Running Hibernate in an Eclipse plugin-based environment - eclipse

Question

Lately, I have been trying to use Hibernate as O-R-Mapper for a project based on Eclipse bundles.
Because of the unique class-loading of Eclipse-bundles, many people advise using Eclipselink instead of Hibernate.
Having tried Eclipselink and being not quite satisfied with it, I do want to know:
Isn't there a way to get Hibernate up and running in my Eclipse Plug-In Project?

Answer 1

Here is a small walk-through of how I got it to work. Please feel free to ask questions and post suggestions on how to improve this:
Download Hibernate 4.2.5 or newer, which comes with OSGi-Support (see Hibernate OSGi Documentation). However, the examples there use Apache Felix as OSGi-implementation and not equinox.
Create a new Plug-In Project from existing jar-archives.
In my case, I added the following jars:
hibernate-core-4.2.5
hibernate-osgi-4.2.5
hibernate-commons-annotations-4.0.2 (i am using annotations)
hibernate-jpa-2.0 (i am using the java persistence api for more flexibility)
hibernate-entitymanager-4.2.5 (also the more generic jpa entitymanager instead of hibernates session)
org.osgi.core-4.3.1 (for the osgi classes)
jboss-logging
jboss-transaction-api
dom4j-1.6.1
antlr-2.7.7
Open the MANIFEST.MF of the project and add the following:
Bundle-Activator: org.hibernate.osgi.HibernateBundleActivator (this is hibernate's bundle activator from the hibernate-osgi bundle)
Bundle-ActivationPolicy: lazy (so that osgi passes the context to the bundle once it is activated)
Eclipse-BuddyPolicy: registered (we need this later to make our entity classes known to hibernate and vice versa)
Also make sure all your jars are on the Bundle-Classpath and all packages of the plug-in are exported.
Now, create a new plug-in project for your hibernate configuration and DAO.
Put your persistence configuration file (persistence.xml or hibernate.cfg.xml) in the META-INF folder at the root of your plugin. Here is an example for the persistence.xml:
<persistence xmlns="http://java.sun.com/xml/ns/persistence"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/persistence persistence_2_1.xsd"
version="1.0">
<persistence-unit name="TheNameOfMyPersistenceUnit" transaction-type="RESOURCE_LOCAL">
<description>My Persistence Unit</description>
<provider>org.hibernate.ejb.HibernatePersistence</provider>
<class>de.eicher.jonas.SomeClass</class>
<class>de.eicher.jonas.AnotherClass</class>
<properties>
<property name="hibernate.dialect" value="org.hibernate.dialect.DerbyDialect"/>
<property name="hibernate.connection.url" value="jdbc:derby:C:/Temp/data;create=true"/>
<property name="hibernate.connection.driver_class" value="org.apache.derby.jdbc.EmbeddedDriver"/>
<property name="hibernate.connection.username" value="sa"/>
<property name="hibernate.connection.password" value=""/>
<property name="org.hibernate.FlushMode" value="commit" />
<property name="hibernate.hbm2ddl.auto" value="update" />
<property name="hibernate.current_session_context_class" value="thread"/>
<property name="hibernate.cache.provider_class" value="org.hibernate.cache.NoCacheProvider"/>
<property name="hibernate.show_sql" value="true"/>
</properties>
</persistence-unit>
</persistence>
Add org.eclipse.core.runtime to your dependencies and create an Activator to get static access to the BundleContext:
import org.eclipse.core.runtime.Plugin;
import org.osgi.framework.BundleContext;
public class HibernateJpaActivator extends Plugin {
private static BundleContext context;
#Override
public void start(BundleContext context)
throws Exception {
HibernateJpaActivator.context = context;
}
public static BundleContext getContext() {
return context;
}
}
In your DAO or Util class, use the following code to get the EntityManagerFactory or EntityManager:
BundleContext context = HibernateJpaActivator.getContext();
ServiceReference serviceReference = context.getServiceReference( PersistenceProvider.class.getName() );
PersistenceProvider persistenceProvider = (PersistenceProvider) context.getService( serviceReference );
emf = persistenceProvider.createEntityManagerFactory( "TheNameOfMyPersistenceUnit", null );
EntityManager em = emf.createEntityManager();
Only a few more things to do, before it works:
Open the MANIFEST.MF and make sure that your bundle receives the BundleContext on activation.
Bundle-ActivationPolicy: lazy
Bundle-Activator: my.package.name.HibernateJpaActivator
Open the plug-in containing your entities and add a dependecy to the plugin with your hibernate jars (the first one we created).
Now we also need the entities to be known in the plugin with the hibernate jars. We can't add a dependency there, because this would produce a cyclic dependency. Fortunately, Eclipse provides us with a workaround:
Open the MANIFEST.MF of your entity-bundle and register your hibernate-jar plugin as a buddy:
Eclipse-RegisterBuddy: org.hibernate4.osgi (the name of your hibernate plugin, the one where you set Eclipse-Buddy-Policy: registered)
Now Hibernate knows our classes and our classes know Hibernate. We also made sure, that Hibernate finds our persistence.xml (or hibernate.cfg.xml) and creates our readily configured EntityMangerFactory (or Session).