JPA and multiple persistence units

I have JPA/EJB3 @Entities in two separate jars that are loaded by my webapp. As strange as this sounds, it's not that easy to define ORM entities in more than one jar file and have your application load them. It seems this is not a common requirement, yet for a really modular application, it seems obvious to me that you might do this.

One supposedly standard way to solve this is to use the jar-file tag in persistence.xml. But it doesn't solve the problem because you have to use absolute URLs, which is not workable for production.

I found the solution in this post, which was to override persistence unit processing. In addition to the following code, I had to set up my Spring configuration in a particular way to make it all work properly:

import org.springframework.orm.jpa.persistenceunit.MutablePersistenceUnitInfo;
import org.springframework.orm.jpa.persistenceunit.PersistenceUnitPostProcessor;

/**
* This merges all JPA entities from multiple jars. To use it, all entities must
* be listed in their respective persistence.xml files using the <class> tag.
*
* @see http://forum.springsource.org/showthread.php?t=61763
*/
public class MergingPersistenceUnitPostProcessor implements PersistenceUnitPostProcessor
{
 Map<String, List<String>> puiClasses = new HashMap<String, List<String>>();

 public void postProcessPersistenceUnitInfo(MutablePersistenceUnitInfo pui)
 {
  List<String> classes = puiClasses.get(pui.getPersistenceUnitName());
  if (classes == null)
  {
   classes = new ArrayList<String>();
   puiClasses.put(pui.getPersistenceUnitName(), classes);
  }
  pui.getManagedClassNames().addAll(classes);

  final List<String> names = pui.getManagedClassNames();
  classes.addAll(pui.getManagedClassNames());
 }
}

To use it, I found that this only works if do all of the following:

• All your entities need to be mentioned in your persistence.xml files. Normally you don't have to do this is if you set up entity scanning in your Spring config
  
• All your persistence units use the same name, and furthermore it has to be the default name, namely "default"
• You have to use a particular entity manager factory, namely LocalContainerEntityManagerFactoryBean
• You have to have define your persistence provider in persistence.xml, in our case org.hibernate.ejb.HibernatePersistence
• Each persistence.xml file needs a unique name, because they are enumerated in the application context.
  

Here is an example of a persistence.xml:

<?xml version="1.0" encoding="UTF-8"?>
<persistence version="1.0"
 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 http://java.sun.com/xml/ns/persistence/persistence_1_0.xsd">
 <persistence-unit name="default" transaction-type="RESOURCE_LOCAL">
 
  <provider>org.hibernate.ejb.HibernatePersistence</provider>
  
  <!-- Entities have to be enumerated here for MergingPersistenceUnitPostProcessor -->
  <class>com.my.Foo</class>
...

Here is an excerpt from my application context. The whole point is that I can now have as many persistence.xml files as I like, in different jar files:

<bean id="pum"
 class="org.springframework.orm.jpa.persistenceunit.DefaultPersistenceUnitManager">
 <property name="persistenceXmlLocations">
  <list>
   <value>classpath*:META-INF/persistence1.xml</value>
   <value>classpath*:META-INF/persistence2.xml</value>
  </list>
 </property>
 <property name="persistenceUnitPostProcessors">
       <bean class="com.my.MergingPersistenceUnitPostProcessor"/>
 </property>
</bean>
<bean id="entityManagerFactory" class="org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean">
 <property name="persistenceUnitManager" ref="pum"/>
</bean>