Java Data Objects: Difference between revisions

Revision as of 02:29, 25 June 2012

Revision as of 13:07, 18 October 2012

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{{No footnotes|date=June 2010}}

'''Java Data Objects''' ('''JDO''') is a specification of [[Java platform|Java]] [[object (computer science)|object]] [[Persistence (computer science)|persistence]]. One of its features is a transparency of the persistentpersistence services to the [[___domain model]]. JDO persistent objects are ordinary [[Java (programming language)|Java programming language]] [[class (computer science)|classes]] ([[Plain Old Java Object|POJOs]]); there's is no requirement for them to implement certain [[interface (Java)|interfaces]] or extend from special classes. JDO 1.0 was developed under the [[Java Community Process]] as [http://www.jcp.org/en/jsr/detail?id=12 JSR 12]. JDO 2.0 was developed under [http://www.jcp.org/en/jsr/detail?id=243 JSR 243] and was released on May 10, 2006. JDO 2.1 was completed in Feb 2008, developed by the [http://db.apache.org/jdo/ Apache JDO] project. JDO 2.2 was released in October 2008. JDO 3.0 was released in April 2010.

Object persistence is defined in the external [[XML]] metafiles, which may have vendor-specific extensions. JDO vendors provide developers with ''enhancers'', which modify compiled Java class files so they can be transparently persisted. (Note that byte-code enhancement is not mandated by the JDO specification, although it is the commonly used mechanism for implementing the JDO specification's requirements.) Currently, JDO vendors offer several options for persistence, e.g. to [[Relational database|RDBMS]], to [[Object database|OODB]], or to [[Computer file|files]].

JDO enhanced classes are portable across different vendors' implementation. Once enhanced, a Java class can be used with any vendor's JDO product.

JDO is integrated with [[Java Platform, Enterprise Edition|Java EE]] in several ways. First of all, the vendor implementation may be provided as a {{clarify-span|JEE Connector}}. Secondly, JDO may work in the context of JEE [[Java transaction service|transaction services]].

== JDO vs. EJB3 vs. JPA ==

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[[Enterprise Java Beans]] 3.0 (EJB3) specification also covered persistence, as had EJB v2 with Entity Beans. There has been standards conflict between the two standards bodies in terms of pre-eminence. JDO has several commercial implementations.

In the end, persistence has been "broken out" of "EJB3 Core", and a new standard formed, the [[Java Persistence API]] (JPA). JPA uses the <code>javax.persistence</code> package, and was first specified in a separate document within the EJB3 spec [http://www.jcp.org/en/jsr/detail?id=220 JSR 220], but was later moved to its own spec [http://www.jcp.org/en/jsr/detail?id=317 JSR 317]. Significantly, <code>javax.persistence</code> will '''not''' require an EJB container, and thus will work within a Java SE environment as well, as JDO always has. JPA, however, is an [[Objectobject-relational mapping]] (ORM) standard, while JDO is both an [[Objectobject-relational mapping]] standard and a transparent object persistence standard. JDO, from an API point of view, is agnostic to the technology of the underlying datastore, whereas JPA is targeted to RDBMS datastores (although there are several JPA providers that support access to non-relational datastores through the JPA API, such as DataNucleus and ObjectDB).

Leading JDO commercial implementations and open source projects also offer a JPA API implementation as an alternative access to their underlying persistence engines, formerly exposed solely via JDO in the original products. There are many open source implementations of JDO.

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== New Features in JDO Version 2 Not Found In Version 1 ==

* Disconnected Objectobject Graphsgraphs concept

* Standardized ORM Mappingmapping Descriptorsdescriptors (for ORM-based JDO Implementationsimplementations)

* JDOQL Enhancements (Single String Form; more String, Map & Math methods support mandated)

* Get e.g. a {{Javadoc:SE|java/sql|Connection}} from <code>javax.jdo.PersistenceManager</code>

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* JDO 1.0 : FOStore

* JDO 2.0 : [[DataNucleus|JPOX]] 1.1

* JDO 2.1 : JPOX 1.2

* JDO 2.2 : [[DataNucleus]] AccessPlatform 1.0.1

* JDO 3.0 : DataNucleus AccessPlatform 2.1.0