Transactions in EJB3

A little pause in the Quest for the Java Portal. I'm creating a JavaEE application, and I got a simple problem, but not well explained on the Internet. The scenario:

• EJB module has:
• Some CMP beans (with @ManyToOne and @OneToMany relationships);
• A Session Facade (stateless local bean).
• WEB module has:
• A jMaki-powered menu tree;
• A JSF-managed bean (that populates the jMaki tree with a call to Session Facade).

It is very simple, but I got a ServletException (caused by a LazyInitializationException) when I try to navigate the relationships on web container.

Googleing this problem, I found that the transaction of a stateless bean (using a "requires" or "requires new" attribute) is closed after the method returns, if the container opened it. Specifically, it happens when the EntityManager is destroyed. This is obvious, but not very intuitive. Solutions:

• Use a stateful bean. The Entity Manager will "survive" along with the session, but I think this will make the transaction survive that long, too. Not the best solution;
• Open the transaction in web container. This is strange, but logical: if I want to access the database (with lazy fetching), I must have an active transaction. EJB container is prepared to reuse the transaction by using the @TransactionAttribute annotation (defaults to "REQUIRED"). Using the brand-new resource injection, it is a matter of creating a "@Resource UserTransaction tx" attribute on my JSF managed bean. It is not nice to inject a transaction on every bean of web container, so, you only need to create a web filter that opens before chaining the operation.
  

As a conclusion, my contribution to JavaEE community. If you want to use Lazy Loading on web containers, you just need to create a Filter like this one:

public class TransactionFilter implements Filter {
 @Resource
 private UserTransaction tx;

 public void doFilter(ServletRequest request,
         ServletResponse response,
         FilterChain chain)
         throws IOException, ServletException {
     try {
         if (tx.getStatus() == STATUS_NO_TRANSACTION) {
             tx.begin();
         }
     } catch (NotSupportedException ex) {
         throw new ServletException(ex);
     } catch (SystemException ex) {
         throw new ServletException(ex);
     }
 
     chain.doFilter(request, response);
 }

 public void destroy() {
 }

 public void init(FilterConfig filterConfig) {
 }

}