How does spring.jpa.hibernate.ddl-auto property exactly work in Spring?

For the record, the spring.jpa.hibernate.ddl-auto property is Spring Data JPA specific and is their way to specify a value that will eventually be passed to Hibernate under the property it knows, hibernate.hbm2ddl.auto.

The values create, create-drop, validate, and update basically influence how the schema tool management will manipulate the database schema at startup.

For example, the update operation will query the JDBC driver's API to get the database metadata and then Hibernate compares the object model it creates based on reading your annotated classes or HBM XML mappings and will attempt to adjust the schema on-the-fly.

The update operation for example will attempt to add new columns, constraints, etc but will never remove a column or constraint that may have existed previously but no longer does as part of the object model from a prior run.

Typically in test case scenarios, you'll likely use create-drop so that you create your schema, your test case adds some mock data, you run your tests, and then during the test case cleanup, the schema objects are dropped, leaving an empty database.

In development, it's often common to see developers use update to automatically modify the schema to add new additions upon restart. But again understand, this does not remove a column or constraint that may exist from previous executions that is no longer necessary.

In production, it's often highly recommended you use none or simply don't specify this property. That is because it's common practice for DBAs to review migration scripts for database changes, particularly if your database is shared across multiple services and applications.

In Spring/Spring-Boot, SQL database can be initialized in different ways depending on what your stack is.

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.

Hibernate property values are: create, update, create-drop, validate and none:

• create – Hibernate first drops existing tables, then creates new tables
• update – the object model created based on the mappings (annotations or XML) is compared with the existing schema, and then Hibernate updates the schema according to the diff. It never deletes the existing tables or columns even if they are no more required by the application
• create-drop – similar to create, with the addition that Hibernate will drop the database after all operations are completed. Typically used for unit testing
• validate – Hibernate only validates whether the tables and columns exist, otherwise it throws an exception
• none – this value effectively turns off the DDL generation

Spring Boot internally defaults this parameter value to create-drop if no schema manager has been detected, otherwise none for all other cases.

"spring.jpa.hibernate.ddl-auto= create-drop" means that when the server is run, the database(table) instance is created. And whenever the server stops, the database table instance is droped.