The Java Persistence API, sometimes referred to as JPA, is a Java programming language framework managing relational data in applications using Java Platform, Standard Edition and Java Platform, Enterprise Edition.

The Java Persistence API originated as part of the work of the JSR 220 Expert Group. JPA 2.0 is the work of the JSR 317 Expert Group.

Persistence in this context covers three areas:

• the API itself, defined in the javax.persistence package
• the Java Persistence Query Language (JPQL)
• object/relational metadata

History

The final release date of the JPA 1.0 specification was 11 May 2006. The JPA 2.0 specification was released 10 Dec, 2009. JPA is a replacement for the much criticized EJB 2.0 and EJB 2.1 entity beans.

Entities

A persistence entity is a lightweight Java class whose state is typically persisted to a table in a relational database. Instances of such an entity correspond to individual rows in the table. Entities typically have relationships with other entities, and these relationships are expressed through object/relational metadata. Object/relational metadata can be specified directly in the entity class file by using annotations, or in a separate XML descriptor file distributed with the application.

The Java Persistence Query Language

The Java Persistence Query Language (JPQL) makes queries against entities stored in a relational database. Queries resemble SQL queries in syntax, but operate against entity objects rather than directly with database tables.

See Wikibooks:Java persistence:JPQL.

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.

Related Technologies

Enterprise JavaBeans

The EJB 3.0 specification (itself part of the Java EE 5 platform) included a definition of the Java Persistence API. However, end-users do not need an EJB container or a Java EE application server in order to run applications that use this persistence API.^[1] Future versions of the Java Persistence API will be defined in a separate JSR and specification rather than in the EJB JSR/specification.

The Java Persistence API replaces the persistence solution of EJB 2.0 CMP (Container Managed Persistence).

Java Data Objects API

The Java Persistence API was developed in part to unify the Java Data Objects API, and the EJB 2.0 Container Managed Persistence (CMP) API. As of 2009 most products supporting each of those APIs support the Java Persistence API.

The Java Persistence API specifies relational persistence (ORM: Object relational mapping) only for RDBMS (although providers exist who support other datastores). The Java Data Objects specification(s) provides relational persistence (ORM), as well as persistence to other types of datastores.

Service Data Object API

The designers^[2] of the Java Persistence API aimed to provide for relational persistence, with many of the key areas taken from object-relational mapping tools such as Hibernate and TopLink. Java Persistence API improved on and replaced EJB 2.0, evidenced by its inclusion in EJB 3.0. The Service Data Objects (SDO) API (JSR 235) has a very different objective to the Java Persistence API and is considered ^[3][4] complementary. The SDO API is designed for service-oriented architectures, multiple data formats rather than only relational data, and multiple programming languages. The Java Community Process manages the Java version of the SDO API; the C++ version of the SDO API is managed via OASIS.

Hibernate

Hibernate provides an open source object-relational mapping framework for Java. Versions 3.2 and later provide an implementation for the Java Persistence API.^[5]

Gavin King founded^[6] Hibernate. He represented JBoss on JSR220,^[7] the JCP expert group charged with developing JPA. This led to ongoing controversy and speculation surrounding the relationship between JPA and Hibernate. Sun Microsystems has stated^[8] that ideas came from several frameworks, including Hibernate and JDO.

JPA 2.0

Development of a new version of JPA, namely JPA 2.0 JSR 317 was started in July 2007. JPA 2.0 was approved as final on December 10, 2009.

The focus of JPA 2.0 was to address features that were present in some of the popular ORM vendors but couldn't gain consensus approval for JPA 1.0.

The main features included in this update are:

• Expanded object/relational mapping functionality
  • support for collections of embedded objects
  • multiple levels of embedded objects
  • ordered lists
  • combinations of access types
• A criteria query API
• standardization of query 'hints'
• standardization of additional metadata to support DDL generation
• support for validation

Vendors supporting JPA 2.0

• DataNucleus (formerly JPOX)
• EclipseLink (formerly Oracle TopLink)
• JBoss Hibernate
• ObjectDB
• OpenJPA
• IBM, via its OpenJPA-based Feature Pack for OSGi Applications and JPA 2.0 for WebSphere Application Server

See also

• JDBC
• JPQL
• iBATIS
• pureQuery
• OpenXava
• SAP NetWeaver Application Server
• JOPS
• .NET Persistence API (NPA)

References

External links

General info

• JCP Persistence page
• GlassFish's Persistence page
• Documentation for the final version of the EJB3 spec (called JSR220)

Documentation

• Persistence in the Java EE 5

Tutorials

• Java EE 6 Persistence API tutorial
• Java EE 6 Persistence API Javadoc
• Hibernate EntityManager User guide
• Hibernate Javadoc: org.hibernate.ejb
• EclipseLink JPA tutorials for Application Servers