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Introducing EclipseLink

06.30.2008
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The Eclipse Persistence Services Project, more commonly known as EclipseLink, is a comprehensive open source persistence solution.  EclipseLink was started by a donation of the full source code and test suites of Oracle's TopLink product. This project brings the experience of over 12 years of commercial usage and feature development to the entire Java community. This evolution into an open source project is now complete and developers will soon have access to the EclipseLink 1.0 release.

We describe EclipseLink as a comprehensive solution because it delivers not one, but a set of persistence services enabling developers to efficiently develop applications that access data in a variety of data sources and formats.  EclipseLink's services currently include object-relational with JPA, object-XML binding in MOXy (with support for JAXB), and a Service Data Objects (SDO) implementation sharing a common mapping core. EclipseLink’s most popular persistence service is dealing with relational databases through JPA. In this article I'll introduce the EclipseLink JPA implementation and some of its advanced features.  We'll delve into the other persistence services in future articles.


JPA and Beyond

EclipseLink 1.0 supports JPA 1.0 and also offers many advanced features. The project's intent is to deliver a standards based solution focussed on JPA but with the abilitity to use advanced features for those applications where they are required. By focusing first on JPA, the EclipseLink project enables broad integration and minimizes coupling. With the recent announcement that EclipseLink will be delivering the reference implementation of JPA 2.0 (JSR 317) the project team will continue to lead the standardization of many of the advanced features. 

Advanced object-relational mappings in EclipseLink JPA offers greater flexibility when dealing with complex or legacy relational schemas. This mapping support has evolved over many years of commercial use dealing with many 'interesting' (i.e., challenging) domain models and relational schemas. The resulting support is supported through JPA where possible and configurable with EclipseLink specific mappings when no cooresponding features exists in JPA.  As mentioned above, some advanced features are making their way into JPA 2.0 and over time you can expect the use of implementation specific features to decline as the specification evolves.


Converters

Converters allow developers to customize how database values are converted into the domain model and how these values are then written back into the database. This supports user-defined types as well as custom conversion logic that can be used with any mapping. Converters are defined once for a persistence unit and can then be used in any entity through their unique name. All converters implement EclipseLink's Converter interface.

@Converter(name="money-converter", converterClass=mypackage.MyMoneyConverter.class)

...

@Convert("money-converter")
private Money balance;

The above example shows how a developer can write their own converter class and then define it under a user provided name.  Once defined, a Converter can be attached to any number of mappings through the @Convert annotation or in an XML mapping file. EclipseLink also provides out of the box converters for handing primitive value and type conversions and some database specific data types.

Published at DZone with permission of its author, Doug Clarke.

(Note: Opinions expressed in this article and its replies are the opinions of their respective authors and not those of DZone, Inc.)

Comments

Vijay Nair replied on Tue, 2008/07/01 - 12:10pm

HI, Does this support out-of the box integration with Coherence as the second level cache or even ehCache ?

 

Didnt see any property setting for that anywhere in the examples

 

Tks..VJ 

Doug Clarke replied on Tue, 2008/07/01 - 3:10pm

VJ,

EclipseLink 1.0 has added cache interceptors to enable us to leverage grid caching solutions such as Oracle's Coherence. The actual implementation for Coherence is not available yet but is being worked on and we hope to attract other grid vendors to integrate their solutions as well. The approach taken allows you to leverage the grid as more then just a simple data cache. You should be able to leverage its distributed data porocessing capabilities for query execution and concurrency protection.

For the 1.1 release I hope to have an implementation that iullustrates how an alternate caching solution such as ehCache could be used. If interested in learning more please post a question to the users mailing list or newsgroup and we can get you connected with the developers working on the solution and the technical details necessary to use it.

I also recommend looking at our cache coordination support that is offerred out of the box. It addresses minimizing stale data in the cache in a clustered deployment.

Doug 

 

Sudhakar Ramasamy replied on Thu, 2008/07/03 - 3:12pm

From what I've read so far I haven't been able to figure out if JPA 2.0 will have a Criteria API.

Will EclipseLink 1.0 have a Criteria API or will it be included in a future release of EclipseLink as part of the JPA 2.0 implementation or as an extension?

 sud

Gordon Yorke replied on Fri, 2008/07/04 - 8:41am

As mentioned in the Early Draft a critera API for JPA 2.0 is planned.  Some of the API should be in the next Draft of the specification.  I would rather not comment on the structure of the criteria API right now as the definition is in the very early stages.

--Gordon 

Mike Keith replied on Fri, 2008/07/04 - 8:44am in response to: Sudhakar Ramasamy

Sud,

JPA 2.0 will have a Java-based expression language (sometimes called a "criteria API").  It will not be as powerful as the existing native TopLink expression API, but will offer all of the current functionality in JP QL, plus more as possible and appropriate. Although EclipseLink already includes support for a Java expression/criteria API, it will also include the new JPA 2.0 API once we have fully specified it. The EclipseLink API will continue to be offered, though, so you can use that now and migrate to the JPA version when it is specified and implemented.

-Mike

Donny A. Wijaya replied on Tue, 2008/07/08 - 10:33pm

I wonder if these annotations would be available via XML mappings? Would they (the annotations) become part of JPA standard or just part of EclipseLink? But anyway the EclipseLink looks great, StoredProc and Converter are what I want to see in the next JPA

Doug Clarke replied on Wed, 2008/07/09 - 4:37am in response to: Donny A. Wijaya

Donny,

Yes, all of the advanced features configured using annotations can also be done using XML.

For the example provided in the article I can have an EclipseLink-ORM.XML file containing:

    <converter name="money-converter" class="mypackage.MoneyConverter"/>

Then when I wish to use it on a mapping:

            <basic name="balance">
<convert>money-converter</convert>
</basic>

In EclipseLink 1.0 these XML files can be used in place of the standard JPA ones or as overrides with just the advanced features specified to enable greater portability between vendors.

It is our goal to continue to drive the advanced features we have added based on user requirements back into future versions of the standards. I will leave it to our expert group memebers to comment on the specifics but you can also get more involved by providing feedback directly to the expert group and reviewing all of the drafts that they publish (http://www.jcp.org/en/jsr/detail?id=317).

Doug

Sudhakar Ramasamy replied on Thu, 2008/07/10 - 2:32pm

Thanks for the response about the Criteria API. Can you shed some light on the integration with Teneo - the EMF to JPA persistence framework.

I'm investigating the feasibilty of an EMF - JFace Databinding - Teneo - EclipseLink - Derby stack for a rich client application and anything that makes this seamless/painless would be very helpful.

-sud 

kalle pallo replied on Tue, 2009/10/27 - 2:01pm

Advanced object-relational mappings in EclipseLink JPA offers greater flexibility when dealing with complex or legacy relational schemas. This mapping support has evolved over many years of commercial use dealing with many 'interesting' (i.e., challenging) domain models and relational schemas. hampaiden valkaisu kotona

Manish Chowdhary replied on Thu, 2012/03/22 - 8:12pm

Anyone have any recent links regarding EclipseLink that they can share? GoECart

Jason Bourne replied on Tue, 2012/10/23 - 2:04am

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