Q:

How do I connect to multiple databases using JPA?

I have an application using Java servlets/JSP's. There are multiple clients using my app, however each client has a separate database. All the databases have the same schema. I would like to determine which database connection to use at the time when a user logs into the system. 
For example client A logs in, I determine that client A belongs to database C, grab the connection for database C and continue on my merry way.
I am using JPA with Hibernate as my JPA provider. Is it possible to do this using multiple persistence units and determining which unit to use at login time? Is there a better/preferred way to do this?
Edited to add: 
I am using annotations and EJB's so the Persistence Context is being set in the EJB with @PersistenceContext(unitName = "blahblah"), can this be determined at login time? Can I change the unitName at runtime?
Thanks

A:

1) Create several persistent units in your persistence.xml with different names.
2) Create necessary number of EntityManagerFactorys (1 per persistence-unit) and specify which persistence-unit should be used for concrete factory:
<bean id="authEntityManagerFactory" class="org.springframework.orm.jpa.LocalEntityManagerFactoryBean">
   <property name="persistenceUnitName" value="SpringSecurityManager"/>
</bean>

3) Create necessary number of TransactionManager s:
<bean id="authTransactionManager" class="org.springframework.orm.jpa.JpaTransactionManager">
   <property name="entityManagerFactory" ref="authEntityManagerFactory" />
</bean>

4) In your DAO's classes specify with which persistence-unit (and so with which EntityManagerFactory) you want to work:
public class AbstractAuthDao<T> { 

   @PersistenceContext (unitName = "SpringSecurityManager")
   protected EntityManager em;

    ...
}

5) In your service-objects specify which TransactionManager should be used (this feature is supported only in Spring 3.0):
@Transactional (value = "authTransactionManager", readOnly = true)
public class UserServiceImpl implements UserService {

   ...
}

6) If you have OpenEntityManagerInViewFilter in your web.xml, then specify in its init-param name of necessary EntityManagerFactory (or create several filters with correspondent init-blocks):
<init-param>
    <param-name>entityManagerFactoryBeanName</param-name>
    <param-value>authEntityManagerFactory</param-value>
</init-param>

