Mybatis-Configuration XML

The MyBatis configuration contains settings and properties that have a dramatic effect on how MyBatis behaves.

properties

These are externalizable, substitutable properties that can be configured in a typical Java Properties file instance, or passed in through sub-elements of the properties element. For example:

The properties can then be used throughout the configuration files to substitute values that need to be dynamically configured. For example:

settings

These are extremely important tweaks that modify the way that MyBatis behaves at runtime.

typeAliases

A type alias is simply a shorter name for a Java type. It's only relevant to the XML configuration and simply exists to reduce redundant typing of fully qualified classnames. For example:

You can also specify a package where MyBatis will search for beans. For example:

typeHandlers

Whenever MyBatis sets a parameter on a PreparedStatement or retrieves a value from a ResultSet, a TypeHandler is used to retrieve the value in a means appropriate to the Java type.

You can override the type handlers or create your own to deal with unsupported or non-standard types.

Handling Enums

If you want to map an Enum, you'll need to use either EnumTypeHandler or EnumOrdinalTypeHandler.

objectFactory

Each time MyBatis creates a new instance of a result object, it uses an ObjectFactory instance to do so. The default ObjectFactory does little more than instantiate the target class with a default constructor, or a parameterized constructor if parameter mappings exist. If you want to override the default behaviour of the ObjectFactory, you can create your own.

plugins

MyBatis allows you to intercept calls to at certain points within the execution of a mapped statement.
By default, MyBatis allows plug-ins to intercept method calls of:

Executor (update, query, flushStatements, commit, rollback, getTransaction, close, isClosed)
ParameterHandler (getParameterObject, setParameters)
ResultSetHandler (handleResultSets, handleOutputParameters)
StatementHandler (prepare, parameterize, batch, update, query)

Using plug-ins is pretty simple given the power they provide.
Simply implement the Interceptor interface, being sure to specify the signatures you want to intercept.

environments

MyBatis can be configured with multiple environments. This helps you to apply your SQL Maps to multiple databases for any number of reasons. For example, you might have a different configuration for your Development, Test and Production environments. Or, you may have multiple production databases that share the same schema, and you’d like to use the same SQL maps for both. There are many use cases.

One important thing to remember though: While you can configure multiple environments, you can only choose ONE per SqlSessionFactory instance.

So if you want to connect to two databases, you need to create two instances of SqlSessionFactory, one for each. For three databases, you’d need three instances, and so on.

databaseIdProvider

MyBatis is able to execute different statements depending on your database vendor. The multi-db vendor support is based on the mapped statements databaseId attribute. MyBatis will load all statements with no databaseId attribute or with a databaseId that matches the current one. In case the same statement is found with and without the databaseId the latter will be discarded.

mappers

Now that the behavior of MyBatis is configured with the above configuration elements, we’re ready to define our mapped SQL statements. But first, we need to tell MyBatis where to find them. Java doesn’t really provide any good means of auto-discovery in this regard, so the best way to do it is to simply tell MyBatis where to find the mapping files. You can use classpath relative resource references, fully qualified url references (including file:/// URLs), class names or package names.