MyBatis

MyBatis
Configuration
Hidden features

Configuration XML
The MyBatis XML configuration file contains settings and properties that
have a dramatic effect on how MyBatis behaves.
The high level structure of the document is ‘configuration’, that includes:
 properties;
 settings;
 typeAliases;
 typeHandlers;
 objectFactory;
 plugins;
 environments (transactionManager, dataSource);
 mappers.

Configuration XML: 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:

Configuration XML: properties
Properties can also be passed into the SqlSessionBuilder.build() methods. For
example:

Configuration XML: 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:

Configuration XML: typeAliases

Configuration XML: 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. The following table describes the default
TypeHandlers.

Configuration XML: 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. For example:

Configuration XML: 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)

Configuration XML: environments

Configuration XML: transactionManager
There are two TransactionManager types that are included with MyBatis:
 JDBC – This configuration simply makes use of the JDBC commit and rollback
facilities directly. It relies on the connection retrieved from the dataSource to
manage the scope of the transaction.
 MANAGED – This configuration simply does almost nothing. It never commits, or
rolls back a connection. Instead, it lets the container manage the full lifecycle of the
transaction (e.g. Spring or a JEE Application Server context). By default it does
close the connection. However, some containers don’t expect this, and thus if you
need to stop it from closing the connection, set the closeConnection property to false.
For example:

Configuration XML: dataSource
The dataSource element configures the source of JDBC Connection objects using the
standard JDBC DataSource interface.
There are three build-in dataSource types (UNPOOLED, POOLED, JNDI)
UNPOOLED – This implementation of DataSource simply opens and closes a
connection each time it is requested. While it’s a bit slower, this is a good choice for
simple applications that do not require the performance of immediately available
connections. Different databases are also different in this performance area, so for some
it may be less important to pool and this configuration will be ideal. The UNPOOLED
DataSource is configured with only five properties:
 driver – This is the fully qualified Java class of the JDBC driver (NOT of the DataSource class if
your driver includes one).
 url – This is the JDBC URL for your database instance.
 username – The database username to log in with.
 password – The database password to log in with.
 defaultTransactionIsolationLevel – The default transaction isolation level for connections.

POOLED – This implementation of dataSource pools JDBC Connection objects to
avoid the initial connection and authentication time required to create a new
Connection instance. This is a popular approach for concurrent web applications to
achieve the fastest response. Properties that can be used to configure the POOLED
dataSource:
 poolMaximumActiveConnections – This is the number of active (i.e. in use) connections
that can exist at any given time. Default: 10
 poolMaximumIdleConnections – The number of idle connections that can exist at any
given time.
 poolMaximumCheckoutTime – This is the amount of time that a Connection can be
“checked out” of the pool before it will be forcefully returned. Default: 20000ms
 poolTimeToWait – This is a low level setting that gives the pool a chance to print a log
status and re‐attempt the acquisition of a connection in the case that it’s taking unusually
long. Default: 20000ms
 poolPingQuery – The Ping Query is sent to the database to validate that a connection is in
good working order and is ready to accept requests.
 poolPingEnabled – This enables or disables the ping query. If enabled, you must also set
the poolPingQuery property with a valid SQL statement. Default: false.
 poolPingConnectionsNotUsedFor – This configures how often the poolPingQuery will be
used. This can be set to match the typical timeout for a database connection, to avoid
unnecessary pings. Default: 0

JNDI – This implementation of DataSource is intended for use with
containers such as Spring or Application Servers that may configure the
DataSource centrally or externally and place a reference to it in a JNDI
context. This DataSource configuration only requires two properties:
 initial_context – This property is used for the Context lookup from the
InitialContext (i.e. initialContext.lookup(initial_context)). This property is optional,
and if omitted, then the data_source property will be looked up against the
InitialContext directly.
 data_source – This is the context path where the reference to the instance of the
DataSource can be found. It will be looked up against the context returned by the
initial_context lookup, or against the InitialContext directly if no initial_context is
supplied.

Configuration XML: mappers
Now that the behaviour 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 class path relative
resource references, or literal, fully qualified url references. For example:

SQL Map XML Files: insert, update, delete

SQL Map XML Files: sql
This element can be used to define a reusable fragment of SQL code that can
be included in other statements. For example:
The SQL fragment can then be included in another statement, for example:

SQL Map XML Files: Parameters
In all of the past statements, you’ve seen examples of simple parameters.
Parameters are very powerful elements in MyBatis. For simple situations,
probably 90% of the cases, there’s not much too them, for example:
The example above demonstrates a very simple named parameter mapping.
The parameterType is set to “int”, so therefore the parameter could be named
anything. Primitive or simply data types such as Integer and String have no
relevant properties, and thus will replace the full value of the parameter
entirely. However, if you pass in a complex object, then the behaviour is a little
different. For example:

SQL Map XML Files: advanced resultMap

resultMap:
 constructor – used for injecting results into the constructor of a class upon
instantiation
o idArg – ID argument; flagging results as ID will help improve overall performance
o arg – a normal result injected into the constructor
 id – an ID result; flagging results as ID will help improve overall performance
 result – a normal result injected into a field or JavaBean property
 association – a complex type association; many results will roll up into this type.
Nested result mappings – associations are resultMaps themselves, or can refer to
one
 collection – a collection of complex types. Nested result mappings – collections are
resultMaps themselves, or can refer to one
 discriminator – uses a result value to determine which resultMap to use
o case – a case is a result map based on some value. Nested result mappings – a case is also a
result map itself, and thus can contain many of these same elements, or it can refer to an
external, resultMap.

id, result
These are the most basic of result mappings. Both id, and result map a
single column value to a single property or field of a simple data type
(String, int, double, Date, etc.).
The only difference between the two is that id will flag the result as an
identifier property to be used when comparing object instances. This helps
to improve general performance, but especially performance of caching and
nested result mapping (i.e. join mapping).

id, result

constructor
While properties will work for most Data Transfer Object (DTO) type
classes, and likely most of your domain model, there may be some cases
where you want to use immutable classes. Often tables that contain
reference or lookup data that rarely or never changes is suited to
immutable classes.
Constructor injection allows you to set values on a class upon instantiation,
without exposing public methods. MyBatis also supports private properties
and private JavaBeans properties to achieve this, but some people prefer
Constructor injection. The constructor element enables this.

constructor

association
The association element deals with a “has-one” type relationship. For example, in
our example, a Blog has one Author. An association mapping works mostly like any
other result. You specify the target property, the column to retrieve the value from,
the javaType of the property (which MyBatis can figure out most of the time), the
jdbcType if necessary and a typeHandler if you want to override the retrieval
of the result values.
Where the association differs is that you need to tell MyBatis how to load the
association. MyBatis can do so in two different ways:
• Nested Select: By executing another mapped SQL statement that returns
the complex type desired.
• Nested Results: By using nested result mappings to deal with repeating
subsets of joined results.

association

collection
The collection element works almost identically to the association. In fact, it’s so
similar, to document the similarities would be redundant. So let’s focus on the
differences. To continue with our example above, a Blog only had one Author. But a
Blog has many Posts. On the blog class, this would be represented by something like:
To map a set of nested results to a List like this, we use the collection element. Just like the
association element, we can use a nested select, or nested results from a join.

discriminator
A discriminator definition specifies column and javaType attributes. The
column is where MyBatis will look for the value to compare. The javaType
is required to ensure the proper kind of equality test is performed
(although String would probably work for almost any situation). For
example:

$ vs #
${id} inserts substring
select * from ${schema}.users
• When you can’t use parameter
select * from users order by ${field}
• For configuration
select * from ${schema}.users
#{id} creates parametrized query
select * from secure.users where id=#{id}

Mapped statement
MappedStatement ms = config.getMappedStatement(statement);
for (ResultMapping m : ms.getResultMaps().get(0).getPropertyResultMappings()) {
if (m.getProperty().equals(userEnteredSortField)) {
return m.getColumn();
}
}
//
MappedStatement ms = session.getConfiguration()
.getMappedStatement(UserService.class.getName() + ".getUser");
BoundSql boundSql = ms.getBoundSql(i); // parameter for the SQL statement

Dynamic SQL
choose, when, otherwise

MyBatis

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