Mybatis返回Map对象的实现
目录
- 一、场景介绍
- 二、MyBATis @MapKey 方式
- 三、Stream 返回 Map<Integer, List<Student>>
- 四、MybatisPlus www.devze.com返回 List<Map<String, Object>>
- 五、jsON 返回 List<Map<String, Object>>
- 六、MybatisPlus 返回 Map<String, Object>
- 七、JSON 返回 Map<String, Object>
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("gradephp")，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=http://www.devze.com"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=1HNrdzbraW, 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, Objec编程t>>
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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