MyBatis一对多demo！！！

最新推荐文章于 2022-07-27 10:35:30 发布

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最新推荐文章于 2022-07-27 10:35:30 发布

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文章标签： string integer class encoding interface join

本文链接：https://blog.youkuaiyun.com/zq9017197/article/details/6220431

本文深入探讨了MyBatis3中ID的概念及其重要性，特别是在一对多关系映射中的应用。通过具体示例，解释了如何正确设置ID以确保数据的唯一性和正确性，并避免查询结果集中出现重复数据的问题。

在学习MyBatis3的过程中，文档上面一直在强调一个id的东西！在做这个实验的时候，也因为没有理解清楚id含义而导致一对多的“多”中也只有一条数据。id和result的唯一不同是id表示的结果将是当比较对象实例时用到的标识属性。这帮助来改进整体表现，特别是缓存和嵌入结果映射。所以不同数据的id应该唯一区别，不然倒置数据结果集只有一条数据。

TeacherMapper.xml

<?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.nantian.mybatis.one2many.TeacherMapper"> <resultMap type="Teacher" id="teacher"> <id property="id" column="t_id" javaType="int" jdbcType="INTEGER"/> <result property="name" column="t_name" javaType="string" jdbcType="VARCHAR"/> <collection property="students" column="t_s_id" ofType="Student"> <id property="id" column="s_id" javaType="int" jdbcType="INTEGER"/> <result property="name" column="s_name" javaType="string" jdbcType="VARCHAR"/> </collection> </resultMap> <select id="one2many" parameterType="int" resultMap="teacher"> select t.t_id,t.t_name,s.t_s_id,s.s_id,s.s_name from teacher t join student s on t.t_id = s.t_s_id where t.t_id = #{id} </select> </mapper>

Teacher

package com.nantian.mybatis.one2one; import java.util.List; import com.nantian.mybatis.one2many.Student; public class Teacher { private int id; private String name; private List<Student> 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; } }

TeacherMapper

package com.nantian.mybatis.one2many; import com.nantian.mybatis.one2one.Teacher; public interface TeacherMapper { public Teacher one2many(int id); }

Student

package com.nantian.mybatis.one2many; public class Student { private int id; private String 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; } }

package com.nantian.mybatis.one2many; import java.util.List; import org.apache.ibatis.session.SqlSession; import com.nantian.mybatis.data.DBUtil; import com.nantian.mybatis.one2one.Teacher; public class T { public static void main(String[] args) { SqlSession sqlSession = DBUtil.getInstance().getSqlSessionFactory() .openSession(); try { TeacherMapper teacherMapper = sqlSession .getMapper(TeacherMapper.class); Teacher teacher = teacherMapper.one2many(100); List<Student> students = teacher.getStudents(); System.out.println(teacher.getId()); System.out.println(teacher.getName()); for (Student student : students) { System.out.println(student.getId()); System.out.println(student.getName()); } } finally { sqlSession.close(); } } }