What is the correct way of accessing a Java Persistence unit from the web tier (no EJB session beans)?

I use JBoss 4.2.0.

I can't get the resource injection annotations @PersistenceUnit or @PersistenceContext to work, so I have done like this:

public class MyServletContextListener implements ServletContextListener {

 public void contextInitialized(ServletContextEvent sce) {
 try {
 EntityManagerFactory emf = Persistence.createEntityManagerFactory("myunit");
 sce.getServletContext().setAttribute(EntityManagerFactory.class.getName(), emf);
 }
 catch (RuntimeException e) {
 throw e;
 }
 catch (Exception e) {
 throw new RuntimeException(e);
 }
 }

 public void contextDestroyed(ServletContextEvent sce) {
 EntityManagerFactory emf = (EntityManagerFactory)sce.getServletContext().getAttribute(
 EntityManagerFactory.class.getName());
 if (emf != null) {
 emf.close();
 }
 }

}

and for each web request I do like this:

EntityManagerFactory emf = (EntityManagerFactory)context.getAttribute(EntityManagerFactory.class.getName());
 EntityManager em = emf.createEntityManager();
 EntityTransaction transaction = em.getTransaction();
 transaction.begin();

 try {
 // process the request here...
 transaction.commit();
 } catch (Exception e) {
 transaction.rollback();
 // log the error
 } finally {
 em.close();
 }

It seems to work, but is it thread-safe and does it give reasonable performance?