[hibernate-dev] v6 and load-event

[Steve Ebersole]

Inline...

On Wed, May 27, 2020 at 8:10 AM Sanne Grinovero <sanne at hibernate.org> wrote:

> At high level I agree, just have 3 more thoughts:
>
> # Regarding the "swap" of information between listeners - could that
> even work? I might have misunderstood something, but wouldn't we
> require listeners to run in some specific order for such swapping to
> work?
>

This is why we allow control over the ordering of the registered
listeners.  And yes, that is and was a hokey solution.  Nothing changes
there really if that is why you are using load listener.



> # The "surround advice" you mention for e.g. timing seems very
> interesting, especially as I'd love us to be able to integrate with
> tracing libraries - but these would need to be able to co-relate the
> pre-load event with some post-load event. How would that work?  I'd
> expect these to need having a single listener implementation which
> implements both PreLoadEventListener and PostLoadEventListener, but
> also they'll likely need some capability to store some information
> contextual to the "event".
>

I was just thinking through this one as well.  My initial thought was
exactly what you proposed - some combination of pre/post listener with some
ability to store state between.  But that gets ugly.

Another option I thought about is easier to illustrate, but basically works
on the principle of "continuation" many surround advice solutions are based
on: https://gist.github.com/sebersole/142765fe2417492061e92726e7cb6bd8

I kept the name LoadEventListener there, but since it changes the contract
anyway I'd probably rename this to something like SurroundLoadEventListener



> # To clarify on my previous comment regarding why I'd consider having
> an actual Event class more maintainable:
> Sure we won't have inline classes widely used for a while, but I
> prefer planning for the long term - also we could start using them
> very soon via multi-release jars, which would simply imply that users
> on newer JDKs would see more benefits than other users.
> But especially, such event instances are passed over and over across
> many methods; so in terms of maintenance and readability, such methods
> would need to pass many parameters rather than one: the example made
> above is oversimplifying our use.  Also while I understand it's
> unlikely, having a "cheap" event objects makes it easier to change the
> exact types being passed on.
> BTW stack space is cheap but forcing many references to be passed when
> one single reference could do might also have some performance
> implications since these are passed many times - I've never tested
> this scientifically though :)   Inline objects would typically be
> allocated on the stack as well, but they don't force the JVM to do so.


> Also while I said that it's unlikely we want to change those types,
> the very coming of inline types might actually encourage us to make
> changes in this area, even though these events have been stable for
> years; for example "String entityName" seems like an excellent
> candidate to become "EntityName typeIdentifier" - and then allow us to
> improve the persister maps, which have been a bottleneck in the past.
> So sure we could remove them and just pass parameters, we'd just need
> to change more code if such a situation arises - I'm just highliting
> the drawbacks for our consideration, not recommending against it :)
>

Maybe its simply a difference of wording, but to me none of this validates
how keeping an event class is more maintainable.  If you want to say that
eventually the overhead of having an actual event class will be less, ok,
but that's different.

For sure though we'd have lots of uses for in-line value types throughout
the code base.  Just not sure this really an argument for keeping the event
impl in-and-of-itself.