JPA

Introduction to JPA

JPA stands for JAVA PERSISTENCE API.

JPA is an object relational mapping API that makes it easier to

• Extract data from relational databases and put into objects
• Extract data from objects and put into a relational database

JPA is a standard

There are two commonly used ==implementations==

• JBoss Hibernate
• Eclipse Link

Hierarchy

When writing applications to access databases - use JDBC

Then use an ORM Implementation - Hibernate on the top of JDBC

Then use JPA API that sits on the top of the ORM implementation

JPA Entity and Mapping

Further with JPA

APIs

Architecture

The Persistence Unit is the entity class along with the persistence.xml file

The Persistence Context, which is a set of entity instances based upon the persistence unit

The Entity Manager which provides access to a persistence context

Entity Object Lifecycle

Persistence - always related to Database

• New - New entities will be managed if persisted.
• Managed - This is the main state. Any changes made to these managed entities will automatically be saved into database.
• Detached
• Removed

JP-QL

JP-QL stands for Java Persistence Query Language.

SELECT e FROM CompactDisc e WHERE e.artist = ‘Abba’

Named Queries cached -

@NamedQuery(name=”findAllCDs”, 
query=”SELECT e FROM CompactDisc e”) 
Query query = em.createNamedQuery(“findAllCDs”);