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
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public class Card implements Serializable{ private Integer id; private String code; //省略set和get方法. } |
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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
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package com.glj.mapper; import com.glj.poji.Card; public interface CardMapper { Card selectCardById(Integer id); } |
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<?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> |
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package com.glj.mapper; import com.glj.poji.Person; public interface PersonMapper { Person selectPersonById(Integer id); } |
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<?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
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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 } |
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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 } |
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<?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> |
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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
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<?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> |
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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
原文地址：https://www.cnblogs.com/pxzbky/p/10050257.html