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3.5. Contextual Sessions
Most applications using Hibernate need some form of "contextual" session, 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; 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
JTAtransactions, where the
JTAtransaction defined both the scope and context of a current session. Given the maturity of the numerous stand-alone
JTA TransactionManagerimplementations, most, if not all, applications should be using
JTAtransaction management, whether or not they are deployed into a
J2EEcontainer. Based on that, the
JTA-based contextual sessions are all you 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.CurrentSessionContextinterface 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
JTAtransaction. 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. 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
Sessioninstance with static methods on this class: it does not open, flush, or close a
The first two implementations provide a "one session - one database transaction" programming model. This is also 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 programmatic transaction demarcation in plain JSE without JTA, you are advised to use the Hibernate
TransactionAPI to hide the underlying transaction system from your code. If you use JTA, you can utilize the JTA interfaces to demarcate transactions. If you execute in an EJB container that supports CMT, transaction boundaries are defined declaratively and you do not need any transaction or session demarcation operations in your code. Refer to Chapter 11, Transactions and Concurrency for more information and code examples.
hibernate.current_session_context_classconfiguration parameter defines which
org.hibernate.context.CurrentSessionContextimplementation should be used. For backwards compatibility, if this configuration parameter is not set but a
org.hibernate.transaction.TransactionManagerLookupis 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 three corresponding short names: "jta", "thread", and "managed".