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

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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

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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