Why does Hibernate re-implement functionality that databases already have?

Question

For example:

@Table(name = "stock", catalog = "mkyong", uniqueConstraints = {
@UniqueConstraint(columnNames = "STOCK_NAME"),
@UniqueConstraint(columnNames = "STOCK_CODE") })

or

@Column(name = "STOCK_NAME", unique = true, nullable = false, length = 20)

Constraints like 'unique', 'nullable', even field length are core database features. Why include this here? Also (although this may hurt some) I'd also wager that a database's implementation of such constraints, particularly mainstream commercial DBs like Oracle, is probably better than whatever the OSS Hibernate devs can come up with.

Is it wise to use this type of stuff in Hibernate, or is it a better practice to put constraints and such in the database? It seems that if you utilize these Hibernate features, you're practically treating the database as a file system, so what's the point? Usage of this is everywhere but I've yet to find the documentation explaining why you'd do this.

Answer 1

Hibernate can create a database schema based on those annotations for you.

Answer 2

It does not implement them - it has the option to validate the data model against the schema, or create it.

The hibernate.hbm2ddl.auto configuration property is the one that allows you to create the schema based on the mappings.

Automatically validates or exports schema DDL to the database when the SessionFactory is created. With create-drop, the database schema will be dropped when the SessionFactory is closed explicitly.

e.g. validate | update | create | create-drop

This is very useful, if you want your data model to be in the central place, rather than the database structure