Hi, all.
I've been implementing some code against JPA and I was trying to figure
out how to handle a transaction rollback. I wasn't sure if JPA is
responsible for rolling back the state of any objects in the persistence
context.
The answer appears to be no, and I found this in the spec:
===
3.3.2 Transaction Rollback
For both transaction-scoped and extended persistence contexts,
transaction rollback causes all pre-existing managed instances and
removed instances[15] to become detached. The instances’ state will be
the state of the instances at the point at which the transaction was
rolled back. Transaction rollback typically causes the persistence
context to be in an inconsistent state at the point of rollback. In
particular, the state of version attributes and generated state (e.g.,
generated primary keys) may be inconsistent. Instances that were
formerly managed by the persistence context (including new instances
that were made persistent in that transaction) may therefore not be
reusable in the same manner as other detached objects—for example, they
may fail when passed to the merge operation.
====
As an app writer, this statement is somewhat disconcerting.
I was wondering if you could give me some guidance for how I can write
an app against JPA that handles rollback correctly.
I'm writing a controller that interacts with entity objects, passing
them around to various methods, storing them in member variables, and so
on. I don't want to detach them, because (a) it is costly for me to copy
all the data into a separate, detached instance, and then during the
merge operation to have them copied back into the instance stored in the
persistence context and (b) I never know if I can access a field or
relationship because I don't know if the field or related instance has
been loaded yet. When I keep them attached, JPA takes care of this for me.
Then I perform a persist() operation and it fails and rolls back. This
means all my objects I've been passing around and storing in various
places are suddenly detached and "may be" in an inconsistent state
(which from the perspective of a program is the same as saying they
*are* in an inconsistent state), and thus are invalid.
So now what do I do? Is there a recommended approach for dealing with
this? How does my application detect this in some global way, and
reloads all the instances it was keeping around from the persistence
context?
The other concern I have is I'd like consumers of my controller to use
entity objects as POJOs without having to know or care if they are
entities. But the rollback semantics means my callers have to deal with
handling a rollback. Alternately I could pay the copy cost of detaching
my objects before passing them up to higher levels, but then my caller
will still have to deal with the semantics of detached objects (e.g.
lazy fields or associated instances may be null). I don't see any way to
expose entities directly to higher levels of my app that are
entity-unaware... Which means I have to wrap my entities into proxy
classes that handle all the JPA semantics internally and don't expose
this to the caller.
I am concerned that the rollback consequences can slip past developers,
who will then build apps that behave very erratically after a
transaction rollback. I know it's in the spec, but having worked with
lots of developers, most of them don't read the spec, but instead
cut-and-paste from examples. It would be great if we did a blog about
this, and provided some code showing how to do it right... I could do
this, but before I do I thought I'd check with you all first for any
thoughts you have on this.
Thanks!
David