8.7. Session-per-request pattern

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This is the most common transaction pattern.
The term request here relates to the concept of a system that reacts to a series of requests from a client/user.
Web applications are a prime example of this type of system, though certainly not the only one.
At the beginning of handling such a request, the application opens a Hibernate Session, starts a transaction, performs all data related work, ends the transaction and closes the Session.
The crux of the pattern is the one-to-one relationship between the transaction and the Session.

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Within this pattern there is a common technique of defining a current session to simplify the need of passing this `Session` around to all the application components that may need access to it.
Hibernate provides support for this technique through the `getCurrentSession` method of the `SessionFactory`.
The concept of a _current_ session has to have a scope that defines the bounds in which the notion of _current_ is valid.
This is the purpose of the `org.hibernate.context.spi.CurrentSessionContext` contract.

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There are 2 reliable defining scopes:

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• First is a JTA transaction because it allows a callback hook to know when it is ending, which gives Hibernate a chance to close the Session and clean up. This is represented by the org.hibernate.context.internal.JTASessionContext implementation of the org.hibernate.context.spi.CurrentSessionContext contract. Using this implementation, a Session will be opened the first time getCurrentSession is called within that transaction.

• Secondly is this application request cycle itself. This is best represented with the org.hibernate.context.internal.ManagedSessionContext implementation of the org.hibernate.context.spi.CurrentSessionContext contract. Here an external component is responsible for managing the lifecycle and scoping of a current session. At the start of such a scope, ManagedSessionContext#bind() method is called passing in the Session. At the end, its unbind() method is called. Some common examples of such external components include:

  *   `javax.servlet.Filter` implementation
  

  • AOP interceptor with a pointcut on the service methods
  • A proxy/interception container </div> </div>

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  The getCurrentSession() method has one downside in a JTA environment. If you use it, after_statement connection release mode is also used by default. Due to a limitation of the JTA specification, Hibernate cannot automatically clean up any unclosed ScrollableResults or Iterator instances returned by scroll() or iterate(). Release the underlying database cursor by calling ScrollableResults#close() or Hibernate.close(Iterator) explicitly from a finally block.

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