Mtbatis------basic application

MyBatis

Mybatis configuration file

template

<?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>
<!--
    mybatis 配置文件的主要的两个职责
        1. 配置数据源
        2. 管理映射文件
        3. 设置类型别名

-->
    <!-- 配置数据源 -->
    <properties resource="jdbc.properties"></properties>
    
    <settings>
        <!-- 解决 查询 返回 Map 字段 值为 null 不显示 字段的问题 -->
        <setting name="callSettersOnNulls" value="true"/>
        <!-- 解决驼峰命名和 数据库蛇形命名相互转换的配置 -->
        <setting name="mapUnderscoreToCamelCase" value="true"/>
    </settings>

    <!-- 设置类型别名 -->
    <typeAliases>
        <!-- <typeAlias type="com.subai.entity.User" alias="user" /> -->

        <!--
            如果 通过 package 设置别名，那么 别名的 名字 默认是 类名 （不区分大小写，推荐使用小写！）
        -->
        <package name="com.subai.entity" />
    </typeAliases>


    <plugins>
        <plugin interceptor="com.github.pagehelper.PageInterceptor">
            <!--合理化配置 分页的参数 ， 当页码小于等于0的时候，查询第一页， 当页码大于总页数的时候，查询最后一页 -->
            <property name="reasonable" value="true"/>
        </plugin>
    </plugins>


    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"></transactionManager>
            <dataSource type="POOLED">
                <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>
        <!--
            如果使用 package 来管理映射文件， 那么需要满足两个要求
                1.  映射文件 和 对应的 接口 名 保持一致
                2.  映射文件 和 接口 需要在 同一个包下

            如果 后期和 Spring 框架 进行整合， 就可以采用通配符的方式进行一次性的配置
        -->
        <package name="com.subai.mapper" />

        <!--  该方式只能应用于注解的开发 -->
        <!--<mapper class="com.subai.mapper.RoleMapper" />-->
        
        <!--<mapper resource="com/subai/mapper/UserMapper.xml" />-->
        <!--<mapper resource="com/subai/mapper/GoodsMapper.xml" />-->
        <!--<mapper resource="com/subai/mapper/OrderMapper.xml" />-->
        <!--<mapper resource="com/subai/mapper/OrderGoodsMapper.xml" />-->
    </mappers>

</configuration>

XML mapper

• sql : a reusable block of statements that can be referenced by other statements
• insert : mapping insert statement
• update : map update statement
• delete : map delete statement
• select : mapping query statement

sql

• The SQL fragment is defined through the sql tag, and the corresponding SQL fragment is introduced through the include tag

<!--  通过 sql 标签定义 -->
<sql id="columns">
    id,  username, password, create_time as createTime
</sql>

<!--  通过 include 标签 引入 -->
<include refid="columns" />

insert

• The first type inserts and gets the primary key

<insert id="save" parameterType="user" useGeneratedKeys="true" keyProperty="id">
    insert into tb_user(username, password, create_time) values (#{username}, md5(#{password}), now())
</insert>

• The second inserts and gets the primary key

 <insert id="save">
    insert into tb_goods(goods_name, goods_price, create_time) values
         (#{goodsName}, #{goodsPrice}, now())

    <selectKey keyProperty="id" resultType="long" order="AFTER">
        select last_insert_id()
    </selectKey>
</insert>

update

<update id="update" parameterType="user">
    update tb_user set
    username = #{username} ,
    password = md5(#{password}) ,
    create_time = #{createTime} ,
    update_time = now()
    where id = #{id}
</update>

delete

<delete id="batchRemove">
    delete from tb_user where id = #{id}  
</delete>

select

• resultType to define the returned result type

<!-- resultType 设置的是查询单条记录的类型 -->
<select id="findUserById" resultType="user">
    select id, username, password, create_time, update_time from tb_user  where id = #{id}
</select>

• ResultMap Advanced Result Mapping
• Allows the user to define the corresponding relationship between the field name of the query and the entity attribute

<resultMap id="userMap" type="user">
    <!--
        result 表示 结果集中的一个字段
               column : 用来设置 查询的字段名
               property : 是用来设置 type类型对应的属性名
               javaType : 用来设置 属性的类型
               jdbcType : 用来设置 数据库的类型
        id 和 result的用法相同，标记 该映射的是一个主键
    -->
    <id column="id" property="id" />
    <result column="username" property="username"/>
    <result column="password" property="password"/>
    <result column="time" property="createTime" />
</resultMap>
<select id="findUserByUsername" parameterType="string" resultMap="userMap">
    <!--
       如果 参数的类型是字面量类型 (8种基本数据类型+包装类 + String)
       且 有且只有一个参数，那么 #{} 中的占位符，可以任意
       如果 参数中使用了 @Param 注解，那么只能使用 注解中提供的值作为占位符
     -->
    select id, username, password, create_time as time from tb_user where username = #{username}
</select>

• The ResultMap advanced result map returns an object

<resultMap id="ogResultMap" type="orderGoods">
    <id property="id" column="id" />
    <result property="goodsId" column="goods_id"/>
    <result property="goodsName" column="goods_name"/>
    <result property="goodsPrice" column="goods_price"/>
    <result property="goodsNum" column="goods_num"/>
    <association property="order" javaType="order">
        <id property="id" column="order_id" />
        <result property="orderNum" column="order_num"/>
        <result property="createTime" column="create_time"/>
        <result property="status" column="status"/>
    </association>
</resultMap>
<select id="findOrderGoodsByOrderIdOrOrderNum" resultMap="ogResultMap">
    select g.id,
           g.order_id,
           g.goods_id,
           g.goods_name,
           g.goods_price,
           g.goods_num,
           t.order_num,
           t.user_id,
           t.create_time,
           t.staus
    from tb_order_goods g
             right join tb_order t on t.id = g.order_id
    <trim prefix="where" prefixOverrides="and |or ">
        <choose>
            <when test="orderNum != null">
                and t.order_num = #{orderNum}
            </when>
            <when test="orderId != null">
                and g.order_id = #{orderId}
            </when>
        </choose>
    </trim>
</select>

• ResultMap Advanced result map returns a collection

<resultMap id="resourceResultMap" type="Resource">
    <id property="id" column="id"></id>
    <result property="resourceName" column="resource_name"></result>
    <result property="resourceDesc" column="resource_desc"></result>
    <result property="star" column="star"></result>
    <result property="createTime" column="create_time"></result>
    <collection property="resourceCommentList" ofType="ResourceComment" >
        <id property="id" column="res_id"></id>
        <result property="userId" column="user_id"></result>
        <result property="content" column="content"></result>
        <result property="star" column="cstar"></result>
    </collection>
</resultMap>
<select id="findResourceById" resultMap="resourceResultMap">
    select r.id,
           r.resource_name,
           r.resource_desc,
           r.star,
           r.create_time,
           c.res_id,
           c.user_id,
           c.content,
           c.star cstar
    from tb_resource r
             join tb_resource_comment c
                  on r.id = c.res_id
    where r.id = #{id}
</select>

Dynamic SQL

• if

<update id="update" parameterType="user">
    update tb_user set
        <if test="username!=null">
            username = #{username} ,
        </if>
        <if test="password!=null">
            password = md5(#{password}) ,
        </if>
        <if test="createTime!=null">
            create_time = #{createTime} ,
        </if>
        update_time = now()
    where id = #{id}
</update>

• choose、when、otherwise

<select id="findUser" resultType="user" parameterType="user">
    select
        id,  username, password, create_time as createTime
    from tb_user
    <!-- where 标签主要适用于 条件的拼接，如果 条件中 多了 and , or ,  那么会自动去掉 多余的 and , or -->
    <where>
        <choose>
            <when test="id!=null">and id = #{id}</when>
            <when test="username!=null">and username = #{username}</when>
            <otherwise>and password = #{password}</otherwise>
        </choose>
    </where>
</select>

• trim、where、set

<select id="findOrderGoodsByOrderIdOrOrderNum" resultMap="ogResultMap">
    select g.id,
           g.order_id,
           g.goods_id,
           g.goods_name,
           g.goods_price,
           g.goods_num,
           t.order_num,
           t.user_id,
           t.create_time,
           t.staus
    from tb_order_goods g
             right join tb_order t on t.id = g.order_id
    <trim prefix="where" prefixOverrides="and |or ">
        <choose>
            <when test="orderNum != null">
                and t.order_num = #{orderNum}
            </when>
            <when test="orderId != null">
                and g.order_id = #{orderId}
            </when>
        </choose>
    </trim>
</select>

• foreach

 <insert id="batchSave">
    insert into tb_user(username, password, create_time) values

    <!--
        collection : 定义要遍历的数据
           如果传入的参数类型是 List, 那么 collection中的值 写成 list
           如果传入的参数类型是 数组 , 那么 collection中的值 写成 array
           如果使用了 @Param 注解指定了名字，那么就使用指定的名字
        item : 代表 遍历过程中 的 每一条数据 对应的变量, 变量名符合标识符命名规范即可

        separator : 用来定义循环中 每一条数据的 分割符号
        index : 用来标记该条记录的索引， 从 0 开始
        nullable : 是否允许为空，
        open :  整个 循环 以 ... 开始
        close:  整个循环 以 ... 结尾
    -->
    <foreach collection="list" item="user" separator=",">
        ( #{user.username} , md5(#{user.password}) ,  now() )
    </foreach>
</insert>

• bind

For fuzzy query, you need to use the concat function to concatenate %.
In the MySQL database, if the query condition uses a function, it will cause the index to fail.
In mybatis, you can use the bind tag to bind variables,
name is the set variable name, value is the variable value to set

<select id="findUserByLikeName" resultType="user">
    <bind name="name" value="'%' + username + '%'" />
    select id,username,password, create_time as createTime from tb_user
    where username like #{name}
</select>

The difference between ${} and #{} in MyBatis

'#{}' is a placeholder, which is mainly used for the placeholder of the value in SQL. When executing, it will be replaced by ? , and when splicing at the bottom layer, quotation marks
will be automatically added before and after the value. Values ​​can also be escaped, which can be used to prevent SQL injection

'${}' is a placeholder, which is mainly used in SQL structures (non-values), and will not be replaced by ? at the time of execution, and the value will be output literally
as part of the SQL structure

'${}' cannot be abused, because there will be a risk of SQL injection, if you can use #{}, never use ${} for placeholders

cache

• Level 1 cache is enabled by default and cannot be disabled
• Enabling the second-level cache will add caching effects to all selects in the mapping file.
• insert, update, delete will update the cache
• On the select tag, useCache defaults to true, and flushCache defaults to false
• On insert, update, delete tags, useCache is false by default, and flushCache is true by default
• useCache , flushCache To take effect, the second-level cache must be enabled