Mybatis中collection和association的使用区别详解

public class user{

private card card_one;

private list<card> card_many;

}

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.

}

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>

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);

}

<?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);

}