JPA EntityManagers

27 01 2009

Been having problems with JPA EntityManagers recently in K2 as we didn’t realise they should not be shared between threads. Fortunately fixing this in K2 doesn’t require very much. Create a proxy to a thread bound entity manager contained within a ThreadBound holder. When these are put into a request scope cache from the CacheManager, the ThreadBound.unbind method is called when the request ends, so we have a hook to perform commit and close down on the EntityManager. The really nice thing about using a Cache from the CacheManager is that it can also be created with a Thread scope rather than a Request scope. In Thread scope, its not unbound… so by changing one switch we can, if the JPA usage is well behaved move from a per request to per thread binding. Fortunately the overhead of creating an EntityManger from most JPA providers is < 1ms.. and with this approach, we don’t need to create it on every request.




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