Wednesday, November 20, 2013

I understand that JPA 2 is a specification and Hibernate is a tool for ORM. Also, I understand that Hibernate has more features than JPA 2. But from a practical point of view, what really is the difference?
I have experience using iBatis and now I'm trying to learn either Hibernate or JPA2. I picked up Pro JPA2 book and it keeps referring to "JPA provider". For example:
If you think a feature should be standardized, you should speak up and request it from your JPA provider
This confuses me so I have a few questions:
  • Using JPA2 alone can I fetch data from DB by simply annotating my POJO's
  • Is JPA2 supposed to be used with a "JPA Provider" e.g TopLink or Hibernate? If so, then what's the benefit of using JPA2 + Hibernate as compared to Hibernate alone?
  • This might be repetition of other questions on SO but can you recommend a good practical JPA2 book. "Pro JPA2" seems more like a bible and reference on JPA2 (It doesn't get into Queries until later half of the book). Is there a book that takes a problem/solution approach to JPA2?
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3 Answers

As you state JPA is just a specification, meaning there is no implementation. You can annotate your classes as much as you would like with JPA annotations, however without an implementation nothing will happen. Think of JPA as the guidelines that must be followed or an interface, while Hibernate's JPA implementation is code that meets the API as defined by the JPA specification and provides the under the hood functionality.
When you use Hibernate with JPA you are actually using the Hibernate JPA implementation. The benefit of this is that you can swap out Hibernate's implementation of JPA for another implementation of the JPA specification. When you use straight Hibernate your locking into the implementation because other ORMs may use different methods/configurations and annotations, therefore you cannot just switch over to another ORM.
For a more detailed explanation of this answer read my blog entry.
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From the Wiki.
Motivation for creating the Java Persistence API
Many enterprise Java developers use lightweight persistent objects provided by open-source frameworks or Data Access Objects instead of entity beans: entity beans and enterprise beans had a reputation of being too heavyweight and complicated, and one could only use them in Java EE application servers. Many of the features of the third-party persistence frameworks were incorporated into the Java Persistence API, and as of 2006 projects like Hibernate (version 3.2) and Open-Source Version TopLink Essentials have become implementations of the Java Persistence API.
As told in the JCP page the Eclipse link is the Reference Implementation for JPA. Have look at this answer for bit more on this.
JPA itself has features that will make up for a standard ORM framework. Since JPA is a part of Java EE spec, you can use JPA alone in a project and it should work with any Java EE compatible Servers. Yes, these servers will have the implementations for the JPA spec.
Hibernate is the most popular ORM framework, once the JPA got introduced hibernate conforms to theJPA specifications. Apart from the basic set of specification that it should follow hibernate provides whole lot of additional stuff.
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