Using EntityHome for entities in long-running contexts

Seam solves the problem of LazyInitializationExceptions (LIE) for Conversations using a Seam Managed Persistence Context (SMPC). Seam's EntityHomes make it trivial to solve this problem for longer scopes - SESSION, APPLICATION and BUSINESS_PROCESS.

How to use it

@Name("currentUserHome")
public class CurentUserHome extends EntityHome<User> {
   
   @Out(scope=ScopeType.SESSION, required=false)
   @In(required=false)
   private Integer currentUserId;

   @Factory(value="currentUser")
   public User getCurrentUser() {
      return getInstance();       
   }
        
   @Override
   protected User createInstance() {
      if (currentUserId != null) {
         return getEntityManager().find(User.class, currentUserId);
      } else {
         return new User();
      }
   }

}

Then, to set the current user, just outject currentUserId into SESSION scope.

How does it work?

Well, if you simply annotate your entity with @Name and @Scope, then, when loaded (using a SMPC) and outjected, it is stored in the desired context. BUT, if you then leave the conversation in which it was loaded, you will get an LIE if you access a property which was not initialised inside that conversation. Ouch.

So, instead of storing the entity in a scope longer than Conversation, we store the id of the entity in the long running scope and the entity in the conversation. Using a @Factory we initialise the entity context variable. If the id is null a new entity is created, otherwise a fresh copy of the entity is loaded and attached to this conversation.