78. Database initialization
An SQL database can be initialized in different ways depending on what your stack is. Or of course you can do it manually as long as the database is a separate process.
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. See below for more detail.
You can set spring.jpa.hibernate.ddl-auto explicitly and the standard Hibernate property values are none, validate, update, create, create-drop. Spring Boot chooses a default value for you based on whether it thinks your database is embedded (default create-drop) or not (default none). An embedded database is detected by looking at the Connection type: hsqldb, h2 and derby are embedded, the rest are not. Be careful when switching from in-memory to a ‘real’ database that you don’t 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]](/images/spring-boot/1.5.9.RELEASE/note.png) |
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You can output the schema creation by enabling the |
In addition, a file named import.sql in the root of the classpath will be 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 probably not something you want to be on the classpath in production. It is a Hibernate feature (nothing to do with Spring).
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 will process 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, e.g. you might choose to set it to the vendor name of the database (hsqldb, h2, oracle, mysql, postgresql etc.).
Spring Boot enables the fail-fast feature of the Spring JDBC initializer by default, so if the scripts cause exceptions the application will fail to start. You can tune that using spring.datasource.continue-on-error.
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In a JPA-based app, you can choose to let Hibernate create the schema or use |
You can also disable initialization by setting spring.datasource.initialize to false.
If you are using Spring Batch then it comes pre-packaged with SQL initialization scripts for most popular database platforms. Spring Boot will detect your database type, and execute those scripts by default, and in this case will switch the fail fast setting to false (errors are logged but do not prevent the application from starting). This is because the scripts are known to be reliable and generally do not contain bugs, so errors are ignorable, and ignoring them makes the scripts idempotent. You can switch off the initialization explicitly using spring.batch.initializer.enabled=false.
Spring Boot supports two higher-level migration tools: Flyway and Liquibase .
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, e.g. ‘1’ or ‘2_1’). By default they live in a folder classpath:db/migration but you can modify that using flyway.locations. You can also add a special {vendor} placeholder to use vendor-specific scripts. Assume the following:
flyway.locations=db/migration/{vendor}Rather than using db/migration, this configuration will set the folder to use according to the type of the database (i.e. db/migration/mysql for MySQL). The list of supported database are available in DatabaseDriver .
See also the Flyway class from flyway-core for details of available settings like schemas etc. In addition Spring Boot provides a small set of properties in FlywayProperties that can be used to disable the migrations, or switch off the location checking. Spring Boot will call Flyway.migrate() to perform the database migration. If you would like more control, provide a @Bean that implements FlywayMigrationStrategy .
![[Tip]](/images/spring-boot/1.5.9.RELEASE/tip.png) |
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If you want to make use of Flyway callbacks , those scripts should also live in the |
By default Flyway will autowire the (@Primary) DataSource in your context and use that for migrations. If you like to use a different DataSource you can create one and mark its @Bean as @FlywayDataSource - if you do that remember to create another one and mark it as @Primary if you want two data sources. Or you can use Flyway’s native DataSource by setting flyway.[url,user,password] in external properties.
There is a Flyway sample so 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 will only be run when your application starts for testing. If you want to be more sophisticated you can use profile-specific configuration to customize flyway.locations so that certain migrations will only run when a particular profile is active. For example, in application-dev.properties you could set flyway.locations to classpath:/db/migration, classpath:/dev/db/migration and migrations in dev/db/migration will only run when the dev profile is active.
To automatically run Liquibase database migrations on startup, add the org.liquibase:liquibase-core to your classpath.
The master change log is by default read from db/changelog/db.changelog-master.yaml but can be set using liquibase.change-log. In addition to YAML, Liquibase also supports JSON, XML, and SQL change log formats.
By default Liquibase will autowire the (@Primary) DataSource in your context and use that for migrations. If you like to use a different DataSource you can create one and mark its @Bean as @LiquibaseDataSource - if you do that remember to create another one and mark it as @Primary if you want two data sources. Or you can use Liquibase’s native DataSource by setting liquibase.[url,user,password] in external properties.
See LiquibaseProperties for details of available settings like contexts, default schema etc.
There is a Liquibase sample so you can see how to set things up.