Detailed explanation of configuration files in MyBatis

---

1. Configuration overview

MyBatisThe configuration of will affect MyBatisthe settings and attribute information of the behavior. The following figure shows the attributes in the configuration file: It

should be noted mybatis-config.xmlthat these attributes must be written in the order above!

Second, the role of each configuration

1. Properties

The function of this configuration is to reference an external configuration file. You can create a new .propertiesfile, write some configuration information for connecting to the database into the file, and then propertiesimport it with attributes. The demonstration is as follows:

Create a new configuration file db.properties::

driver=com.mysql.cj.jdbc.Driver
url=jdbc:mysql://localhost:3306/MyBatis_DB?serverTimezone=GMT
username=root
password=root

Then mybatis-config.xmlwrite the attribute:

    <!-- properties 引入外部配置文件 -->
    <properties resource="db.properties">
        <!-- 
        也可以在这里直接写
         <property name="xxxx" value="xxxx" />
         -->
    </properties>

If an attribute is configured in more than one place, MyBatis will be loaded in the following order:

• First read the properties specified in the properties element body.
• Then read the property file under the classpath according to the resource attribute in the properties element, or read the property file according to the path specified by the url attribute, and overwrite the previously read property with the same name.
• Finally, the attribute passed as the method parameter is read, and the attribute with the same name read before is overwritten.

2. Settings

settingsIt is a Mybatisglobal configuration, MyBatissome default configuration has been stipulated in it, if other configuration is needed, settingsjust write it in the label.

An settingsexample of a fully configured element is as follows:

<settings>
  <setting name="cacheEnabled" value="true"/>
  <setting name="lazyLoadingEnabled" value="true"/>
  <setting name="multipleResultSetsEnabled" value="true"/>
  <setting name="useColumnLabel" value="true"/>
  <setting name="useGeneratedKeys" value="false"/>
  <setting name="autoMappingBehavior" value="PARTIAL"/>
  <setting name="autoMappingUnknownColumnBehavior" value="WARNING"/>
  <setting name="defaultExecutorType" value="SIMPLE"/>
  <setting name="defaultStatementTimeout" value="25"/>
  <setting name="defaultFetchSize" value="100"/>
  <setting name="safeRowBoundsEnabled" value="false"/>
  <setting name="mapUnderscoreToCamelCase" value="false"/>
  <setting name="localCacheScope" value="SESSION"/>
  <setting name="jdbcTypeForNull" value="OTHER"/>
  <setting name="lazyLoadTriggerMethods" value="equals,clone,hashCode,toString"/>
</settings>

The following table lists several more important configurations:

Set name	description	Effective value	Defaults
cacheEnabled	Globally enable or disable any cache configured in all mapper configuration files	true | false	true
lazyLoadingEnabled	Delayed load global switch	true | false	false
logImpl	Specify MyBatisthe specific implementation of the log used, if not specified, it will be automatically searched	SLF4J | LOG4J | LOG4J2 | JDK_LOGGING | COMMONS_LOGGING | NO_LOGGING	Not set

3. typeAliases type alias

When writing MyBatisa xmldocument, you returnTypemust fill in the full class name of the entity class every time you fill in it. For example,

this is really super troublesome. typeAliasesThe function is only to give an alias for these full class names to simplify writing.

There are three ways of aliasing:

• Alias ​​a single entity class

    <typeAliases>
        	<typeAlias alias="User" type="com.wzq.domain.User" />
    </typeAliases>
  

• Specify a package name and convert all entity classes under the package to aliases (the default is the lowercase of the first letter of the class name)

      <typeAliases>
       	<package name="com.wzq.domain"/>
      </typeAliases>
  

• Use annotations ( you need to write in the previous step ), you can add annotations to the class name to alias

  @Alias("User")
  public class User{
        
         
  	...
  }
  

4. typeHandlers type processor

MyBatisStatements provided pretreatment ( PreparedStatementtime) concentrated parameters or values taken from a result, the acquired values are converted in a suitable manner type processor Javatype. The following table describes some of the default type processors.

5. objectFactory object factory

Every MyBatistime a new instance of the result object is created, it will use an object factory ( ObjectFactory) to instantiate instances to complete the work. What the default object factory needs to do is to instantiate the target class, either through the default no-argument constructor, or through the existing parameter mapping to call the constructor with parameters. If you want to override the default behavior of the object factory, you can do so by creating your own object factory.

6, plugins

MyBatisThe plug-in function is still very powerful, and there are many extensions, such as:

• mybatis-generator-core
• mybatis-plus
• Universalmapper
• and many more

7, environments environment configuration

MyBatisMultiple environments can be configured, but: Although multiple environments can be configured, only one environment can be selected for each SqlSessionFactory instance.

<!-- default字段定义了当前使用的是哪一套配置 -->
<environments default="development">
  <environment id="development">
    <!-- 使用JDBC的事务管理 -->
    <transactionManager type="JDBC">
      <property name="..." value="..."/>
    </transactionManager>
    <!-- type = "POOLED" 规定了这里有数据库链接池 -->
    <dataSource type="POOLED">
      <property name="driver" value="${driver}"/>
      <property name="url" value="${url}"/>
      <property name="username" value="${username}"/>
      <property name="password" value="${password}"/>
    </dataSource>
  </environment>
  <!-- 另一套配置 -->
  <environment id="test">
    <transactionManager type="JDBC">
      <property name="..." value="..."/>
    </transactionManager>
    <dataSource type="POOLED">
      <property name="driver" value="${driver}"/>
      <property name="url" value="${url}"/>
      <property name="username" value="${username}"/>
      <property name="password" value="${password}"/>
    </dataSource>
  </environment>
</environments>

8, mappers mapper

mappersThe role is: register the binding Mapperfile, it has four ways to bind

Method 1: Useresource

    <mappers>
        <mapper resource="com/wzq/mapper/UserMapper.xml"/>
    </mappers>

Method 2: Useclass

    <mappers>
        <mapper class="com.wzq.mapper.UserMapper"/>
    </mappers>

But here needs Mapperto Mapper.xmlbe in the same package with the file, and have the same name, so is the third way!

Method 3: Register all mapper interface implementations in the package as mappers

	<mappers>
  		<package name="org.mybatis.builder"/>
	</mappers>

The fourth method is to use urlbinding, which is less used, so it will not be listed here!