typeAliases

我们还可以使用typeAliases定义一些值，让SQL Mapper XML中引用。 比如我们所有的查找操作，返回的都是"org.example.model.AllType"。在SQL Mapper XML（AllTypeMapper.xml）中如下使用。

<select id="findAll" resultType="org.example.model.AllType">
      select * from all_type
</select>
<select id="find" resultType="org.example.model.AllType">
    select * from all_type where info_int = #{info_int}
</select>

如果我们觉得这个值太长，可以在Mybatis配置文件（mybatis-config.xml）中新增如下字段

    <typeAliases>
        <typeAlias type="org.example.model.AllType" alias="AllType"/>
    </typeAliases>

然后将在SQL Mapper XML中org.example.model.AllType替换成AllType即可。

    <select id="findAll" resultType="AllType">
        select * from all_type
    </select>
    <select id="find" resultType="AllType">
        select * from all_type where info_int = #{info_int}
    </select>

settings

除了这些简化配置的功能，Mybatis配置文件还可以给框架设置一些属性。比如我们希望执行过程可以输出到终端，则需要加入如下配置即可

    <settings><setting name="logImpl" value="STDOUT_LOGGING"/></settings>

这样我们执行代码时就可以看大SQL语句模板

Logging initialized using ‘class org.apache.ibatis.logging.stdout.StdOutImpl’ adapter. Opening JDBC Connection ==> Preparing: insert into all_type(info_int, info_tint, info_sint) values (?, ?, ?) , (?, ?, ?) , (?, ?, ?) , (?, ?, ?) , (?, ?, ?) , (?, ?, ?) , (?, ?, ?) , (?, ?, ?) , (?, ?, ?) , (?, ?, ?) ==> Parameters: 100(Integer), 100(Byte), 100(Short), 101(Integer), 101(Byte), 101(Short), 102(Integer), 102(Byte), 102(Short), 103(Integer), 103(Byte), 103(Short), 104(Integer), 104(Byte), 104(Short), 105(Integer), 105(Byte), 105(Short), 106(Integer), 106(Byte), 106(Short), 107(Integer), 107(Byte), 107(Short), 108(Integer), 108(Byte), 108(Short), 109(Integer), 109(Byte), 109(Short) <== Updates: 10 Closing JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@78a287ed]

字段顺序

经过这番修改，我们的配置文件最后如下

<?xml version="1.0" encoding="UTF-8" ?>
<!-- mybatis-config-2.xml -->
<!DOCTYPE configurationPUBLIC "-//mybatis.org//DTD Config 3.0//EN""https://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration><properties><property name="driver" value="com.mysql.cj.jdbc.Driver"/><property name="username" value="root"/><property name="password" value="fangliang"/></properties><settings><setting name="logImpl" value="STDOUT_LOGGING"/></settings><typeAliases><typeAlias type="org.example.model.AllType" alias="AllType"/></typeAliases><environments default="development"><environment id="development"><transactionManager type="JDBC"/><dataSource type="POOLED"><property name="driver" value="${driver}"/><property name="url" value="jdbc:mysql://localhost:3306/testdb?useSSL=true&amp;useUnicode=true&amp;characterEncoding=utf8"/><property name="username" value="${username}"/><property name="password" value="${password}"/></dataSource></environment></environments><mappers><mapper resource="mybatis/mapper/AllTypeMapper-2.xml"/></mappers>

</configuration>

这儿特别需要注意的是：configuration下字段是有顺序的。 假如我们将settings放在properties前，如下（错误的）

<configuration><settings><setting name="logImpl" value="STDOUT_LOGGING"/></settings><properties><property name="driver" value="com.mysql.cj.jdbc.Driver"/><property name="username" value="root"/><property name="password" value="fangliang"/></properties>
    ……
<configuration>

就会报错：

The content of element type “configuration” must match “(properties?,settings?,typeAliases?,typeHandlers?,objectFactory?,objectWrapperFactory?,reflectorFactory?,plugins?,environments?,databaseIdProvider?,mappers?)”.

这句话的意思是：configuration下字段需要按如下顺序排列。

1. properties
2. settings
3. typeAliases
4. typeHandlers
5. objectFactory
6. objectWrapperFactory
7. reflectorFactory
8. plugins
9. environments
10. databaseIdProvider
11. mappers

参考资料

• https://mybatis.org/mybatis-3/zh_CN/configuration.html
• https://mybatis.org/mybatis-3/zh_CN/sqlmap-xml.html