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236 APPENDIX A n MORE ADVANCED FEATURES APPENDIX A n MORE ADVANCED FEATURES manager.close(); factory.close(); } } While the configuration of an EJB 3 application server falls well outside the scope of this book (which is a shame, because the topic is interesting see Pro EJB: Java Persistence API, by Mike Keith and Merrick Schincariol (Apress, 2006), for a good introduction to the subject), the use of an EntityManager deployed into an EJB 3 application server is straightforward. Typically in such an environment, the container manages the EntityManager. Listing A-4 demonstrates how to obtain a reference to an EntityManager in such an environment only very simple changes would be necessary in Listing A-3 to support this. Note that in this environment, there is no need to interact with the EntityManagerFactory the container manages the appropriate interaction with the factory in a way that is transparent to the user code. Listing A-4. Obtaining an EntityManager from the Container by Injection public class Ejb3Example { @PersistenceContext(unitName=”sampleManager”,type=EXTENDED) EntityManager manager; // } As Listing A-4 demonstrates, the combination of container-managed EntityManagers, annotations, and resource injection makes the acquisition of an EntityManagerobject trivially simple (and remember, the EntityManageris essentially the same as a Hibernate Sessionobject). Hibernate provides a couple of additional features to facilitate the transition of Hibernate 3 code to EJB 3. Where your application uses a Configuration (or an AnnotationConfiguration) object to programmatically configure the Hibernate application, there is now an alternative Ejb3Configuration class that can be used in a similar manner to provide the configuration information for the EJB 3 objects without the need for a configuration.xml file. The element of the Hibernate Tools Ant task conversely allows the configuration of the tasks to be drawn from the classpath s META-INF/configuration.xml file, instead of from an explicitly identified Hibernate configuration or properties file. Managed Versioning and Optimistic Locking While we have saved versioning for this appendix s discussion of advanced features, it is actually quite straightforward to understand and apply. Consider the following scenario: Client A loads and edits a record. Client B loads and edits the same record. Client A commits its edited record data. Client B commits its differently edited record data. While the scenario is simple, the problems it presents are not. If Client A establishes a transaction, then Client B may not be able to load and edit the same record. Yet in a web
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