Mybatis一对多

0.前期准备

数据库配置：

CREATE TABLE tb_clazz
(
  id   INT AUTO_INCREMENT
    PRIMARY KEY,
  CODE VARCHAR(18) NULL,
  name VARCHAR(18) NULL,
  CONSTRAINT TB_CLAZZ_id_uindex
  UNIQUE (id)
)
CREATE TABLE tb_student
(
  id       INT AUTO_INCREMENT
    PRIMARY KEY,
  NAME     VARCHAR(18) NULL,
  sex      CHAR(3)     NULL,
  age      INT         NULL,
  clazz_id INT         NULL,
  CONSTRAINT TB_STUDENT_id_uindex
  UNIQUE (id),
  CONSTRAINT TB_STUDENT_tb_clazz_id_fk
  FOREIGN KEY (clazz_id) REFERENCES tb_clazz (id)
)

CREATE INDEX TB_STUDENT_tb_clazz_id_fk
  ON tb_student (clazz_id);

一个clazz对应多个student。

log4j.properties需要进行配置：

log4j.rootLogger=ERROR, stdout

#log4j.logger.org.fkit.mapper.UserMapper=DEBUG
log4j.logger.org.fkit.mapper=DEBUG
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%5p [%t] - %m%n

在mybatis-config.xml中配置一下mapper和懒加载：

<setting name="lazyLoadingEnabled" value="true"/>
<setting name="aggressiveLazyLoading" value="false"/>

配置pojo类：

//Student.java
package org.fkit.domain;

public class Student {
    private Integer id;
    private String name;
    private String sex;
    private Integer age;
    private Clazz clazz;
//getter,setter

//Clazz.java
package org.fkit.domain;

import java.util.List;

public class Clazz {
    private Integer id;
    private String code;
    private String name;
    private List<Student> students;
//getter,setter

1.配置StudentMapper和ClazzMapper

//StudentMapper.xml

<?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="org.fkit.mapper.StudentMapper">
    <select id="selectStudentById" parameterType="int" resultMap="studentResultMap">
        SELECT * from tb_clazz c,tb_student s
        where c.id = s.clazz_id and s.id = #{id}
    </select>

    <select id="selectStudentByClazzId" parameterType="int" resultMap="studentResultMap">
        select * from tb_student where clazz_id = #{id}
    </select>
    <resultMap id="studentResultMap" type="org.fkit.domain.Student">
        <id property="id" column="id"/>
        <result property="name" column="name"/>
        <result property="sex" column="sex"/>
        <result property="age" column="age"/>
        <association property="clazz" javaType="org.fkit.domain.Clazz">
            <id property="id" column="id"/>
            <result property="code" column="code"/>
            <result property="name" column="name"/>
        </association>
    </resultMap>

</mapper>

//StudentMapper.java
package org.fkit.mapper;

import org.fkit.domain.Student;

public interface StudentMapper {
    Student selectStudentById(Integer id);

    Student selectStudentByClazzId(Integer id);
}

//ClazzMapper.xml
<?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="org.fkit.mapper.ClazzMapper">
    <select id="selectClazzById" parameterType="int" resultMap="clazzResultMap">
        SELECT * FROM tb_clazz where id = #{id}
    </select>
    
    <resultMap id="clazzResultMap" type="org.fkit.domain.Clazz">
        <id property="id" column="id"/>
        <result property="code" column="code"/>
        <result property="name" column="name"/>
        <collection property="students" javaType="ArrayList"
                    column="id" ofType="org.fkit.domain.Student"
                    select="org.fkit.mapper.StudentMapper.selectStudentByClazzId"
                    fetchType="lazy">
            <id property="studentId" column="id"/>
            <result property="name" column="name"/>
            <result property="sex" column="sex"/>
            <result property="age" column="age"/>
        </collection>
    </resultMap>
</mapper>

//ClazzMapper.java
package org.fkit.mapper;

import org.fkit.domain.Clazz;

public interface ClazzMapper {
    Clazz selectClazzById(Integer id);
}

测试代码：

//OneToMany.java
package org.fkit.test;

import org.apache.ibatis.session.SqlSession;

import org.fkit.domain.Clazz;
import org.fkit.factory.FKSqlSessionFactory;
import org.fkit.mapper.ClazzMapper;


public class OneToMany {
    public static void main(String args[])throws Exception {
        SqlSession sqlSession = FKSqlSessionFactory.getSqlSession();

        OneToMany oneToMany = new OneToMany();
        oneToMany.testSelectClazzById(sqlSession);
        sqlSession.commit();

        sqlSession.close();

    }

    public void testSelectClazzById(SqlSession sqlSession) {
        ClazzMapper clazzMapper = sqlSession.getMapper(ClazzMapper.class);
        Clazz clazz = clazzMapper.selectClazzById(1);
        System.out.println(clazz.getId() + " " + clazz.getCode());
        System.out.println(clazz.getStudents());
    }
}
//output:
DEBUG [main] - ==>  Preparing: SELECT * FROM tb_clazz where id = ? 
DEBUG [main] - ==> Parameters: 1(Integer)
DEBUG [main] - <==      Total: 1
1 j1601
DEBUG [main] - ==>  Preparing: select * from tb_student where clazz_id = ? 
DEBUG [main] - ==> Parameters: 1(Integer)
DEBUG [main] - <==      Total: 2
[Student{id=1, name='jack', sex='男', age=22, clazz=Clazz{id=1, code='null', name='jack', students=null}}, Student{id=2, name='rose', sex='女', age=18, clazz=Clazz{id=2, code='null', name='rose', students=null}}]

可以看出，懒加载会只加载需要使用的(PS:如果做接口，json传数据，则冷加载相当于失效，但是可以在pojo类上加上@JsonIgnoreProperties(“fieldname”)）。