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} catch (HibernateException ex) { if (tx != null) tx.rollback(); throw ex; } finally { session.close(); } } catch (HibernateException ex) { ... // Throw application specific exception }
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First, we get a new Session using our utility class and then start a database transaction. We load the Item from the database, using its identifier value, obtaining a pessimistic lock (this prevents two simultaneous bids for the same item). If the end date of the auction is earlier than the current date, we forward to an error page. Usually you ll want more sophisticated error handling for this exception, with a qualified error message. Using an HQL query, we check if there is a higher bid for the current item in the database. If there is, we forward to an error message. If all checks are successful, we place the new bid by adding it to the item. We don t have to save it manually; it will be saved using transitive persistence (cascading from the Item to Bid). Committing the database transaction flushes the current state of the Session to the database.
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If any method in the inner try-catch block threw an exception, we have to roll back the transaction and rethrow the exception. The Session must always be closed, freeing database resources. The outer try-catch block is responsible for exceptions thrown by Session.close() and Transaction.rollback() and, of course, the rethrown inner exception. The first thing wrong with this implementation is the clutter caused by all the session, transaction, and exception-handling code. Since this code is typically identical for all actions, we d like to centralize it somewhere. One option is to place it in the execute() method of an abstract superclass of our actions. We also have a problem with lazy initialization if we access the new bid on the showSuccess.jsp page: By the time we get to the JSP, the Hibernate session is already closed, so we can t access unfetched lazy associations. We encourage you to think about this issue; we made the experience that this might not be obvious for new Hibernate users. A great solution to both problems is the thread-local session pattern. The thread-local session A thread-local session is a single session instance associated with a particular request. It lets you implement a persistence context, similar to the JTA notion of a transaction context. Any components called in the same request will share the same session and persistence context. It s especially useful to include a JSP in the persistence context. The JSP pulls information from the domain model by navigating the object graph beginning at some persistent object in the session or request scope for example, the newly created bid that was placed in the request scope by our action. However, the object graph might include uninitialized associations (proxies or collections) that must be traversed (and initialized) while rendering the view. In our example, the JSP might list all items the current bidder has for sale by calling newBid.getBidder().getItems().iterator(). (This sounds a little contrived, but it s certainly possible.) Since the items association is lazy, it would be unfetched at this point. But we already closed the Hibernate session at the end of the action s execute() method, so Hibernate will throw a LazyInitializationException when the association is accessed the database connection is no longer available, and the object graph is detached, so Hibernate can t fetch the collection. It s possible to write our action to ensure that all needed associations are fully initialized before forwarding
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to the view (we discuss this later), but a more convenient approach is to leave the session open until the view is completely rendered.
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Why can t Hibernate open a new connection (or session) if it has to lazy-load associations First, we think it s a better solution to fully initialize all required objects for a specific use case using eager fetching (this approach is less vulnerable to the n+1 selects problem). Furthermore, opening new database connections (and ad hoc database transactions!) implicitly and transparently to the developer exposes the application to transaction isolation issues. When do you close the session and end the ad hoc transaction after each lazy association is loaded We strongly prefer transactions to be clearly and explicitly demarcated by the application developer. If you want to enable lazy fetching for a detached instance, you can use lock() to attach it to a new session.
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The thread-local session pattern allows you to have a single Hibernate session per request, spanning the view and potentially multiple action executes(). Java provides the ThreadLocal class for implementing thread scoped variables. The threadlocal session pattern combines a ThreadLocal with an interceptor or servlet filter that closes the Session at the end of the request, after the view is rendered and just before the response is sent to the client. First, we enhance the HibernateUtil helper. Instead of opening a new Session when getSession() is called, it returns a Session kept in a ThreadLocal variable, that is, it returns the Session associated with the current thread. The HibernateUtil class is also a good place to implement generic exception handling; hence we add some other static helper methods that wrap exceptions. The full code of our improved HibernateUtil is shown in listing 8.3.
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