[hibernate-dev] SessionEventsListener feature (HHH-8654)

I wanted to highlight a new feature in 4.3 as it came about from performance testing efforts. Its a way to hopefully help track down potential performance problems in applications that use Hibernate. In this way it is similar to statistics, but it operates per-Session (though certainly custom impls could role the metrics up to a SF level). It revolves around the SessionEventsListener[1] interface which essentially defines a number of start/end pairs for the interesting events (for example starting to prepare a JDBC statement and ending that preparation). Multiple SessionEventsListener instances can be associated with the Session simultaneously. You can add them programatically to a Session using Session#addEventsListeners(SessionEventsListener...) method. They can also be added to the Session up-front via the SessionFactory#withOptions API for building Sessions. Additionally there are 2 settings that allow SessionEventsListener impls to be applied to all Sessions created: * 'hibernate.session.events.auto' allows you to name any arbitrary SessionEventsListener class to apply to all Sessions. * 'hibernate.session.events.log' refers to a particular built-in implementation of SessionEventsListener that applies some timings across the start/end pairs (org.hibernate.engine.internal.LoggingSessionEventsListener). In fact this listener is added by default if (a) stats are enabled and (b) the log level (currently INFO) of LoggingSessionEventsListener is enabled. Below[2] is some sample output of LoggingSessionEventsListener. There is also a org.hibernate.EmptySessionEventsListener (no-op) class to help develop custom ones. Anyway, as much as anything I wanted to point it out so people can try it out and to get feedback. I think the API covers most of the interesting events. If you feel there are any missing, lets discuss here or on a Jira issue. [1] https://gist.github.com/sebersole/7438250 [2] 14:40:20,017 INFO LoggingSessionEventsListener:275 - Session Metrics { 9762 nanoseconds spent acquiring 1 JDBC connections; 0 nanoseconds spent releasing 0 JDBC connections; 1020726 nanoseconds spent preparing 4 JDBC statements; 1442351 nanoseconds spent executing 4 JDBC statements; 0 nanoseconds spent executing 0 JDBC batches; 0 nanoseconds spent executing 0 L2C puts; 0 nanoseconds spent executing 0 L2C hits; 0 nanoseconds spent executing 0 L2C misses; 2766689 nanoseconds spent executing 1 flushes (flushing a total of 3 entities and 1 collections); 1096552384585007 nanoseconds spent executing 2 partial-flushes (flushing a total of 3 entities and 3 collections) }