文章目录
summary：
MyBatis3中实现一对多的查询比较简单，可以自动完成。但插入操作要复杂一些，需要相关的DAO配合完成，这点不如Hibernate。
场景描述：
类：Mail和Attachment类
关系描述：一封邮件（Mail）可以有0个或多个附件（Attachment），附件（Attachment）仅对应一封邮件。
表格：mail表（主键：id_mail）和attachment表（外键：id_mail）。
POJO：
Mail.java
public class Mail implements Serializable {
private static final long serialVersionUID = 7427977743354005783L;
private Integer id;
private String sender;
private String subject;
private String content;
private String fromAddress;
...
getters and setters...
}
Attachment.java
public class Attachment implements Serializable {
private static final long serialVersionUID = -1863183546552222728L;
private String id;
private String mailId;
private String name;
private String relativePath;
...
getters and setters...
}
SqlMapConfig:
<?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>
<properties resource="test/properties/mysql.properties"></properties>
<typeAliases>
<typeAlias type="test.model.Mail" alias="Mail"/>
<typeAlias type="test.model.Attachment" alias="Attachment"/>
</typeAliases>
<environments default="development">
<environment id="development">
<transactionManager type="JDBC" />
<dataSource type="UNPOOLED">
<property name="driver" value="${db_driver}" />
<property name="url" value="${db_url}" />
<property name="username" value="${db_user}" />
<property name="password" value="${db_password}"/>
</dataSource>
</environment>
</environments>
<mappers>
<mapper resource="test/data/MailMapper.xml"/>
<mapper resource="test/data/AttachmentMapper.xml"/>
</mappers>
</configuration>
Mappers
MailMapper.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="test.data.MailMapper"> <cache /> <resultMap type="Mail" id="result_base"> <id property="id" column="id_mail" /> <result property="sender" column="sender"/> <result property="fromAddress" column="from_address" /> <result property="subject" column="subject"/> <result property="content" column="content"/> <result property="sendTime" column="send_time" /> .... </resultMap> <!--这里是关键，一对多映射的“魔法”几乎都在<collection>的配置里。select=...中"test.data.AttachmentMapper"对应于AttachmentMapper中
的namespace-->
<resultMap type="Mail" id="result" extends="result_base">
<collection property="attachments" javaType="ArrayList" column="id_mail" ofType="Attachment"
select="test.data.AttachmentMapper.selectByMailId"/>
</resultMap>
<insert id="insert" parameterType="Mail" useGeneratedKeys="true" keyProperty="id_note">
insert into note(sender, from_address, subject, content, send_time)
values(#{sender}, #{fromAddress}, #{subject}, #{content}, #{sendTime})
<selectKey keyProperty="id_mail" resultType="int">
select LAST_INSERT_ID()
</selectKey>
</insert>
<select id="selectById" parameterType="int" resultMap="result" >
select * from mail where id_mail = #{id}
</select>
<select id="selectAllMails" resultMap="result">
select * from note Note
</select>
<!--这里可以获得刚插入表格的id，为后面attachment的插入提供了mailId字段-->
<select id="selectLastId" resultType="int">
select LAST_INSERT_ID()
</select>
</mapper>
AttachmentMapper.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="test.data.AttachmentMapper">
<cache />
<resultMap type="Attachment" id="result">
<result property="id" column="id_accessory" />
<result property="name" column="name" />
<result property="relativePath" column="relative_path" />
<result property="mailId" column="id_mail" />
</resultMap>
<insert id="insert" parameterType="Attachment">
insert into attachments(id_mail, name, relative_path) values(#{mailId}, #{name}, #{relativePath})
</insert>
<!--MailMapper中的ResultMap调用这个方法来进行关联-->
<select id="selectByMailId" parameterType="int" resultMap="result">
select id, id_mail, name, relative_path
from attachments where id_note = #{id}
</select>
</mapper>
DAO
AttachmentDAO
public class AttachmentDAO {
private SqlSessionFactory sqlSessionFactory;
public AttachmentDAO(){
this.sqlSessionFactory = TestConnectionFactory.getSqlSessionFactory();
}
public void insert(Attachment attachment){
SqlSession session = sqlSessionFactory.openSession();
AttachmentMapper attachmentMapper = session.getMapper(AttachmentMapper.class);
try {
attachmentMapper.insert(attachment);
session.commit();
} finally {
session.close();
}
}
}
MailDAO
public class MailDAO {
private SqlSessionFactory sqlSessionFactory;
public MailDAO(){
sqlSessionFactory = TestConnectionFactory.getSqlSessionFactory();
}
public void insertMailOnly(Mail mail){
SqlSession session = sqlSessionFactory.openSession();
MailMapper mailMapper = session.getMapper(MailMapper.class);
try {
mailMapper.insert(mail);
session.commit();
} finally {
session.close();
}
}
//inset
public void insertMail(Mail mail){
SqlSession session = sqlSessionFactory.openSession();
MailMapper mailMapper = session.getMapper(MailMapper.class);
AttachmentMapper attachmentMapper = session.getMapper(AttachmentMapper.class);
try{
mailMapper.insert(mail);
//这里必须commit，再执行Attachment的插入操作。否则会导致null pointer异常
session.commit();
//获得最近插入到note表的id
int mailId = mailMapper.selectLastId();
for(Attachment attach : mail.getAttachments()){
attach.setMailId(String.valueOf(mailId));
attachmentMapper.insert(attach);
}
session.commit();
}finally{
session.close();
}
}
public ArrayList<Mail> selectAllMails(){
ArrayList<Mail> mailList = null;
SqlSession session = sqlSessionFactory.openSession();
MailMapper mailMapper = session.getMapper(MailMapper.class);
try {
mailList = mailMapper.selectAllMails();
session.commit();
} finally {
session.close();
}
return mailList;
}
public Mail selectMailById(int i){
Mail mail = null;
SqlSession session = sqlSessionFactory.openSession();
MailMapper mailMapper = session.getMapper(MailMapper.class);
try {
mail = mailMapper.selectById(i);
session.commit();
} finally {
session.close();
}
return mail;
}
public int selectLastId(){
int id = -1;
SqlSession session = sqlSessionFactory.openSession();
MailMapper mailMapper = session.getMapper(MailMapper.class);
try {
id = mailMapper.selectLastId();
session.commit();
} finally {
session.close();
}
return id;
}
}