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=""/>
</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 =
<if test="sname!=null and !"".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},)
</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！