Java EE APIs operate in the "Convention over Configuration" or "Configuration by Exception" mode:
- APIs ship with suitable defaults
- Defaults can be overridden / supplemented via annotations
- Annotations can be overridden / supplemented via XML descriptors
- XML descriptors can be overridden via dedicated extension points, hooks (not available for all APIs)
JPA entities in particular can be deployed with minimalistic set (@Entity / @ID) of annotations first and overridden on demand:
"XML metadata may be used as an alternative to these annotations, or to override or augment annotations (...)"[Chapter 11, Page 421, JSR-388: Java Persistence 2.1]
In JPA, supplementing, or even replacing annotations with orm.xml eases realization of following use cases:
- Product development: generic mappings can be partially adjusted during installation without recompiling the code
- Data migrations: entities are read from one persistence unit fully relying on annotations and writing to a persistence unit augmented by orm.xml
- Integration of external libraries / third party code: JavaBeans without existing source code can be augmented with orm.xml and turned into entities
- Separated read / write databases: a dedicated
EntityManageris used for reading, another for writing. The database instances used behind the
EntityManagermay require mapping adjustments
However: premature configuration is the root of (all) evil. Most of the Java EE projects come without any configuration or XML and they fully rely on conventions "salted" with only a few annotations.
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