Using Java Persistence with Tomcat and no EJBs

May 08, 2008 📬 Get My Weekly Newsletter

The project I'm working on is deployed under Tomcat and isn't using EJBs. The codebase is using JDBC for database access and I'm looking into using some O/R mapping. Hibernate is great, but Java Persistence is more desirable, as it's more of a standard. Getting it to work with EJB3 is dead simple. Getting it to work without EJB was a bit more problematic. The entire application is being deployed as a WAR file. As such, the JPA configuration artifacts weren't getting picked up. Setting aside how absolutely horrendous Java Enterprise configuration is, here's what ended up working for me:
  • Create a persistence.xml file as per standard documentation leaving out the jta-data-source stanza (I could not figure out how to get Hibernate/JPA to find my configured data source)
  • Create your hibernate.cfg.xml, being sure to include JDBC conncetion info. This will result in hibernate managing connections for you, which is fine
  • Create a persistence jar containing:
    • Hibernate config at root
    • persistence.xml in META-INF
    • All classes with JPA annotations in root (obviously in their java package/directory structure)
  • This goes into WEB-INF/lib of the war file (being careful to omit the JPA-annotated classes from WEB-INF/classes
The first two steps took a while to get to and aren't super clear from the documentation. To use JPA, this (non-production quality) code works:
EntityManagerFactory emf = 
    Persistence.createEntityManagerFactory("name used in persistence.xml");
EntityManager em = emf.createEntityManager(); 

Query query = em.createQuery("from Account where name = :name");
query.setParameter("name",itsAccountName);
List results = query.getResultList();

// do stuff with your results


em.close();
emf.close();
The EntityManagerFactory is supposed to survive the life of application and not be created/destroyed on every request. I also believe there might be some transaction issues with this, but I can't figure out from the documentation what they are and if they are a big deal for a single-database application. Update: Turns out, it's not quite this simple. Since this configuration is running outside an EJB container, and given Bug $2382, you can query all day long, but you cannot persist. To solve this, you must work in a transaction, as so:
EntityManagerFactory emf = 
    Persistence.createEntityManagerFactory("name used in persistence.xml");
EntityManager em = emf.createEntityManager(); 
EntityTransaction tx = em.getTransaction();

tx.begin();
Query query = em.createQuery("from Account where name = :name");
query.setParameter("name",itsAccountName);
List results = query.getResultList();

// modify your results somehow via persist() 

// or merge()


tx.commit();
em.close();
emf.close();
Again, this is not production code as no error handling has been done at all, but you get the point.