[hibernate-issues] [Hibernate-JIRA] Commented: (HHH-3220) Patch to prevent "org.hibernate.AssertionFailure: possible non-threadsafe access to the session" error caused by stateless sessions

[Dan Bisalputra (JIRA)]

    [ http://opensource.atlassian.com/projects/hibernate/browse/HHH-3220?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=31707#action_31707 ] 

Dan Bisalputra commented on HHH-3220:
-------------------------------------

I was originally trying to implement a history/undo function in which a Hibernate interceptor would save the previous state of dirty objects so that any change made to the database could be undone.  Stateless sessions were handy for saving the previous state of a given object without triggering cascaded saves of its associations.  I wrote the patch because I was getting the assertion error even though I was accessing the database using a single thread.

I abandoned this approach for two reasons: 1) it was too slow because every transaction effectively had twice as much work to do, and 2) there was only a single history trail, which meant that it might be necessary to undo a more recent change in a different part of the database before undoing the desired change.

The solution I finally used was to restrict undoability to a small list of objects whose editing operations were tricky to undo by hand. Each undoable object is associated with a persistent list of history entries (designed along the lines of the Swing UndoableEdit class).  This approach does not give a comprehensive history like the first approach, but it is faster and allows undos to occur without affecting unrelated objects.

So in the end, I did not do anything clever with stateless sessions, I just avoided the problem by re-scoping my design to use Hibernate in a less exotic way.

> Patch to prevent "org.hibernate.AssertionFailure: possible non-threadsafe access to the session" error caused by stateless sessions
> -----------------------------------------------------------------------------------------------------------------------------------
>
>                 Key: HHH-3220
>                 URL: http://opensource.atlassian.com/projects/hibernate/browse/HHH-3220
>             Project: Hibernate Core
>          Issue Type: Patch
>          Components: core
>    Affects Versions: 3.2.6
>         Environment: Hibernate 3.2.6, Apache Derby on Mac OSX & PC
>            Reporter: Dan Bisalputra
>            Priority: Minor
>         Attachments: StatelessSession-patch
>
>
> When performing a query in a stateless session, the query loads objects in a two-phase process in which a temporary persistence context is populated with empty objects in the first phase, then the objects' member data are read from the database in the second phase.  If one of the objects contains an association or a collection, it performs a recursive call to the session's get() method.  The get() method clears the temporary persistence context, so if the parent object contains any other associations to be read in the second phase, Hibernate throws an assertion because they are not found in the persistence context.
> This patch solves the problem by only clearing the persistence context when the recursion ends.  It passes all the unit tests for our application, but I have not tested it with any of the Hibernate unit tests.

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