MyBatis(3)：优化MyBatis配置文件

一、连接数据库的配置单独放在一个properties文件中

1，创建一个database.properties

driver=com.mysql.jdbc.Driver
url=jdbc:mysql://localhost:3306/mybatis?
useSSL=true&useUnicode=true&charsetEncoding=UTF-8
username=root
password=123456

2，在mybatis核心配置文件中引入properties配置文件，并用${}表达式引入其中的值

<configuration>

<span class="hljs-comment">&lt;!--配置文件修改--&gt;</span>
<span class="hljs-tag">&lt;<span class="hljs-name">properties</span> <span class="hljs-attr">resource</span>=<span class="hljs-string">"database.properties"</span>/&gt;</span>

<span class="hljs-tag">&lt;<span class="hljs-name">environments</span> <span class="hljs-attr">default</span>=<span class="hljs-string">"development"</span>&gt;</span>
    <span class="hljs-tag">&lt;<span class="hljs-name">environment</span> <span class="hljs-attr">id</span>=<span class="hljs-string">"development"</span>&gt;</span>
        <span class="hljs-tag">&lt;<span class="hljs-name">transactionManager</span> <span class="hljs-attr">type</span>=<span class="hljs-string">"JDBC"</span>/&gt;</span>
        <span class="hljs-tag">&lt;<span class="hljs-name">dataSource</span> <span class="hljs-attr">type</span>=<span class="hljs-string">"POOLED"</span>&gt;</span>
            <span class="hljs-tag">&lt;<span class="hljs-name">property</span> <span class="hljs-attr">name</span>=<span class="hljs-string">"driver"</span> <span class="hljs-attr">value</span>=<span class="hljs-string">"${driver}"</span>/&gt;</span>
            <span class="hljs-tag">&lt;<span class="hljs-name">property</span> <span class="hljs-attr">name</span>=<span class="hljs-string">"url"</span> <span class="hljs-attr">value</span>=<span class="hljs-string">"${url}"</span>/&gt;</span>
            <span class="hljs-tag">&lt;<span class="hljs-name">property</span> <span class="hljs-attr">name</span>=<span class="hljs-string">"username"</span> <span class="hljs-attr">value</span>=<span class="hljs-string">"${username}"</span>/&gt;</span>
            <span class="hljs-tag">&lt;<span class="hljs-name">property</span> <span class="hljs-attr">name</span>=<span class="hljs-string">"password"</span> <span class="hljs-attr">value</span>=<span class="hljs-string">"${password}"</span>/&gt;</span>
        <span class="hljs-tag">&lt;/<span class="hljs-name">dataSource</span>&gt;</span>
    <span class="hljs-tag">&lt;/<span class="hljs-name">environment</span>&gt;</span>
<span class="hljs-tag">&lt;/<span class="hljs-name">environments</span>&gt;</span>

<span class="hljs-tag">&lt;<span class="hljs-name">mappers</span>&gt;</span>
    <span class="hljs-tag">&lt;<span class="hljs-name">mapper</span> <span class="hljs-attr">resource</span>=<span class="hljs-string">"com/kuang/dao/userMapper.xml"</span>/&gt;</span>
<span class="hljs-tag">&lt;/<span class="hljs-name">mappers</span>&gt;</span>


<span class="hljs-comment">&lt;!--配置文件修改--&gt;</span>
<span class="hljs-tag">&lt;<span class="hljs-name">properties</span> <span class="hljs-attr">resource</span>=<span class="hljs-string">"database.properties"</span>/&gt;</span>

<span class="hljs-tag">&lt;<span class="hljs-name">environments</span> <span class="hljs-attr">default</span>=<span class="hljs-string">"development"</span>&gt;</span>
    <span class="hljs-tag">&lt;<span class="hljs-name">environment</span> <span class="hljs-attr">id</span>=<span class="hljs-string">"development"</span>&gt;</span>
        <span class="hljs-tag">&lt;<span class="hljs-name">transactionManager</span> <span class="hljs-attr">type</span>=<span class="hljs-string">"JDBC"</span>/&gt;</span>
        <span class="hljs-tag">&lt;<span class="hljs-name">dataSource</span> <span class="hljs-attr">type</span>=<span class="hljs-string">"POOLED"</span>&gt;</span>
            <span class="hljs-tag">&lt;<span class="hljs-name">property</span> <span class="hljs-attr">name</span>=<span class="hljs-string">"driver"</span> <span class="hljs-attr">value</span>=<span class="hljs-string">"${driver}"</span>/&gt;</span>
            <span class="hljs-tag">&lt;<span class="hljs-name">property</span> <span class="hljs-attr">name</span>=<span class="hljs-string">"url"</span> <span class="hljs-attr">value</span>=<span class="hljs-string">"${url}"</span>/&gt;</span>
            <span class="hljs-tag">&lt;<span class="hljs-name">property</span> <span class="hljs-attr">name</span>=<span class="hljs-string">"username"</span> <span class="hljs-attr">value</span>=<span class="hljs-string">"${username}"</span>/&gt;</span>
            <span class="hljs-tag">&lt;<span class="hljs-name">property</span> <span class="hljs-attr">name</span>=<span class="hljs-string">"password"</span> <span class="hljs-attr">value</span>=<span class="hljs-string">"${password}"</span>/&gt;</span>
        <span class="hljs-tag">&lt;/<span class="hljs-name">dataSource</span>&gt;</span>
    <span class="hljs-tag">&lt;/<span class="hljs-name">environment</span>&gt;</span>
<span class="hljs-tag">&lt;/<span class="hljs-name">environments</span>&gt;</span>

<span class="hljs-tag">&lt;<span class="hljs-name">mappers</span>&gt;</span>
    <span class="hljs-tag">&lt;<span class="hljs-name">mapper</span> <span class="hljs-attr">resource</span>=<span class="hljs-string">"com/kuang/dao/userMapper.xml"</span>/&gt;</span>
<span class="hljs-tag">&lt;/<span class="hljs-name">mappers</span>&gt;</span>
</configuration>

二、为实体类定义别名，简化sql映射xml文件中的引用

1，配置别名

<typeAliases>

<span class="hljs-comment">&lt;!--配置指定类的别名--&gt;</span>
<span class="hljs-tag">&lt;<span class="hljs-name">typeAlias</span> <span class="hljs-attr">type</span>=<span class="hljs-string">"com.kuang.pojo.User"</span> <span class="hljs-attr">alias</span>=<span class="hljs-string">"User"</span>/&gt;</span>


<span class="hljs-comment">&lt;!--配置指定类的别名--&gt;</span>
<span class="hljs-tag">&lt;<span class="hljs-name">typeAlias</span> <span class="hljs-attr">type</span>=<span class="hljs-string">"com.kuang.pojo.User"</span> <span class="hljs-attr">alias</span>=<span class="hljs-string">"User"</span>/&gt;</span>
</typeAliases>

2，优化完毕后我们在mapper映射文件中就可以使用别名了

<?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="com.Shandx.dao.UserDao">



<span class="hljs-tag">&lt;<span class="hljs-name">select</span> <span class="hljs-attr">id</span>=<span class="hljs-string">"selectUser"</span> <span class="hljs-attr">resultType</span>=<span class="hljs-string">"User"</span>&gt;</span>
select * from user
<span class="hljs-tag">&lt;/<span class="hljs-name">select</span>&gt;</span>


<span class="hljs-tag">&lt;<span class="hljs-name">select</span> <span class="hljs-attr">id</span>=<span class="hljs-string">"selectUser"</span> <span class="hljs-attr">resultType</span>=<span class="hljs-string">"User"</span>&gt;</span>
select * from user
<span class="hljs-tag">&lt;/<span class="hljs-name">select</span>&gt;</span>
</mapper>

三，解决属性和字段名不一致

字段名对应的是数据库，属性名对应的是实体类，如果我们的字段名和属性名不一致时，我们就获取不到数据；譬如：

那么属性名和字段名不同的值将获取不到

那么解决方法有三种
第一种是修改set方法名字，我们不推荐使用
第二种是给sql语句取别名，但是只是在字段少的时候使用

第三种方法是结果集映射ResultMap ，这个方法我们推荐使用
【前提是我们要在配置好指定类的别名】

<!--设置结果的映射类型-->
<resultMap id="UserMap" type="User">
    <!--
    一般通过id标签来映射主键
    column = 数据库的列名
    property = 结果集对应的数据库列名的映射名
    -->
    <id column="id" property="id"/>
    <result column="name" property="name"/>
    <result column="pwd" property="password"/>
</resultMap>
<select id="selectUser" resultMap="UserMap">

select * from user

</select>