Mybatis中collection和association的使用区别详解
这篇文章主要介绍了Mybatis中collection和association的使用区别详解，小编觉得挺不错的，现在分享给大家，也给大家做个参考。一起跟随小编过来看看吧最近一直把collection和association弄混，所以为了增强自己的记忆，就撸一个关系出来算是总结罢了
1. 关联-association
2. 集合-collection
比如同时有user.java和card.java两个类
user.java如下:
public class user{
private card card_one;
private list<card> card_many;
}
在映射card_one属性时用association标签, 映射card_many时用collection标签.
所以association是用于一对一和多对一，而collection是用于一对多的关系
下面就用一些例子解释下吧
association-一对一
人和身份证的关系
下面是pojo
public class card implements serializable{
private integer id;
private string code;
//省略set和get方法.
}
public class person implements serializable{
private integer id;
private string name;
private string sex;
private integer age;
//人和身份证是一对一的关系
private card card;
//省略set/get方法.
}
下面是mapper和实现的接口
package com.glj.mapper;
import com.glj.poji.card;
public interface cardmapper {
card selectcardbyid(integer id);
}
<?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.glj.mapper.cardmapper">
<select id="selectcardbyid" parametertype="int" resulttype="com.glj.poji.card">
select * from tb_card where id = #{id}
</select>
</mapper>
package com.glj.mapper;
import com.glj.poji.person;
public interface personmapper {
person selectpersonbyid(integer id);
}
<?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.glj.mapper.personmapper">
<resultmap type="com.glj.poji.person" id="personmapper">
<id property="id" column="id"/>
<result property="name" column="name"/>
<result property="sex" column="sex"/>
<result property="age" column="age"/>
<association property="card" column="card_id"
select="com.glj.mapper.cardmapper.selectcardbyid"
javatype="com.glj.poji.card">
</association>
</resultmap>
<select id="selectpersonbyid" parametertype="int" resultmap="personmapper">
select * from tb_person where id = #{id}
</select>
</mapper>
personmapper.xml 还使用association的分步查询。
同理多对一，也是一样
只要那个pojo出现private card card_one;
即使用association
collection 一对多和association的多对一关系
学生和班级的一对多的例子
pojo类
package com.glj.pojo;
import java.io.serializable;
import java.util.list;
public class clazz implements serializable{
private integer id;
private string code;
private string name;
//班级与学生是一对多的关系
private list<student> students;
//省略set/get方法
}
package com.glj.pojo;
import java.io.serializable;
public class student implements serializable {
private integer id;
private string name;
private string sex;
private integer age;
//学生与班级是多对一的关系
private clazz clazz;
//省略set/get方法
}
<?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.glj.mapper.clazzmapper">
<select id="selectclazzbyid" parametertype="int" resultmap="clazzresultmap">
select * from tb_clazz where id = #{id}
</select>
<resultmap type="com.glj.pojo.clazz" id="clazzresultmap">
<id property="id" column="id"/>
<result property="code" column="code"/>
<result property="name" column="name"/>
<!-- property: 指的是集合属性的值, oftype：指的是集合中元素的类型 -->
<collection property="students" oftype="com.glj.pojo.student"
column="id" javatype="arraylist"
fetchtype="lazy" select="com.glj.mapper.studentmapper.selectstudentbyclazzid">
<id property="id" column="id"/>
<result property="name" column="name"/>
<result property="sex" column="sex"/>
<result property="age" column="age"/>
</collection>
</resultmap>
</mapper>
package com.glj.mapper;
import com.glj.pojo.clazz;
public interface clazzmapper {
clazz selectclazzbyid(integer id);
}
clazzmapper使用到了集合-collection 即为一对多，一个班级面对多个学生
<?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.glj.mapper.studentmapper">
<select id="selectstudentbyid" parametertype="int" resultmap="studentresultmap">
select * from tb_clazz c,tb_student s where c.id = s.id and s.id = #{id}
</select>
<select id="selectstudentbyclazzid" parametertype="int" resultmap="studentresultmap">
select * from tb_student where clazz_id = #{id}
</select>
<resultmap type="com.glj.pojo.student" id="studentresultmap">
<id property="id" column="id"/>
<result property="name" column="name"/>
<result property="sex" column="sex"/>
<result property="age" column="age"/>
<association property="clazz" javatype="com.glj.pojo.clazz">
<id property="id" column="id"/>
<result property="code" column="code"/>
<result property="name" column="name"/>
</association>
</resultmap>
</mapper>
package com.glj.mapper;
import com.glj.pojo.student;
public interface studentmapper {
student selectstudentbyid(integer id);
}
studentmapper则是与班级为多对一关系，所以使用了关联-association
嗯，希望我以后又不记得二者的关系时,能感谢现在总结的自己
附上一张mybatis的类型别名图
以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持CodeAE代码之家。
原文链接：http://199604.com/709
http://www.zzvips.com/article/171005.html
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