[[jdbc-initializing-datasource]]
= Initializing a `DataSource`

The `org.springframework.jdbc.datasource.init` package provides support for initializing
an existing `DataSource`. The embedded database support provides one option for creating
and initializing a `DataSource` for an application. However, you may sometimes need to initialize
an instance that runs on a server somewhere.


[[jdbc-initializing-datasource-xml]]
== Initializing a Database by Using Spring XML

If you want to initialize a database and you can provide a reference to a `DataSource`
bean, you can use the `initialize-database` tag in the `spring-jdbc` namespace:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<jdbc:initialize-database data-source="dataSource">
		<jdbc:script location="classpath:com/foo/sql/db-schema.sql"/>
		<jdbc:script location="classpath:com/foo/sql/db-test-data.sql"/>
	</jdbc:initialize-database>
----

The preceding example runs the two specified scripts against the database. The first
script creates a schema, and the second populates tables with a test data set. The script
locations can also be patterns with wildcards in the usual Ant style used for resources
in Spring (for example,
`classpath{asterisk}:/com/foo/{asterisk}{asterisk}/sql/{asterisk}-data.sql`). If you use a
pattern, the scripts are run in the lexical order of their URL or filename.

The default behavior of the database initializer is to unconditionally run the provided
scripts. This may not always be what you want -- for instance, if you run
the scripts against a database that already has test data in it. The likelihood
of accidentally deleting data is reduced by following the common pattern (shown earlier)
of creating the tables first and then inserting the data. The first step fails if
the tables already exist.

However, to gain more control over the creation and deletion of existing data, the XML
namespace provides a few additional options. The first is a flag to switch the
initialization on and off. You can set this according to the environment (such as pulling a
boolean value from system properties or from an environment bean). The following example gets a value from a system property:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<jdbc:initialize-database data-source="dataSource"
		enabled="#{systemProperties.INITIALIZE_DATABASE}"> <1>
		<jdbc:script location="..."/>
	</jdbc:initialize-database>
----
<1> Get the value for `enabled` from a system property called `INITIALIZE_DATABASE`.


The second option to control what happens with existing data is to be more tolerant of
failures. To this end, you can control the ability of the initializer to ignore certain
errors in the SQL it runs from the scripts, as the following example shows:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<jdbc:initialize-database data-source="dataSource" ignore-failures="DROPS">
		<jdbc:script location="..."/>
	</jdbc:initialize-database>
----

In the preceding example, we are saying that we expect that, sometimes, the scripts are run
against an empty database, and there are some `DROP` statements in the scripts that
would, therefore, fail. So failed SQL `DROP` statements will be ignored, but other failures
will cause an exception. This is useful if your SQL dialect doesn't support `DROP ... IF
EXISTS` (or similar) but you want to unconditionally remove all test data before
re-creating it. In that case the first script is usually a set of `DROP` statements,
followed by a set of `CREATE` statements.

The `ignore-failures` option can be set to `NONE` (the default), `DROPS` (ignore failed
drops), or `ALL` (ignore all failures).

Each statement should be separated by `;` or a new line if the `;` character is not
present at all in the script. You can control that globally or script by script, as the
following example shows:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<jdbc:initialize-database data-source="dataSource" separator="@@"> <1>
		<jdbc:script location="classpath:com/myapp/sql/db-schema.sql" separator=";"/> <2>
		<jdbc:script location="classpath:com/myapp/sql/db-test-data-1.sql"/>
		<jdbc:script location="classpath:com/myapp/sql/db-test-data-2.sql"/>
	</jdbc:initialize-database>
----
<1> Set the separator scripts to `@@`.
<2> Set the separator for `db-schema.sql` to `;`.

In this example, the two `test-data` scripts use `@@` as statement separator and only
the `db-schema.sql` uses `;`. This configuration specifies that the default separator
is `@@` and overrides that default for the `db-schema` script.

If you need more control than you get from the XML namespace, you can use the
`DataSourceInitializer` directly and define it as a component in your application.

[[jdbc-client-component-initialization]]
=== Initialization of Other Components that Depend on the Database

A large class of applications (those that do not use the database until after the Spring context has
started) can use the database initializer with no further
complications. If your application is not one of those, you might need to read the rest
of this section.

The database initializer depends on a `DataSource` instance and runs the scripts
provided in its initialization callback (analogous to an `init-method` in an XML bean
definition, a `@PostConstruct` method in a component, or the `afterPropertiesSet()`
method in a component that implements `InitializingBean`). If other beans depend on the
same data source and use the data source in an initialization callback, there
might be a problem because the data has not yet been initialized. A common example of
this is a cache that initializes eagerly and loads data from the database on application
startup.

To get around this issue, you have two options: change your cache initialization strategy
to a later phase or ensure that the database initializer is initialized first.

Changing your cache initialization strategy might be easy if the application is in your control and not otherwise.
Some suggestions for how to implement this include:

* Make the cache initialize lazily on first usage, which improves application startup
  time.
* Have your cache or a separate component that initializes the cache implement
  `Lifecycle` or `SmartLifecycle`. When the application context starts, you can
  automatically start a `SmartLifecycle` by setting its `autoStartup` flag, and you can
  manually start a `Lifecycle` by calling `ConfigurableApplicationContext.start()`
  on the enclosing context.
* Use a Spring `ApplicationEvent` or similar custom observer mechanism to trigger the
  cache initialization. `ContextRefreshedEvent` is always published by the context when
  it is ready for use (after all beans have been initialized), so that is often a useful
  hook (this is how the `SmartLifecycle` works by default).

Ensuring that the database initializer is initialized first can also be easy. Some suggestions on how to implement this include:

* Rely on the default behavior of the Spring `BeanFactory`, which is that beans are
  initialized in registration order. You can easily arrange that by adopting the common
  practice of a set of `<import/>` elements in XML configuration that order your
  application modules and ensuring that the database and database initialization are
  listed first.
* Separate the `DataSource` and the business components that use it and control their
  startup order by putting them in separate `ApplicationContext` instances (for example, the
  parent context contains the `DataSource`, and the child context contains the business
  components). This structure is common in Spring web applications but can be more
  generally applied.
