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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