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?
文章目录
-
-
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- 1.数据库思想:连表查询
- 2.面向对象思想
-
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同样的,首先给出实体类,工具类,Mybatis核心配置文件的代码以便更好理解
Student.javaStudent实体类
package com.MLXH.pojo;
public class Student {
private int id;
private String name;
public Student() {
}
public Student(int id, String name) {
this.id = id;
this.name = name;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
@Override
public String toString() {
return "Student{" +
"id=" + id +
", name='" + name + '\'' +
'}';
}
}
TeacherTeacher实体类
package com.MLXH.pojo;
import java.util.List;
public class Teacher {
private int id;
private String name;
//一个老师对应对个学生
private List<Student> students;
public Teacher() {
}
public Teacher(int id, String name, List<Student> students) {
this.id = id;
this.name = name;
this.students = students;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public List<Student> getStudents() {
return students;
}
public void setStudents(List<Student> students) {
this.students = students;
}
@Override
public String toString() {
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
public class MyBatisUtils {
//类变量不需要设置默认值；
private static 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 = new SqlSessionFactoryBuilder().build(inputStream);
} catch (IOException e) {
e.printStackTrace();
}
}
//设置SqlSessionFactory公共的方法
public static SqlSessionFactory getSqlSessionFactory(){
return sqlSessionFactory;
}
//获得一个带事务自动提交功能的SqlSession公共的方法
public static SqlSession getSqlSession(){
//自动提交事务
return sqlSessionFactory.openSession(true);
}
}
Mybatis核心配置文件:mybatis-config.xml
<configuration>
<properties resource="database.properties"/>
<settings>
<setting name="logImpl" value="LOG4J"/>
settings>
<typeAliases>
<package name="com.MLXH.pojo"/>
typeAliases>
<environments default="development">
<environment id="development">
<transactionManager type="JDBC"/>
<dataSource type="POOLED">
<property name="driver" value="${driver}"/>
<property name="url" value="${url}"/>
<property name="username" value="${username}"/>
<property name="password" value="${password}"/>
dataSource>
environment>
environments>
<mappers>
<mapper resource="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;
public interface TeacherDao {
//获得一个老师下的所有学生信息； 老师是包含学生的集合；
Teacher getTeacher(int id);
Teacher getTeacherTwo(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
<mapper namespace="com.MLXH.dao.TeacherDao">
<select id="getTeacherTwo" resultMap="TeacherStudent2">
select * from mybatis.teacher where id = #{id}
select>
<resultMap id="TeacherStudent2" type="Teacher">
<collection property="students" javaType="ArrayList" ofType="Student" column="id" select="T2"/>
resultMap>
<select id="T2" resultType="Student">
select * from mybatis.student where tid = #{id}
select>
mapper>
测试
@Test
public void getTeacher(){
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
<mapper namespace="com.MLXH.dao.TeacherDao">
<select id="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>
<resultMap id="TeacherStudent" type="Teacher">
<result property="id" column="id"/>
<result property="name" column="tname"/>
<collection property="students" ofType="Student">
<id property="id" column="sid"/>
<result property="name" column="sname"/>
collection>
resultMap>
mapper>
测试
@Test
public void getTeacherTwo(){
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…