Understanding JPA, 1

原文： http://www.javaworld.com/javaworld/jw-01-2008/jw-01-jpa1.html?page=1

Numerous projects have sought to make the datapersistence layer a good object-oriented citizen in Java applicationdevelopment. The Java Persistence API, an outgrowth of JSR 220,collects and standardizes that work. In this first half of a two-partintroduction to the Java Persistence API, learn how JPA tames theprocess of writing code that manipulates data, making it a more naturalpiece of your object-oriented architecture.

Why JPA? Afundamental question for many Java developers is "Why JPA? Why do Ineed to know how to use this API when object-relational mapping toolslike Hibernate and Toplink are already available?" The answer is thatJPA is not a new technology; rather, it has collected the best ideasfrom existing persistence technologies like Hibernate, TopLink, andJDO. The result is a standardized specification that helps you build apersistence layer that is independent of any particular persistenceprovider.

Likeit or not, data is an integral part of any application, even coolobject-oriented ones. Everything stops at the persistence layer, whereJava developers traditionally have had to write complex SQL queries,which can become unmanageable as the application grows. How good itwould be if you could just treat these queries as objects, and applyobject-oriented programming concepts like encapsulation, abstraction,inheritance, and polymorphism to them! With such an abstraction, youcould hide the complexity of the underlying data store.

In fact, the Javacommunity has produced numerous object-oriented approaches to datapersistence: EJB, JDO, Hibernate, and Toplink are all worthy solutionsthat have tackled this problem. The Java Persistence API, or JPA, is astandard persistence API introduced as part of the Java EE 5 platform.The JPA specification was first introduced as part of JSR 220: EJB 3.0, with the goal of simplifying the EJB entity beans programming model. Although it all started with entity beans and is packaged with Java EE 5.0, JPA can be used outside the container in a Java SE environment.

Inthis article, you will see how elegantly data persistence can behandled in an object-oriented manner just with the help of JPAannotations. The article is intended for readers new to JPA whounderstand the basic concepts of relational database management systemsand are familiar with Java 5 annotations. JPA requires Java 5 orhigher, as it makes heavy use of new Java language features such asannotations and generics.

OpenJPA and the example application

Concepts in this article are explained and demonstrated using OpenJPA,an open source JPA implementation from Apache. I chose OpenJPA overother vendor product because it has been integrated with the Weblogic,WebSphere, and Geronimo application servers. The current version ofOpenJPA at the time of writing is 1.0.1; there's a link to the binarydistribution in the Resources section. If you're going to use a different setup, obviously you'll need to check out the documentation first.

Theremainder of this article will explore various object-oriented JPAconcepts with a sample application that implements functionality for asimple development scenario. Imagine there's a superstore called XYZwith both an online and retail customer base. To begin, you will seehow to apply CRUD operations to this customer model using JPA. In thelater sections, you'll see how you can extend the CRUD operations usingobject inheritance -- the JPA way.

The code package for this article includes sample code for entity listeners and all three inheritance types (single table, joined, and table per class) discussed in the article, implemented in the context of the example application.