Detach an entity from JPA/EJB3 persistence context

What would be the easiest way to detach a specific JPA Entity Bean that was acquired through an EntityManager. Alternatively, could I have a query return detached objects in the first place so they would essentially act as ‘read only’?

The reason why I want to do this is becuase I want to modify the data within the bean – with in my application only, but not ever have it persisted to the database. In my program, I eventually have to call flush() on the EntityManager, which would persist all changes from attached entities to the underyling database, but I want to exclude specific objects.

Answer

Unfortunately, there’s no way to disconnect one object from the entity manager in the current JPA implementation, AFAIR.

EntityManager.clear() will disconnect all the JPA objects, so that might not be an appropriate solution in all the cases, if you have other objects you do plan to keep connected.

So your best bet would be to clone the objects and pass the clones to the code that changes the objects. Since primitive and immutable object fields are taken care of by the default cloning mechanism in a proper way, you won’t have to write a lot of plumbing code (apart from deep cloning any aggregated structures you might have).