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);
}

<?  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);
}

<?  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
}

<?  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);
}

<?  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); 
}