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From "Howard W. Smith, Jr." <>
Subject Re: Entity cant be refreshed with new list values
Date Sat, 19 Oct 2013 20:10:27 GMT
On Sat, Oct 19, 2013 at 11:33 AM, Mark Struberg <> wrote:

> >'did not touch'? i think you are referring to checking size() or isEmpty()
> Yes, sometimes you even need to iterate through those lists and touch all
> the lazy loading fields of those 1:n entities if you know you need them in
> the frontend.

interesting... for some of my @Entity classes, i have used this approach,
but for the most part, I am quite amazed that this is not always necessary
for some of the @Entity classes. I am using eclipselink query results
cache, but I did some playing with that in trying to tinker/improve
performance. i saw Christopher Schultz on tomcat user list mention
something about thrashing (or abusing) the cache... (dont' remember how he
said it), so that kinda motivated me to review my use of query results
cache. there is blog out there about tuning eclipselink jpa that I
read/reviewed at least twice, i think that kinda motivated me or reinforced
my decision to use query results cache (as much as possible).

there is one huge table that can be quite slow when i'm using PrimeFaces
autocomplete to search all rows...for PointOfContact data (and related
data). i think i had to tinker the use of query results cache and size() /
isEmpty() to improve the performance.

> >It is interesting that you refer to it as a restriction. do you
> > prefer/recommend this approach, or another approach?
> In those cases I'd go d' accord with Romain and suggest using DTOs.


> > I think I read somewhere how size() and isEmpty() will retrieve the
> > data, but now I know, you must do this inside @EJB. right?
> Yes, you can think of an @Stateless EJB as a CDI bean which has (amongst
> others) a CDI @Transactional interceptor [1] automatically applied to it.
> What happens is kind of what I show on page 31 in [2]. Simplified, but you
> get the idea I hope.
> This makes it obvious that once the method you invoke on your (outermost)
> EJB returns, then the interceptor will commit the transaction and close the
> EntityManager again. Please note that the EntityManager you get with
> @PersistenceContext is NOT a native JPA EntityManager but just a facade
> which delegates to the _real_ EM (per Thread and EJB invocation). And this
> 'real' EM will gets closed in the Transactional interceptor, which means
> all the entities loaded by it will automatically get detached in that
> moment.
> I hope it's clear now why lazy loading and other stuff _outside_ the EJB
> does not work as expected ;)
> You should nontheless be able to change values in the entites and those
> changes must get reflected to the database if you later em.merge it inside
> an EJB. Oh btw, an error which also happens pretty often is that em.merge
> gets used the wrong way:
> SomeStuff mergedAndManagedEntity = em.merge(detachedEntity);
> the PARAMETER entity will NOT become managed! Only the return value is
> managed!

okay, understood. i think i have had this understanding for some time now
(while developing JSF/JavaEE app, of course).

> >are you recommending this for only OpenJPA users?
> > I'm using
> EclipseLink and (Apache) Derby. I think I read
> > about this before
> somewhere; is the 'optlock' part of
> > Java EE implementation or just for
> OpenJPA users?
> @Version is a *very* important part of the JPA spec itself.
> I'm not sure why, but this is something which is often missing in
> applications.
> Maybe because almost none of the JavaEE samples on the Oracle page and in
> JavaEE talks mention this and other important stuff (like the DTO and
> EntityManager details explained above).
> I've had this discussion with Adam Bien in the past (whom I enjoy sharing
> conference slots with regularly). I recommend watching his talks because
> they are very enthusiastic and show a lot of the power of JavaEE.
> Especially if you have some junior devs and like them to get the basic
> ideas and power of JavaEE or if you like to get a good overview about new
> features of the latest JavaEE version.
> But he also tends to over-simply his samples and leaves out lots of very
> important parts - like the Oracle/Sun docs do as well.
> Adams argument is pretty straight forward. Of course he knows all those
> nasty details, but if he would go into those details then he could only
> explain one or two single features of JavaEE during each sessions. And he
> would utterly confuse listeners by explaining details which they do not get
> anyway. I can completely understand his position, but I believe for
> creating *real* applications, you need to know about all those gory details
> :/
> That's the reason why I regularly do 'Pitfalls in JavaEE' kind of lectures
> on javaeesummit and other events.

I have read some of Adam Bien's blog post, and I recognized all what you
mentioned above, and no respect to him, of course. Before I began Java EE
development, I studied the Java EE 6 tutorial from beginning to end (I
think I skipped the last 2 or 3 sections, or left them as
for-later-reference only), i gained my fundamental knowledge of Java EE via
that tutorial.

after migrating my app to CDI and TomEE, i learned a lot more from you all
(Apache committers)!

> Oki, back to @Version.
> This is what provides you with locking.
> There are 2 ways of locking in the database
> a.) pessimistic locking, also known as database row locking (if you are
> happy it only locks the row in a table. Could also lock the whole table
> depending on indexes, etc). This requires an open database connection, so
> nothing you usually like to do in a web application. See JPA
> b.) optimistic locking. This is what you usually use in web apps. There is
> no database locking involved. Imagine you have a CUSTOMER table with the
> following columns:
> * id* name
> * birthdate

definitely interesting...i read about this on oracle documents site, i
think, and I think i vaguely remember @Version. definitely is good to know.
also, when i read about pessimistic and optimistic locking, it left me a
bit confused. honestly, i would love to have row locking, and I thought
about Derby's implementation/option of row-level locking, but it seems as
though my app is locking entire table during (any/every JPA) transaction.

honestly, i think i have improved the performance of most of the
app...except database access. endusers are 'not' complaining at all, but i
love tuning performance, and i know it can be improved.

> Now imagine a secretary A opens customer 'Jack' with id=1 and changes the
> name to 'Jill'. He lets the edit page open for around 4 hourse because he
> went to lunch. In the meantime another secretary B opened the same row,
> changed the birthdate and stores it. Finally secretary A saves his edit
> page. What happens is
> UPDATE CUSTOMER set name='Jill', birthdate='old' where id =1;
> which means that all the changes done by secretary B get overwritten and
> are ultimately lost!

i have definitely experienced this in my web app, and sometimes, i instruct
endusers to 'not' touch same record/row, if you see (or know) another
enduser is modifying that data. honestly, this does not happen much,
because it is quite seldom that endusers are working on the same data, but
sometimes, it does/can happen. And for that reason, I definitely need to
start using @Version.

> With optimistic locking you would not have this problem. You just need an
> additional 'version' integer column
> * id
> * version
> * name
> * birthdate
> and the update performed by JPA will look like this:
> UPDATE CUSTOMER set name='Jill', birthdate='old', version=:oldversion+1
> where id = 1 and version = :oldversion;
> where the oldversion is the version number (e.g. 1) at the time you did
> load it from the database.
> If anyone has updated the row and incremented the version in the meantime
> then your update will fail and you could detect this as
> OptimisticLockException and force the 2nd user to first reload the values
> from the DB again and then apply his changes on top of the new data. That
> way you do not loose information.

very interesting, thanks!

> I've now uploaded one of my half day lectures to my people account [3].
> Hope you can find some valuable info in there
> [1]
> [2]
> [3]
will definitely have to take a look at those links/references, thanks!

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