Observer and JSF ManagedBean

Question

I have a drop down menu from which user selects an item(in my case project he is working on), most of the data displayed by a webpage depends on that selection. So I have couple of view scoped beans that call EJB beans which do database queries that depend on the selected project.

I want to cache most of that data in order to reduce Database queries, but when user changes project it has to notify other beans that change has happened and new data needs to be fetched.

So I had an idea:

• projectChangeManager (session scoped managed bean), saves which project is selected, notifies it's subscribers when project changes. Has @PreDestory method implemented where I cleanup observers.

• project observers (view scoped managed beans), gets data from EJB based on project selection, has onProjectChange() method which fetches new data from EJB. Has @PreDestory method implemented where projectChangeManager.detach(this) is called to unsubscribe from projectChangeManager.

Is this reasonable approach in JSF? Or is it better not to implement observer pattern but just when user changes project I fetch all cached data and save it in session bean and then in ViewScoped bean I just access that data from SessionScoped bean? Or is there a better way?

Answer 1

To me this looks like overengineering. Initially, I would probably just let every managed bean call the repository/EJB whenever it needs data. Then rely on caching in the persistence layer (JPA/Hibernate/whatever you use).

If this turns out to be a performance problem, you can consider some hand-rolled caching solution.

Even in that case I still don't quite see the advantage of your observer approach. Your second approach (cache in session bean, access from ViewScoped bean) looks simpler and should work just as well.

Finally, if you decide to use caching, consider how to avoid a stale cache. If you cache per session, changes in one session will not be visible in another session. Incidentally, I think this is another reason to leave caching to the persistence layer.