Mybatis中一个对象包含多个其他对象的处理

对应上一篇博客中,多个学生有同一个班主任的查询,那么要是想查询一个班主任下的所有学生,这样一对多的情况该如何处理呢?

package com.MLXH.pojo;publicclassStudent{privateint id;private String name;publicStudent(){}publicStudent(int id, String name){this.id= id;this.name= name;}publicintgetId(){return id;}publicvoidsetId(int id){this.id= id;}public StringgetName(){return name;}publicvoidsetName(String name){this.name= name;}@Overridepublic StringtoString(){return"Student{"+"id="+ id+", name='"+ name+'\''+'}';}}

TeacherTeacher实体类

package com.MLXH.pojo;import java.util.List;publicclassTeacher{privateint id;private String name;//一个老师对应对个学生private List<Student> students;publicTeacher(){}publicTeacher(int id, String name, List<Student> students){this.id= id;this.name= name;this.students= students;}publicintgetId(){return id;}publicvoidsetId(int id){this.id= id;}public StringgetName(){return name;}publicvoidsetName(String name){this.name= name;}public List<Student>getStudents(){return students;}publicvoidsetStudents(List<Student> students){this.students= students;}@Overridepublic StringtoString(){return"Teacher{"+"id="+ id+", name='"+ name+'\''+", students="+ students+'}';}}

工具类:主要是为了更为方便的获得sqlSession对象并设置了自动提交事务

package com.MLXH.utils;import org.apache.ibatis.io.Resources;import org.apache.ibatis.session.SqlSession;import org.apache.ibatis.session.SqlSessionFactory;import org.apache.ibatis.session.SqlSessionFactoryBuilder;import java.io.IOException;import java.io.InputStream;//mybatis的工具类，重复的代码的提纯publicclassMyBatisUtils{//类变量不需要设置默认值；privatestatic SqlSessionFactory sqlSessionFactory;static{//在maven中，所有的资源文件一般都放在resources目录下，我们可以直接拿到。try{
            String resource="mybatis-config.xml";
            InputStream inputStream= Resources.getResourceAsStream(resource);
            sqlSessionFactory=newSqlSessionFactoryBuilder().build(inputStream);}catch(IOException e){
            e.printStackTrace();}}//设置SqlSessionFactory公共的方法publicstatic SqlSessionFactorygetSqlSessionFactory(){return sqlSessionFactory;}//获得一个带事务自动提交功能的SqlSession公共的方法publicstatic SqlSessiongetSqlSession(){//自动提交事务return sqlSessionFactory.openSession(true);}}

Mybatis核心配置文件:mybatis-config.xml

<?xml version="1.0" encoding="UTF-8" ?><!DOCTYPE configuration
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-config.dtd"><configuration><!--配置文件修改--><propertiesresource="database.properties"/><!--Mybatis设置--><settings><!--默认日志实现--><!--Log4j实现--><settingname="logImpl"value="LOG4J"/></settings><!--配置别名--><typeAliases><packagename="com.MLXH.pojo"/></typeAliases><environmentsdefault="development"><environmentid="development"><transactionManagertype="JDBC"/><dataSourcetype="POOLED"><propertyname="driver"value="${driver}"/><propertyname="url"value="${url}"/><propertyname="username"value="${username}"/><propertyname="password"value="${password}"/></dataSource></environment></environments><mappers><!--class对应的是一个接口类--><!--resource对应的是一个接口类的映射文件--><mapperresource="com/MLXH/dao/TeacherMapper.xml"/></mappers></configuration>

database.properties配置文件:数据库所需数据

driver = com.mysql.jdbc.Driver
url = jdbc:mysql://localhost:3306/mybatis?useSSL=true&useUnicode=true&characterEncoding=utf-8
username = root
password = 123456

TeacherDao接口

package com.MLXH.dao;import com.MLXH.pojo.Teacher;publicinterfaceTeacherDao{//获得一个老师下的所有学生信息； 老师是包含学生的集合；
    TeachergetTeacher(int id);

    TeachergetTeacherTwo(int id);}

针对于TeacherDao接口的实现mapper文件:我们使用如下的方式来进行查询

1.数据库思想:连表查询

<?xml version="1.0" encoding="UTF-8" ?><!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd"><mappernamespace="com.MLXH.dao.TeacherDao"><!--数据库思想--><selectid="getTeacherTwo"resultMap="TeacherStudent2">
        select * from mybatis.teacher where id = #{id}</select><resultMapid="TeacherStudent2"type="Teacher"><collectionproperty="students"javaType="ArrayList"ofType="Student"column="id"select="T2"/></resultMap><selectid="T2"resultType="Student">
        select * from mybatis.student where tid = #{id}</select></mapper>

测试

@TestpublicvoidgetTeacher(){
        SqlSession sqlSession= MyBatisUtils.getSqlSession();
        TeacherDao mapper= sqlSession.getMapper(TeacherDao.class);
        Teacher teacher= mapper.getTeacher(1);

        System.out.println(teacher.getName());
        System.out.println(teacher.getStudents());}

2.面向对象思想

<?xml version="1.0" encoding="UTF-8" ?><!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd"><mappernamespace="com.MLXH.dao.TeacherDao"><!--一对多的处理--><!--面向对象方式解决--><selectid="getTeacher"resultMap="TeacherStudent">
        select s.name sname,s.id sid, t.name tname
        from mybatis.student as s,mybatis.teacher as t
        where s.tid = t.id and t.id = #{id}</select><resultMapid="TeacherStudent"type="Teacher"><resultproperty="id"column="id"/><resultproperty="name"column="tname"/><collectionproperty="students"ofType="Student"><idproperty="id"column="sid"/><resultproperty="name"column="sname"/></collection></resultMap></mapper>

测试

@TestpublicvoidgetTeacherTwo(){
        SqlSession sqlSession= MyBatisUtils.getSqlSession();
        TeacherDao mapper= sqlSession.getMapper(TeacherDao.class);
        Teacher teacher= mapper.getTeacherTwo(1);

        System.out.println(teacher.getName());
        System.out.println(teacher.getStudents());}

结果:
总结:

• 一对多：collection 集合
• 两种解决方式：
  –面对对象的思想：关联对象
  –SQL语句思想：联表查询

多对一使用 association :关联
一对多使用 collection :集合
可以理解为:多对一,为多个对象关联一个对象,一对多为一个对象有一个集合,这个集合包含了多个对象,这些更加便于理解和记忆…