Primer for OpenJPA-优快云博客

OpenJPA is the open source implementation of Java Persistence API. It has been integrated with the Weblogic, WebSphere, and Geronimo application servers. JPA provides an agnostic Java-based API for storing and retrieving information to a backend database. It has a canonical query language named Java Persistence Query Language, or JPQL, that blends with the programming methods of Java and eliminates the need to tailor database queries for a particular database. However, JPA also supports native SQL which can be used for quick ports with a known backend database.

You'll need three artifacts to implement a JPA-compliant program:

An entity class
A persistence.xml file
A class through which you will insert, update, or find an entity

Since JPA is all about entity-relationship mapping, need to take a look at the design of the entity object. The entity object is nothing but a POJO class marked as an entity with the @Entity annotation.

JPA allows persistent classes to inherit from non-persistent classes, persistent classes to inherit from other persistent classes, and non-persistent classes to inherit from persistent classes.
The entity class should have a default no-argument constructor.
The entity class should not be final.
Persistent classes cannot inherit from certain natively-implemented system classes such asjava.net.Socketandjava.lang.Thread.
If a persistent class inherits from a non-persistent class, the fields of the non-persistent super class cannot be persisted.

JPA provides callback methods for performing actions at different stages of persistence operations. JPA provides listeners for these kinds of activities before and after each fetch, insert, or update operation. The callback methods can be annotated as any of the following: