CHAPTER 2 Starting a project This completes our
CHAPTER 2 Starting a project This completes our first example with managed EJB components and integrated JPA. You can probably already see how automatic transaction demarcation and EntityManager injection can improve the readability of your code. Later, we ll show you how stateful session beans can help you implement sophisticated conversations between the user and the application, with transactional semantics. Furthermore, the EJB components don t contain any unnecessary glue code or infrastructure methods, and they re fully reusable, portable, and executable in any EJB 3.0 container. NOTE Packaging of persistence units We didn t talk much about the packaging of persistence units you didn t need to package the Hello World example for any of the deployments. However, if you want to use features such as hot redeployment on a full application server, you need to package your application correctly. This includes the usual combination of JARs, WARs, EJB-JARs, and EARs. Deployment and packaging is often also vendor-specific, so you should consult the documentation of your application server for more information. JPA persistence units can be scoped to JARs, WARs, and EJB-JARs, which means that one or several of these archives contains all the annotated classes and a META-INF/persistence. xml configuration file with all settings for this particular unit. You can wrap one or several JARs, WARs, and EJB-JARs in a single enterprise application archive, an EAR. Your application server should correctly detect all persistence units and create the necessary factories automatically. With a unit name attribute on the @PersistenceContext annotation, you instruct the container to inject an EntityManager from a particular unit. Full portability of an application isn t often a primary reason to use JPA or EJB 3.0. After all, you made a decision to use Hibernate as your JPA persistence provider. Let s look at how you can fall back and use a Hibernate native feature from time to time. 2.2.4 Switching to Hibernate interfaces You decided to use Hibernate as a JPA persistence provider for several reasons: First, Hibernate is a good JPA implementation that provides many options that don t affect your code. For example, you can enable the Hibernate second-level data cache in your JPA configuration, and transparently improve the performance and scalability of your application without touching any code. Second, you can use native Hibernate mappings or APIs when needed. We discuss the mixing of mappings (especially annotations) in chapter 3, section 3.3,
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