Current (Contextual) Sessions

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New to Hibernate 3.0.1 is the SessionFactory.getCurrentSession() method. It allows application developers to delegate tracking of current sessions to Hibernate itself. This is fairly trivial functionality, but stuff just about any user of Hibernate had to implement themselves, or rely on third party stuff to do for them. Let's take a look at how this is implemented in Hibernate and how it might be useful.

Context Scope

I said that SessionFactory.getCurrentSession() tracks the current session on behalf of the application developer. What exactly does that mean? What is the scope in which a session is considered current? The transaction! More specifically, a JTA transaction.

Another dimension to scoping the current session is to which factory it belongs. Because Hibernate implements this internal to the SessionFactory, the current sessions are inherently tracked by that given factory. Internally, the SessionFactory maintains a Map of sessions keyed by JTA transaction. There is little overhead in this since the Map is built lazily, and only utilized during getCurrentSession() calls: if you don't use this feature, the map is never even built.

Example Usage

Imagine a simple scenario coordinating efforts between three DAOs:

public Bid myExposedCmtEjbServiceMethod(Long itemId, Double amount) {
    ItemDAO itemDAO = new ItemDAO( getSessionFactory() );
    BidDAO bidDAO = new BidDAO( getSessionFactory() );
    UserDAO userDAO = new UserDAO( getSessionFactory() );

    Item item = itemDAO.load( itemId );
    User bidder = userDAO.load( getCurrentUsername() );
    return bidDAO.create( item, amount, user );
}

How should each of the DAOs utilize the same session to perform their work? The typical pattern is to use ThreadLocals or similiar contextual storage (perhaps a JBoss TransactionLocal) to maintain the current session within that context. Furthermore, how do we know when this current session should be cleaned up?

The usual pattern to implement these functionalities is that a top-level service/method is defined as the service controller which is responsible for opening a session at the start, binding it to the contextual storage (so other collaborators can find it), and cleaning up the session at the end of the service processing. A slight twist on this is to use method interception to apply those behaviours (or aspects) on top of the service controller method. Either way, this can be a lot of work to setup requiring that we either:

modify all the service controller points to perform the open-bind-cleanup functionality

wrapping all our services (sometimes spuriously) in proxies so that we can intercept the method execution and apply those behavioural aspects

So instead, lets look at using the SessionFactory.getCurrentSession() approach:

public class ItemDAO {
    private SessionFactory sf;

    public ItemDAO(SessionFactory sf) { this.sf = sf; }

    public Item load(Long itemId) {
        return ( Item ) sf.getCurrentSession().load( Item.class, itemId );
    }

    ...
}

Here, each of the DAO collaborators simply use the getCurrentSession() method; the things collaborating with the DAOs do not need to perform anything extra and we do not need to generate proxies and method interceptors just to apply the notion of contextual sessions.

So now, by using getCurrentSession() we can easily scope the notion of a current session to the JTA transaction and reuse the same session throughout that JTA transaction. But how do we clean up the session? And how do we manage flushing of the session state with the database?

Auto flush and close

Two new configuration options introduced in Hibernate3 are extremely powerful, especially when combined with the SessionFactory.getCurrentSession(). Both of these are available in the JTA environments, as well as scenarios where application is utilizing the Hibernate transaction-abstraction API.

The first is flush_before_completion, which forces a flush of the session just prior to transaction completion (think Synchronization.beforeCompletion()...). With this setting enabled, we do not have to worry about flushing the session after we are done in order to synchronize in-memory state with the database; Hibernate does it for us (just prior the transaction commit).

The second is auto_close_session, which forces the session to be closed after transaction completion. In JTA environments, this setting has an additional effect; it forces Hibernate to release JDBC connections much more aggresively. Basically, Hibernate will obtain a connection, use it, and then immediately release it back to the datasource. This allows better integration into JTA environments which implement some form of connection containment check (i.e. the JBoss CachedConnectionManager).