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ResultMap中association和collection的区别

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我是靠谱客的博主 纯真睫毛膏，最近开发中收集的这篇文章主要介绍ResultMap中association和collection的区别，觉得挺不错的，现在分享给大家，希望可以做个参考。

概述

mybatis mapper配置文件结果集映射resultMap中collection属性(一对多关系结果集映射)和association属性(多对一关系结果集映射)理解：
collection的使用有两种resultMap和select，必须手动指定一种
association的使用和collection使用类似，只是right join需换成left join
rd_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 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.}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);}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><?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>?1234567package com.glj.mapper;  import com.glj.poji.Person;  public interface PersonMapper { Person selectPersonById(Integer id);}package com.glj.mapper;
 
import com.glj.poji.Person;
 
public interface PersonMapper {
 Person selectPersonById(Integer id);
}?12345678910111213141516171819<?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><?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associationcollection 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;
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}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
}?1234567891011121314151617181920212223<?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><?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>?1234567package com.glj.mapper; import com.glj.pojo.Clazz; public interface ClazzMapper { Clazz selectClazzById(Integer id);}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><?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>?1234567package com.glj.mapper; import com.glj.pojo.Student; public interface StudentMapper { Student selectStudentById(Integer id); }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

最后

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