My scenario is that I don't want to do my entity manager interaction in a session bean - I want to use a layering approach that has Session beans <-> Business Delegates <-> DAOs <-> JPA entities.

I'm getting frustrated that I cannot inject an entity manager into a DAO class inside the ejb container becasue it is not an EJB. I could use a jndi lookup, but then I jhave to worry about closing the entity manager.

What I really want is to be able to access the EntityManager that was injected into the session bean that was my entry point to the container, but

I do not want to pollute my BD and DAO API's with entity manager parameters.

So, can I use ThreadLocal's? Do the folk at JBoss have a paradigm for doing this, or am I doing something completely stupid?

The example code that follows is significantly trimmed. I've also left out the BD, and ignore the fact that a JPA entity is being returned to the client.

I have a session bean with a simple getter method to load an entity. It delegates to a DAO

@Stateful
@Remote
public class StatefulColumnGroupServiceBean {

 @PersistenceContext(unitName="MyApp", type=PersistenceContextType.EXTENDED)
 protected EntityManager em;

 public ColumnGroupEntity getColumnGroup(int iThreadNumber, long id) throws ServerException {
 EntityManagerResource.getInstance().set(em);
 ColumnGroupEntity cgvo = null;
 try {
 cgvo = ColumnGroupDAO.getInstance().getColumnGroup(id);
 }
 catch (DAOException e) {
 log.error(e);
 throw new ServerException( MessageFactory.getMessage("Bean.CannotFind", "ColumnGroup") + id, e );
 }
 return cgvo;
 }
}

public class ColumnGroupDAO {

 public ColumnGroupEntity getColumnGroup( long id ) throws DAOException {
 EntityManager em = EntityManagerResource.getInstance().get();

 ColumnGroupEntity res = null;
 try {
 res = em.find(ColumnGroupEntity.class, id);
 }
 catch (RuntimeException e) {
 throw new DAOException(MessageFactory.getMessage("DAO.FindError", getClass().getName()) + id, e);
 }
 return res;
 }


 protected ColumnGroupDAO() {
 }

 public static ColumnGroupDAO getInstance() {
 if( instance == null ) {
 synchronized( ColumnGroupDAO.class ) {
 if( instance == null ) {
 instance = new ColumnGroupDAO();
 }
 }
 }
 return instance;
 }

 private static ColumnGroupDAO instance = null;

}

public class EntityManagerResource extends ThreadLocalResource<EntityManager> {

 private static EntityManagerResource instance = null;

 public static EntityManagerResource getInstance( ) {
 if( instance == null ) {
 synchronized( EntityManagerResource.class ) {
 if( instance == null ) {
 instance = new EntityManagerResource( );
 }
 }
 }
 return instance;
 }

 protected EntityManagerResource() {
 }
}

public abstract class ThreadLocalResource<T> {

 private final ThreadLocal<T> resource = new ThreadLocal( );

 public void set( T em ) {
 resource.set(em);
 }

 public T get( ) {
 return resource.get();
 }
}