What is a JPA implementation?

Question

I\'m getting started with JPA, and I\'m confused as to what exactly the JPA implementation(EclipseLink, TopLink, Hibernate, etc.) does.

I understand the theoretica

Answer 1

It's the same as Java, it has a specification (Java SE, Java EE) and implementation, most people use the reference implementation (By Sun / Oracle) which is a blueprint for others to make their own "better" and "advanced" implementation of the specification (APIs, Interfaces and documentation / JavaDoc if you need a one word definition, although it is much more than that)

So the Java / JDK you are most likely using is just an implementation (that is very popular). There are many more implementations out there, some have more Garbage Collection configuration options, some claim to be faster, and some are just not worth using.

So the implementation basically does everything, it has the code, where the JPA is an API (implied by it's name, Java Persistance API) e.g. again, more about interfaces and documentation than real implentation of the interface.

Think of it as the abstact class that Hibernate's JPA implementation / EclipseLink extends / implements

Regarding JPA, as opposed to Hibernate which has a single implementation, the idea is to decouple the interface from the implementation, so that the "best" implementation will win and that everyone can participate in the game, not just the interface creator.

This allows for instance for Google App Engine to support JPA, as it just needs to follow an API (DataNucleus is the implementation) if it has to use Hibernate, it would have required to modify the Hiberanate release to support Google's Big Table format (the fact that JPA in GAE is not that great is another story)