Release connection after transaction if using Hibernate and open-in-view is on (turned off by default) #34138
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Even though those two actions trigger under the same condition, they don't have to be executed in one uninterrupted block of code. Connection only can be released if connection holder was successfully removed, but that's all
Releasing connection before transaction cleanup may break connection reset. It certainly does in case of Hibernate, because HibernateJpaDialect needs active connection to reset isolation level and read only flag. New unit test simply makes sure cleanup and connection release happen in expected order.
In case of pre-bound entity manager it makes sense to release connection after transaction (in some cases at least). But if entity manager is not pre-bound, then it is going to be closed immediately after transaction cleanup. And that releases the connection. So there's no point in releasing the connection manually, if it's going to be released next moment anyway.
…f open-in-view This commit introduces a new setting - releaseConnectionAfterTransaction, which controls if HibernateJpaDialect#releaseJdbcConnection will release connections when called. ReleaseJdbcConnection method is only called if session is pre-bound, so the new setting actually doesn't do anything unless open-in-view is on. And in case if open-in-view is on, turning releaseConnectionAfterTransaction on might help with connection pool starvation.
…Adapter Users don't create HibernateJpaDialect themselves, HibernateJpaVendorAdapter does that. So, this change is needed so that releaseConnectionAfterTransaction could be configured via application settings.
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One of the issues with open-in-view is that connections are not released for a long time, which can lead to connection pool starvation.
Sometimes, controller method even makes no queries for seconds and doesn't really need to keep the connection for itself all that time. For example that is often the case, when application makes REST call after making database query.
This pull request gives user setting, which tells transaction manager, that it should release JDBC connection after transaction is finished (successfully or not). Connection release only happens if the setting is turned on and if open-in-view is enabled. If open-in-view is disabled, then entity manager is closed after transaction is finished and connection is released anyway.
What this pull request changes in existing behaviour
because if connection is released, transaction manager can't reset isolation level and read-only flag. I think the only reason nobody noticed that before is that nobody tried to override JpaDialect#releaseJdbcConnection for HibernateJpaDialect
What this pull request adds
P. S. I think those changes are ok, because the behaviour is copied from HibernateTransactionManager. Hibernate 6 is officially only supported through JpaTransactionManager, so it would be nice to port this HibernateTransactionManager feature.