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Switching to Hibernate interfaces
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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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Starting a Java Persistence project
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Object/relational mapping metadata, but here we want to show how you can use a Hibernate API in your JPA application, when needed. Obviously, importing a Hibernate API into your code makes porting the code to a different JPA provider more difficult. Hence, it becomes critically important to isolate these parts of your code properly, or at least to document why and when you used a native Hibernate feature. You can fall back to Hibernate APIs from their equivalent JPA interfaces and get, for example, a Configuration, a SessionFactory, and even a Session whenever needed. For example, instead of creating an EntityManagerFactory with the Persistence static class, you can use a Hibernate Ejb3Configuration:
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Ejb3Configuration cfg = new Ejb3Configuration(); EntityManagerFactory emf = cfg.configure("/custom/hibernate.cfg.xml") .setProperty("hibernate.show_sql", "false") .setInterceptor( new MyInterceptor() ) .addAnnotatedClass( hello.Message.class ) .addResource( "/Foo.hbm.xml") .buildEntityManagerFactory(); AnnotationConfiguration hibCfg = cfg.getHibernateConfiguration();
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The Ejb3Configuration is a new interface that duplicates the regular Hibernate Configuration instead of extending it (this is an implementation detail). This means you can get a plain AnnotationConfiguration object from an Ejb3Configuration, for example, and pass it to a SchemaExport instance programmatically. The SessionFactory interface is useful if you need programmatic control over the second-level cache regions. You can get a SessionFactory by casting the EntityManagerFactory first:
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HibernateEntityManagerFactory hibEMF = (HibernateEntityManagerFactory) emf; SessionFactory sf = hibEMF.getSessionFactory();
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The same technique can be applied to get a Session from an EntityManager:
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HibernateEntityManager hibEM = (HibernateEntityManager) em; Session session = hibEM.getSession();
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This isn t the only way to get a native API from the standardized EntityManager. The JPA specification supports a getDelegate() method that returns the underlying implementation:
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Starting a project
Session session = (Session) entityManager.getDelegate();
Or you can get a Session injected into an EJB component (although this only works in the JBoss Application Server):
@Stateless public class MessageHandlerBean implements MessageHandler { @PersistenceContext Session session; ... }
In rare cases, you can fall back to plain JDBC interfaces from the Hibernate Session:
Connection jdbcConnection = session.connection();
This last option comes with some caveats: You aren t allowed to close the JDBC Connection you get from Hibernate this happens automatically. The exception to this rule is that in an environment that relies on aggressive connection releases, which means in a JTA or CMT environment, you have to close the returned connection in application code. A better and safer way to access a JDBC connection directly is through resource injection in a Java EE 5.0. Annotate a field or setter method in an EJB, an EJB listener, a servlet, a servlet filter, or even a JavaServer Faces backing bean, like this:
@Resource(mappedName="java:/HelloWorldDS") DataSource ds;
So far, we ve assumed that you work on a new Hibernate or JPA project that involves no legacy application code or existing database schema. We now switch perspectives and consider a development process that is bottom-up. In such a scenario, you probably want to automatically reverse-engineer artifacts from an existing database schema.
Reverse engineering a legacy database
Your first step when mapping a legacy database likely involves an automatic reverse-engineering procedure. After all, an entity schema already exists in your database system. To make this easier, Hibernate has a set of tools that can read a schema and produce various artifacts from this metadata, including XML mapping files and Java source code. All of this is template-based, so many customizations are possible. You can control the reverse-engineering process with tools and tasks in your Ant build. The HibernateToolTask you used earlier to export SQL DDL from
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