85. Database Initialization

An SQL database can be initialized in different ways depending on what your stack is. Of course, you can also do it manually, provided the database is a separate process.

85.1 Initialize a Database Using JPA

JPA has features for DDL generation, and these can be set up to run on startup against the database. This is controlled through two external properties:

  • spring.jpa.generate-ddl (boolean) switches the feature on and off and is vendor independent.

  • spring.jpa.hibernate.ddl-auto (enum) is a Hibernate feature that controls the behavior in a more fine-grained way. This feature is described in more detail later in this guide.

85.2 Initialize a Database Using Hibernate

You can set spring.jpa.hibernate.ddl-auto explicitly and the standard Hibernate property values are none , validate , update , create , and create-drop . Spring Boot chooses a default value for you based on whether it thinks your database is embedded. It defaults to create-drop if no schema manager has been detected or none in all other cases. An embedded database is detected by looking at the Connection type. hsqldb , h2 , and derby are embedded, and others are not. Be careful when switching from in-memory to a ‘real’ database that you do not make assumptions about the existence of the tables and data in the new platform. You either have to set ddl-auto explicitly or use one of the other mechanisms to initialize the database.

Note

You can output the schema creation by enabling the org.hibernate.SQL logger. This is done for you automatically if you enable the debug mode.

In addition, a file named import.sql in the root of the classpath is executed on startup if Hibernate creates the schema from scratch (that is, if the ddl-auto property is set to create or create-drop ). This can be useful for demos and for testing if you are careful but is probably not something you want to be on the classpath in production. It is a Hibernate feature (and has nothing to do with Spring).

85.3 Initialize a Database

Spring Boot can automatically create the schema (DDL scripts) of your DataSource and initialize it (DML scripts). It loads SQL from the standard root classpath locations: schema.sql and data.sql , respectively. In addition, Spring Boot processes the schema-${platform}.sql and data-${platform}.sql files (if present), where platform is the value of spring.datasource.platform . This allows you to switch to database-specific scripts if necessary. For example, you might choose to set it to the vendor name of the database ( hsqldb , h2 , oracle , mysql , postgresql , and so on).

Note

Spring Boot automatically creates the schema of an embedded DataSource . This behaviour can be customized by using the spring.datasource.initialization-mode property. For instance, if you want to always initialize the DataSource regardless of its type:

spring.datasource.initialization-mode=always

By default, Spring Boot enables the fail-fast feature of the Spring JDBC initializer. This means that, if the scripts cause exceptions, the application fails to start. You can tune that behavior by setting spring.datasource.continue-on-error .

Note

In a JPA-based app, you can choose to let Hibernate create the schema or use schema.sql , but you cannot do both. Make sure to disable spring.jpa.hibernate.ddl-auto if you use schema.sql .

85.4 Initialize a Spring Batch Database

If you use Spring Batch, it comes pre-packaged with SQL initialization scripts for most popular database platforms. Spring Boot can detect your database type and execute those scripts on startup. If you use an embedded database, this happens by default. You can also enable it for any database type, as shown in the following example:

spring.batch.initialize-schema=always

You can also switch off the initialization explicitly by setting spring.batch.initialize-schema=never .

85.5 Use a Higher-level Database Migration Tool

Spring Boot supports two higher-level migration tools: Flyway and Liquibase.

85.5.1 Execute Flyway Database Migrations on Startup

To automatically run Flyway database migrations on startup, add the org.flywaydb:flyway-core to your classpath.

The migrations are scripts in the form V<VERSION>__<NAME>.sql (with <VERSION> an underscore-separated version, such as ‘1’ or ‘2_1’). By default, they are in a folder called classpath:db/migration , but you can modify that location by setting spring.flyway.locations . This is a comma-separated list of one or more classpath: or filesystem: locations. For example, the following configuration would search for scripts in both the default classpath location and the /opt/migration directory:

spring.flyway.locations=classpath:db/migration,filesystem:/opt/migration

You can also add a special {vendor} placeholder to use vendor-specific scripts. Assume the following:

spring.flyway.locations=classpath:db/migration/{vendor}

Rather than using db/migration , the preceding configuration sets the folder to use according to the type of the database (such as db/migration/mysql for MySQL). The list of supported databases is available in DatabaseDriver.

FlywayProperties provides most of Flyway’s settings and a small set of additional properties that can be used to disable the migrations or switch off the location checking. If you need more control over the configuration, consider registering a FlywayConfigurationCustomizer bean.

Spring Boot calls Flyway.migrate() to perform the database migration. If you would like more control, provide a @Bean that implements FlywayMigrationStrategy.

Flyway supports SQL and Java callbacks. To use SQL-based callbacks, place the callback scripts in the classpath:db/migration folder. To use Java-based callbacks, create one or more beans that implement Callback . Any such beans are automatically registered with Flyway . They can be ordered by using @Order or by implementing Ordered . Beans that implement the deprecated FlywayCallback interface can also be detected, however they cannot be used alongside Callback beans.

By default, Flyway autowires the ( @Primary ) DataSource in your context and uses that for migrations. If you like to use a different DataSource , you can create one and mark its @Bean as @FlywayDataSource . If you do so and want two data sources, remember to create another one and mark it as @Primary . Alternatively, you can use Flyway’s native DataSource by setting spring.flyway.[url,user,password] in external properties. Setting either spring.flyway.url or spring.flyway.user is sufficient to cause Flyway to use its own DataSource . If any of the three properties has not be set, the value of its equivalent spring.datasource property will be used.

There is a Flyway sample so that you can see how to set things up.

You can also use Flyway to provide data for specific scenarios. For example, you can place test-specific migrations in src/test/resources and they are run only when your application starts for testing. Also, you can use profile-specific configuration to customize spring.flyway.locations so that certain migrations run only when a particular profile is active. For example, in application-dev.properties , you might specify the following setting:

spring.flyway.locations=classpath:/db/migration,classpath:/dev/db/migration

With that setup, migrations in dev/db/migration run only when the dev profile is active.

85.5.2 Execute Liquibase Database Migrations on Startup

To automatically run Liquibase database migrations on startup, add the org.liquibase:liquibase-core to your classpath.

By default, the master change log is read from db/changelog/db.changelog-master.yaml , but you can change the location by setting spring.liquibase.change-log . In addition to YAML, Liquibase also supports JSON, XML, and SQL change log formats.

By default, Liquibase autowires the ( @Primary ) DataSource in your context and uses that for migrations. If you need to use a different DataSource , you can create one and mark its @Bean as @LiquibaseDataSource . If you do so and you want two data sources, remember to create another one and mark it as @Primary . Alternatively, you can use Liquibase’s native DataSource by setting spring.liquibase.[url,user,password] in external properties. Setting either spring.liquibase.url or spring.liquibase.user is sufficient to cause Liquibase to use its own DataSource . If any of the three properties has not be set, the value of its equivalent spring.datasource property will be used.

See LiquibaseProperties for details about available settings such as contexts, the default schema, and others.

There is a Liquibase sample so that you can see how to set things up.