mybatis一篇文章搞懂

mybatis核心流程

• 每一个基于mybatis的应用都是建议一个sqlSessionFactory的实例为核心
• sqlSessionFactory的实例可以通过sqlSessionFactoryBuilder获得
• sqlSessionFactoryBuilder可以通过xml配置文件或预先配置好的configuration实例来构建出sqlSessionFactory实例
• 工厂设计模式里面需要获取sqlSession里面提供了在数据库执行sql命令所需的所有方法

使用流程

• 创建mybatis-config.xml配置文件
• 创建xxxMapper.xml配置文件
• 创建sqlSessionFactory创建sqlSession对象
• 用sqlSession执行crud
  例子:
  1.创建mybatis-config.xml配置文件

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"/>
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.cj.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://127.0.0.1:3306/test?useUnicode=true&characterEncoding=utf-8&useSSL=false"/>
                <property name="username" value="root"/>
                <property name="password" value="123"/>
            dataSource>
        environment>
    environments>

    <mappers>
        <mapper resource="mapper/VideoMapper.xml"/>
    mappers>
configuration>

2.创建mapper

<mapper namespace="net.xdclass.online_class.dao.VideoMapper">
mapper>

3.通过创建sqlSessionFactory来创建sqlSession对象从而操作写在mapper方法

String resource = "config/mybatis-config.xml";
// 读取配置文件
InputStream inputStream = Resources.getResourceAsStream(resource);
// 构建Session工厂
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
// 获取sqlSession
try(SqlSession sqlSession = sqlSessionFactory.openSession()) {	
	VideoMapper videoMapper = sqlSession.getMapper(Video.Class);
	// mapper.xml中的sql方法
	Video video = videoMapper.selectById();
}

1.简单查询

1.1 Mapper接口

// @Param(“别名”) mapper.xml中用
 Video selectById(@Param("video_id") int videoId);

1.2 video.xml

<!--
namespace:命名空间，一般需要保持全局唯一，最好是和dao的java的接口的相对位置保持一致，可以映射sql语句 到对应的方法名称和参数，返回类型。
mybatis是使用接口动态代理
-->
<mapper namespace ="xx.xx.dao.Mapper">
  <!--
  statement sql
  id:当前mapper下需要唯一,直接为方法名 selectById,
  resultType sql查询结果集的封装 即表映射的实体类
  -->
  <select id ="selectById" resultType="xx.xx.xx.po"
  select * from video where id = #{video_id}
  </select>
</mapper>

1.3 mybatis配置驼峰字段映射java对象和数据库字段

• 利用别名
• mybatis自带配置(mybatis-config.xml顶部，有顺序性 properties在第一个)

<! -- 下划线自动映射驼峰字段 -->
<settings>
 <setting name ="mapUnderscoreToCamelCase" value="true"/>
setting>

2 Mybatis3.X复杂Sql查询

Mybatis3.X的sql语句返回结果有两种

• resultType:查询出的字段在对应的po中必须和它有相同的字段对应,或者基本数据类型，适合简单查询
• resultMap:需要自定义字段或者多表查询，一对多等关系，比resultType更强大，适合复杂查询

2.1 单表简单一对一resultMap

<resultMap id ="VideoOrderResultMap" type ="Video">

<id column = "id" property = "id" jdbcType ="INTERGER"/>
<result column="video_title" property="title" jdbcType ="VARCHAR"/>
<result column="summary" property="summary"  jdbcType="VARCHAR" />
<result column="cover_img"  property="coverImg"  jdbcType="VARCHAR" />
resultMap>
<select id = "selectBaseFieldByIdWithResultMap" resultMap ="VideoResultMap">
	select id,title as video_title ,summary,cover_id from video where id = #{vodeo_id}
select>

2.2 单表复杂对象一对一resultMap结果映射之association(1对1连表)

• association:映射到POJO的某个复杂类型属性，比如订单order包含user对象

 <resultMap id="VideoOrderResultMap" type="VideoOrder">
        <id column="id" property="id"/>

        <result column="user_id" property="userId"/>
        <result column="out_trade_no" property="outTradeNo"/>
        <result column="create_time" property="createTime"/>
        <result column="state" property="state"/>
        <result column="total_fee" property="totalFee"/>
        <result column="video_id" property="videoId"/>
        <result column="video_title" property="videoTitle"/>
        <result column="video_img" property="videoImg"/>


        
        <association property="user" javaType="User">
            <id property="id"  column="user_id"/>
            <result property="name" column="name"/>
            <result property="headImg" column="head_img"/>
            <result property="createTime" column="create_time"/>
            <result property="phone" column="phone"/>
        association>

    resultMap>

    
    <select id="queryVideoOrderList" resultMap="VideoOrderResultMap">

        select

         o.id id,
         o.user_id ,
         o.out_trade_no,
         o.create_time,
         o.state,
         o.total_fee,
         o.video_id,
         o.video_title,
         o.video_img,
         u.name,
         u.head_img,
         u.create_time,
         u.phone
         from video_order o left join user u on o.user_id = u.id

    select>

代码:

// resultmap association关联查询
VideoOrderMapper videoOrderMapper = sqlSession.getMapper(VideoOrderMapper.class);
List<VideoOrder> videoOrderList = videoOrderMapper.queryVideoOrderList();
System.out.println(videoOrderList.toString());

2.3 ResultMap复杂对象一对多查询结果映射之collection

<resultMap id="UserOrderResultMap" type="User">

        <id property="id"  column="id"/>
        <result property="name" column="name"/>
        <result property="headImg" column="head_img"/>
        <result property="createTime" column="create_time"/>
        <result property="phone" column="phone"/>

        
        <collection property="videoOrderList" ofType="VideoOrder">

            
            <id column="order_id" property="id"/>
            <result column="user_id" property="userId"/>
            <result column="out_trade_no" property="outTradeNo"/>
            <result column="create_time" property="createTime"/>
            <result column="state" property="state"/>
            <result column="total_fee" property="totalFee"/>
            <result column="video_id" property="videoId"/>
            <result column="video_title" property="videoTitle"/>
            <result column="video_img" property="videoImg"/>
        collection>
    resultMap>

    <select id="queryUserOrder" resultMap="UserOrderResultMap">
        select
        u.id,
        u.name,
        u.head_img,
        u.create_time,
        u.phone,
        o.id order_id,
        o.out_trade_no,
        o.user_id,
        o.create_time,
        o.state,
        o.total_fee,
        o.video_id,
        o.video_title,
        o.video_img
        from user u left join video_order o on u.id = o.user_id

    select>

// resultmap association关联查询
VideoOrderMapper videoOrderMapper = sqlSession.getMapper(VideoOrderMapper.class);

//resultmap collection测试

List<User> userList = videoOrderMapper.queryUserOrder();

System.out.println(userList.toString());

2.4 复杂ResultMap复杂对象查询总结

<resultMap id="唯一的标识" type="映射的pojo对象">
  <id column="表的主键字段,或查询语句中的别名字段" jdbcType="字段类型" property="映射pojo对象的主键属性" />
  <result column="表的一个字段" jdbcType="字段类型" property="映射到pojo对象的一个属性"/>

  <association property="pojo的一个对象属性" javaType="pojo关联的pojo对象">
    <id column="关联pojo对象对应表的主键字段" jdbcType="字段类型" property="关联pojo对象的属性"/>
    <result  column="表的字段" jdbcType="字段类型" property="关联pojo对象的属性"/>
  association>

  
  <collection property="pojo的集合属性名称" ofType="集合中单个的pojo对象类型">
    <id column="集合中pojo对象对应在表的主键字段" jdbcType="字段类型" property="集合中pojo对象的主键属性" />
    <result column="任意表的字段" jdbcType="字段类型" property="集合中的pojo对象的属性" />  
  collection>
resultMap>

2.5 例子：三表联查

<resultMap id="VideoDetailResultMap" type="Video">

        <id column="id" jdbcType="INTEGER" property="id"/>

        <result column="title" jdbcType="VARCHAR" property="title"/>
        <result column="summary" jdbcType="VARCHAR" property="summary"/>
        <result column="cover_img" jdbcType="VARCHAR" property="coverImg"/>
        <result column="price" jdbcType="INTEGER" property="price"/>
        <result column="point" jdbcType="DOUBLE" property="point"/>
        <result column="create_time" jdbcType="TIMESTAMP" property="createTime"/>


        <collection property="chapterList" ofType="Chapter">
            <id column="chapter_id" jdbcType="INTEGER" property="id"/>
            <result column="chapter_title" jdbcType="VARCHAR" property="title"/>
            <result column="ordered" jdbcType="INTEGER" property="ordered"/>
            <result column="chapter_create_time" jdbcType="TIMESTAMP" property="createTime"/>

            <collection property="episodeList" ofType="Episode">
                <id column="episode_id" jdbcType="INTEGER" property="id"/>
                <result column="num" jdbcType="INTEGER" property="num"/>
                <result column="episode_title" jdbcType="VARCHAR" property="title"/>
                <result column="episode_ordered" jdbcType="INTEGER" property="ordered"/>
                <result column="play_url" jdbcType="VARCHAR" property="playUrl"/>
                <result column="free" jdbcType="INTEGER" property="free"/>
                <result column="episode_create_time" jdbcType="TIMESTAMP" property="createTime"/>
            collection>

        collection>
        
resultMap>


<select id="findDetailById" resultMap="VideoDetailResultMap">

        select
        v.id, v.title,v.summary,v.cover_img,v.price,v.point,v.create_time,
        c.id as chapter_id, c.title as chapter_title, c.ordered,c.create_time as chapter_create_time,
        e.id as episode_id,e.num, e.title as episode_title,e.ordered as episode_ordered,e.play_url,e.free,e.create_time as episode_create_time

        from video v

        left join chapter c on v.id=c.video_id

        left join episode e on c.id= e.chapter_id

        where v.id = #{video_id}
        order by c.ordered,e.num asc


select>

接口：

Video findDetailById(@Param("video_id" int videoId);

3mybatis的缓存和懒加载

1.一级缓存是sqlSession级别，同一个sqlSession中执行相同的sql第一次回查数据库，并写入缓存，第二次直接从缓存中取，两此查询中间发生了增加，删除，修改，则缓存失效。
2.二级缓存是namespace级别，第一次调用某个namespace下的sql查询数据，查询数据回放在该mapper对应的二级缓存中，第二次调用同一个namespace的mapper映射文件，会去对应的二级缓存中取结果。失效策略是执行同个namespace下的mapper映射文件时，出现crud时，并执行了commit操作胡，会清空该二级缓存，实现二级缓存的时候，，po是可序列化的，建议实现serializabale.默认是最近最少实现lru算法。
懒加载是先从单表查询，需要时再从关联表中查询，resultMap的association和collection有延迟加载的功能