Mybatis框架中实现双向一对多关系映射

学习过Hibernate框架的伙伴们很容易就能简单的配置各种映射关系(Hibernate框架的映射关系在我的blogs中也有详细的讲解)，但是在Mybatis框架中我们又如何去实现

一对多的关系映射呢？ 其实很简单

首先我们照常先准备前期的环境(具体解释请  参考初识Mybatis进行增、删、改、查 blogs )这里我就直接上代码了

主配置文件：Configuration.xml

<?xml version="1.0" encoding="UTF-8" ?>

<!DOCTYPE configuration
    PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
    "http://mybatis.org/dtd/mybatis-3-config.dtd">

<configuration>
  <environments default="development">
    <environment id="development">
      <transactionManager type="JDBC">
        <property name="" value=""/>
      </transactionManager>
      <dataSource type="UNPOOLED">
        <property name="driver" value="oracle.jdbc.OracleDriver"/>
        <property name="url" value="jdbc:oracle:thin:@localhost:1521:orcl"/>
        <property name="username" value="practice"/>
        <property name="password" value="123"/>
      </dataSource>
    </environment>
  </environments>
  
   <mappers>
    <mapper resource="config/Student.xml"/>
    <mapper resource="config/Grade.xml"/>
  </mappers> 

</configuration>

背景：学生和班级是一个典型的一对多的关系，一个班级可以对应着多个学生，所以我们随即创建了学生对象和班级对象

学生类：Student

package entity;
/*
 * 学生类
 * */
public class Student {
    //学生编号
    private Integer sid;
    //学生名称
    private String sname;
    //学生性别
    private String sex;
    
    public Student() {
    }
    public Student(String sname, String sex) {
        this.sname = sname;
        this.sex = sex;
    }
    public Integer getSid() {
        return sid;
    }
    public void setSid(Integer sid) {
        this.sid = sid;
    }
    public String getSname() {
        return sname;
    }
    public void setSname(String sname) {
        this.sname = sname;
    }
    public String getSex() {
        return sex;
    }
    public void setSex(String sex) {
        this.sex = sex;
    }
    
}

班级类：Grade

package entity;

import java.util.HashSet;
import java.util.Set;

/*
 * 班级类
 * */
public class Grade {
    //班级编号
    private Integer gid;
    //班级名称
    private String gname;
    //班级描述
    private String gdesc;
    //班级下的学生信息
    private Set<Student> stus=new HashSet<Student>();
    
    public Set<Student> getStus() {
        return stus;
    }
    public void setStus(Set<Student> stus) {
        this.stus = stus;
    }
    public Grade() {
    }
    public Grade(Integer gid, String gname, String gdesc) {
        this.gid = gid;
        this.gname = gname;
        this.gdesc = gdesc;
    }
    public Integer getGid() {
        return gid;
    }
    public void setGid(Integer gid) {
        this.gid = gid;
    }
    public String getGname() {
        return gname;
    }
    public void setGname(String gname) {
        this.gname = gname;
    }
    public String getGdesc() {
        return gdesc;
    }
    public void setGdesc(String gdesc) {
        this.gdesc = gdesc;
    }
    
}

实体类准备完了的话，我们就可以开始看配置文件了，也是最关键的一部分

首先讲简单点的学生实体类对应的配置文件

Student.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="Student">

  <resultMap type="entity.Student" id="StudentResult">
    <id column="sid" jdbcType="INTEGER" property="sid"/>
    <result column="sname" jdbcType="VARCHAR" property="sname"/>
    <result column="sex" jdbcType="VARCHAR" property="sex"/>
  </resultMap>

</mapper>

然后就是最关键的班级实体的配置文件了

Grade.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="Grade">

  <resultMap type="entity.Grade" id="GradeResult">
    <id column="gid" jdbcType="INTEGER" property="gid"/>
    <result column="gname" jdbcType="VARCHAR" property="gname"/>
    <result column="gdesc" jdbcType="VARCHAR" property="gdesc"/>
    <!-- 一对多关系 -->
    <collection property="stus" resultMap="Student.StudentResult"></collection>
  </resultMap>
     
    <!-- 查询所有信息 -->
    <select id="selectAllInfo" resultMap="GradeResult">
        <!-- select sid,sname,sex,g.gid,gname,gdesc from Student s,Grade g where s.gid=g.gid -->
        select sid,sname,sex,g.gid,gname,gdesc from Student s left join Grade g on s.gid=g.gid
    </select>
</mapper>

以上就是对配置文件的解释了

接下来我们就可以进行一道测试了

/*
     * 1.1 查询所有的班级和班级下的所有学生(一对多)
     * */
    @Test
    public void selectAllStu() throws Exception{
        //通过配置文件获取到数据库连接信息
        Reader reader = Resources.getResourceAsReader("config/Configuration.xml");
        //通过配置信息构建一个SessionFactory工厂
        SqlSessionFactory sqlsessionfactory=new SqlSessionFactoryBuilder().build(reader);
        //通过SessionFaction打开一个回话通道
        SqlSession session = sqlsessionfactory.openSession();
        /*SqlSession session =MybatisUtil.getSession();*/
        //调用配置文件中的sql语句
        List<Grade> list = session.selectList("Grade.selectAllInfo");
        //遍历查询出来的结果
        for (Grade grade : list) {
            System.out.println("班级："+grade.getGname());
            for (Student stu : grade.getStus()) {
                System.out.println("学生："+stu.getSname());
            }
        }
        
        session.close();
    }
    

执行后，查询出来的结果是

 

以上是第一种一对多关系映射的方式，下面是第二种一对多映射的方法，其他的所有步骤和上面的都是一样的只有相对应的配置文件不同，所以我就只贴小配置了

Grade.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="Grade">

  <resultMap type="entity.Grade" id="GradeResult">
    <id column="gid" jdbcType="INTEGER" property="gid"/>
    <result column="gname" jdbcType="VARCHAR" property="gname"/>
    <result column="gdesc" jdbcType="VARCHAR" property="gdesc"/>
    <!-- 一对多关系 -->
    <!-- <collection property="stus" resultMap="Student.StudentResult"></collection>  -->
    <collection property="stus" javaType="entity.Student">
        <id property="sid" column="sid"/>
        <result property="sname" column="sname"/>
        <result property="sex" column="sex"/>
    </collection>
  </resultMap>
     
    <!-- 查询所有信息 -->
    <select id="selectAllInfo" resultMap="GradeResult">
        <!-- select sid,sname,sex,g.gid,gname,gdesc from Student s,Grade g where s.gid=g.gid -->
        select sid,sname,sex,g.gid,gname,gdesc from Student s left join Grade g on s.gid=g.gid
    </select>
    
    <!-- 新增班级并同时新增班级下的学生 -->
    <!--useGeneratedKeys=true 表明采用主键生成策略
        keyProperty="gid"  表明将生成的主键添加到parameterType类中的那个属性值中去
      -->
    <!-- <insert id="" useGeneratedKeys="true" keyProperty="gid" parameterType="entity.Grade">
        
    </insert> -->
</mapper>

接下来就可以在多的一方配置一的关联关系了

Student.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="Student">

  <resultMap type="entity.Student" id="StudentResult">
    <id column="sid" jdbcType="INTEGER" property="sid"/>
    <result column="sname" jdbcType="VARCHAR" property="sname"/>
    <result column="sex" jdbcType="VARCHAR" property="sex"/>
    <!-- 多对一 -->
   <!--  <association property="grade" resultMap="Grade.GradeResult"></association> -->
    <association property="grade" javaType="entity.Grade">
        <id property="gid" column="gid"/>
        <result property="gname" column="gname"/>
        <result property="gdesc" column="gdesc"/>
    </association>
  </resultMap>
  
    <!-- 使用别名 -->
    <sql id="cloums">
        s.sid,s.sname,s.sex ,g.gid,g.gname,g.gdesc
    </sql>
    <!-- 多对一查询学生的班级 -->
    <select id="selectGradeByStu" resultMap="StudentResult">
        select <include refid="cloums"/> from Student s ,Grade g where s.gid=g.gid
    </select>
    
    <!-- 简单查询所有信息 -->
     <select id="selectAllStu"  resultMap="StudentResult">
        select sid,sname,sex,gid from Student 
    </select> 
    
    <!--动态拼接Sql  -->
     <select id="selectAllStuByWhere" parameterType="entity.Student"  resultMap="StudentResult">
        select sid,sname,sex,gid from Student where 1=1
        <if test="sname!=null and !&quot;&quot;.equals(sname.trim())">
            <!-- and sname like '%'|| #{sname}|| '%' --> <!-- 模糊查询 -->
            and sname like '%${sname}%'<!-- 模糊查询 -->
            <!-- and sname = #{sname} -->
        </if>
     </select>
     
     <!-- 新增学生信息 -->
     <insert id="InsertStuInfo" parameterType="entity.Student" >
         insert into Student values(SEQ_NUM.Nextval,#{sname},#{sex},1)
     </insert>
     
     <!-- 删除学生信息 -->
     <insert id="DeleteStuBySid" parameterType="int">
         delete from Student where sid=#{sid}
     <!--或者是     delete from Student where sid=#{_parameter} -->
     </insert>
     
     <!-- 根据SID修改学生信息 -->
     <update id="UpdateStuBySid" parameterType="entity.Student" >
         update Student set sname=#{sname},sex=#{sex} where sid=#{sid}
     </update>
     
     
</mapper>

这样就已经完成了Mybatis框架中的简单的双向一对多的配置了，怎么样，是不是没有想象中的难呢？希望对大家有点帮助吧！