2.5. Contextual Sessions

Most applications using Hibernate need some form of "contextual" sessions, where a given session is in effect throughout the scope of a given context. However, across applications the definition of what constitutes a context is typically different; and different contexts define different scopes to the notion of current. Applications using Hibernate prior to version 3.0 tended to utilize either home-grown ThreadLocal-based contextual sessions, helper classes such as HibernateUtil, or utilized third-party frameworks (such as Spring or Pico) which provided proxy/interception-based contextual sessions.
Starting with version 3.0.1, Hibernate added the SessionFactory.getCurrentSession() method. Initially, this assumed usage of JTA transactions, where the JTA transaction defined both the scope and context of a current session. The Hibernate team maintains that, given the maturity of the numerous stand-alone JTA TransactionManager implementations out there, most (if not all) applications should be using JTA transaction management whether or not they are deployed into a J2EE container. Based on that, the JTA-based contextual sessions is all you should ever need to use.
However, as of version 3.1, the processing behind SessionFactory.getCurrentSession() is now pluggable. To that end, a new extension interface (org.hibernate.context.CurrentSessionContext) and a new configuration parameter (hibernate.current_session_context_class) have been added to allow pluggability of the scope and context of defining current sessions.
See the Javadocs for the org.hibernate.context.CurrentSessionContext interface for a detailed discussion of its contract. It defines a single method, currentSession(), by which the implementation is responsible for tracking the current contextual session. Out-of-the-box, Hibernate comes with three implementations of this interface.
  • org.hibernate.context.JTASessionContext - current sessions are tracked and scoped by a JTA transaction. The processing here is exactly the same as in the older JTA-only approach. See the Javadocs for details.
  • org.hibernate.context.ThreadLocalSessionContext - current sessions are tracked by thread of execution. Again, see the Javadocs for details.
  • org.hibernate.context.ManagedSessionContext - current sessions are tracked by thread of execution. However, you are responsible to bind and unbind a Session instance with static methods on this class, it does never open, flush, or close a Session.
The first two implementations provide a "one session - one database transaction" programming model, also known and used as session-per-request. The beginning and end of a Hibernate session is defined by the duration of a database transaction. If you use programatic transaction demarcation in plain JSE without JTA, you are adviced to use the Hibernate Transaction API to hide the underlying transaction system from your code. If you use JTA, use the JTA interfaces to demarcate transactions. If you execute in an EJB container that supports CMT, transaction boundaries are defined declaratively and you don't need any transaction or session demarcation operations in your code. Refer to Chapter 11, Transactions And Concurrency for more information and code examples.
The hibernate.current_session_context_class configuration parameter defines which org.hibernate.context.CurrentSessionContext implementation should be used. Note that for backwards compatibility, if this config param is not set but a org.hibernate.transaction.TransactionManagerLookup is configured, Hibernate will use the org.hibernate.context.JTASessionContext. Typically, the value of this parameter would just name the implementation class to use; for the three out-of-the-box implementations, however, there are two corresponding short names, "jta", "thread", and "managed".