Question

I have following code in my application each time I want to do some action with hibernate:

Session session = HibernateUtil.getSessionFactory().openSession();
session.beginTransaction();
session.save(obj); //or delete, update etc.
session.getTransaction().commit();
session.close();

This is of course not a good practice. What is the best way to access hibernate and not to have situation like this? Should I use some ‘Util’ class that contains static methods that do the same job? Maybe singleton is better? I also need to wrap this code in thread or SwingWorker in order not to freeze my GUI while accessing database.

Answer 1

Yes, that code is pretty bad.

1. it opens a transaction for every database access. This is inefficient, and probably incorrect (usually, an entire group of actions needs to be atomic. For instance consider

   public void buy(String accountId) {
       Account account = load(accountId);
       account.balance -= 100;
       save(account);
   }
   

   If load and save are in their own transactions, and some evil user buys 2 items at the same time, the following may happen:

   • thread 1 reads the balance (=1000)
   • thread 2 reads the balance (=1000)
   • thread 1 writes the balance (=900)
   • thread 2 writes the balance (=900)

   so the user bought twice, but only paid once 😉

2. it does not finish the transaction if save() throws an exception. In fact, the transaction will remain open indefinitely, including all the locks it holds …

Complexities such as these are the reason most people demarcate transactions declaratively rather than reinventing a square wheel, for instance with Spring or EJB.