybatis返回Map对象的实现_Java

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<integer, student>，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<integer, student> findmapbyids(list<integer> ids) {
        list<student> students = mapper.listbyids(ids);
        map<integer, student> map = students.stream().collect(collectors.tomap(student::getid, function.identity()));
        return map;
    }
}
---------------------------------------------------------------
@test
public void test() {
        list<integer> list = arrays.aslist(1,2,3);
        map<integer, student> 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<integer, student> findmapbyids(list<integer> ids);
}
-------------------------------------------------------------
<?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="com.study.student.mapper.studentmapper">
    
    <select id="findmapbyids" resulttype="com.study.student.entity.student">
        select *
        from student s
        <where>
            and s.id in
            <foreach collection="ids" item="id" open="(" separator="," close=")">
                #{id}
            </foreach>
        </where>
    </select>

</mapper>
-------------------------------------------------------------
@service
public class studentserviceimpl implements studentservice {

    @autowired
    private studentmapper mapper;

    @override
    public map<integer, student> findmapbyids(list<integer> ids) {
        /*
        list<student> students = mapper.listbyids(ids);
        map<integer, student> map = students.stream().collect(collectors.tomap(student::getid, function.identity()));
        return map;
        */
        return mapper.findmapbyids(ids);
    }
}
-------------------------------------------------------------
@test
public void test() {
        list<integer> list = arrays.aslist(1,2,3);
        map<integer, student> 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<student> listbyids(list<integer> ids);

    /**
     * @mapkey 的 value 是 grade 这种非唯一字段
     */
    @mapkey("grade")
    map<integer, student> findmapbyids(list<integer> 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<grade:integer, list<student>> 怎么办呢？

@test
public void test2() {
        list<integer> list = arrays.aslist(1,2,3,4,5,6);
        map<integer, student> 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<integer, list<student>>

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<student> listbyids(list<integer> ids);
}
-------------------------------------------------------------
<?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="com.study.student.mapper.studentmapper">
    
    <select id="listbyids" resulttype="cn.al.admin.entity.finance.student">
        select *
        from student s
        <where>
            and s.id in
            <foreach collection="ids" item="id" open="(" separator="," close=")">
                #{id}
            </foreach>
        </where>
    </select>

</mapper>
-------------------------------------------------------------
@service
public class studentserviceimpl implements studentservice {

    @autowired
    private studentmapper mapper;

    @override
    public map<integer, list<student>> getmapbyids(list<integer> ids) {
        list<student> students = mapper.listbyids(ids);
        return students.stream().collect(collectors.groupingby(student::getgrade));
    }
}
-------------------------------------------------------------
@test
public void test3() {
        list<integer> list = arrays.aslist(1,2,3,4,5,6);
        map<integer, list<student>> 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<map<string, object>>

如果希望查询返回 list<map<string, object>> 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<student> {
}
------------------------------------------------------------
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<studentmapper, student> implements studentservice {

    @override
    public list<map<string, object>> getmaplist(list<integer> ids) {
        lambdaquerywrapper<student> wrapper = new lambdaquerywrapper<>();
        wrapper.in(student::getid, ids);
        return this.getbasemapper().selectmaps(wrapper);
    }
}
------------------------------------------------------------
@test
public void test4() {
        list<integer> list = arrays.aslist(1,2,3,4,5,6);
        list<map<string, object>> 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<map<string, object>>

如果希望查询返回 list<map<string, object>> 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<studentmapper, student> implements studentservice {

    @override
    public list<map<string, object>> getmaplist(list<integer> ids) {
        list<student> students = this.listbyids(ids);
        return students.stream().map(beanutil::beantomap).collect(collectors.tolist());
    }
}
------------------------------------------------------------
@test
public void test5() {
        list<integer> list = arrays.aslist(1,2,3,4,5,6);
        list<map<string, object>> 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<string, object>

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<studentmapper, student> implements studentservice {

    @override
    public map<string, object> getmapbyid(integer id) {
        lambdaquerywrapper<student> wrapper = new lambdaquerywrapper<>();
        wrapper.eq(student::getid, id);
        return this.getmap(wrapper);
    }
}
------------------------------------------------------------
@test
public void test6() {
        map<string, object> 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<string, object>

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<studentmapper, student> implements studentservice {

    @override
    public map<string, object> getmapbyid(integer id) {
        student student = this.getbyid(id);
        return beanutil.beantomap(student);
    }
}
------------------------------------------------------------
@test
public void test7() {
    map<string, object> map = studentservice.getmapbyid(6);
    system.out.println(map);
}
输出：
{id=6, name=小智, gender=male, grade=1, photourl=url6}

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