Re: [eclipselink-users] Stale Data Between GUIs Using EclipseLink JPA

["Tim Hollosy" <hollosyt@xxxxxxxxx>]

I'd like to chime in here, speaking from experience going from a real
production JDBC/RCP app to EclipseLink.

You need to be smart about how you create your EntityManager.

Here's how we did it, I'll use RCP terms, but it'd probably be
translatable to Swing.

The main plugin had it's own EMF.

Each View had it's own EntityManager, it kept this EM for it's life.
These views once created, weren't created and destroyed often.

Each Editor had it's own EntityManager, it tracked changes using the
UnitOfWork API, (a great thing we got for free).

Also, by using EclipseLink we could reuse our Entities directly in the
UI and easily hook everything from text boxes to drop down boxes, to
table controls directly using DataBinding API's designed to work with
JavaBeans.

After the conversion, I would never ever ever go back to straight
JDBC. The code is so much cleaner and so much easier to write.

./tch



On Mon, Jan 12, 2009 at 4:10 PM, James Mello <james.mello@xxxxxxxxx> wrote:
> Usually ORM solutions can be faster. Quite a bit of your poor performance may be related to you getting and releasing an EntityManager per database operation. This is a classic "anti" pattern that is discussed in a hibernate FAQ.
>
> Also, realize that one reason an SQL solution may be faster too is that you currently have a contrived flat object representation. When moving to more complex objects, the required SQL becomes more difficult to write and things like Lazy fetch and fetch groups really start to shine in both development cost and performance. Finally, I would add that you really only need to perform good enough. If the eclipselink solution is taking say 100ms to do something versus 10ms, if it isn't performance critical, err on the side of ease of development Using a JPA solution you can do post development profiling and thunk to native SQL where necessary reducing bother code complexity and the amount of classes you need.
>
> -----Original Message-----
> From: eclipselink-users-bounces@xxxxxxxxxxx [mailto:eclipselink-users-bounces@xxxxxxxxxxx] On Behalf Of Liford, Michael W.
> Sent: Monday, January 12, 2009 11:59 AM
> To: EclipseLink User Discussions
> Subject: RE: [eclipselink-users] Stale Data Between GUIs Using EclipseLink JPA
>
> Thanks for the reply, James.
>
> I read a message in a newsgroup that recommended getting a new EntityManager for every database operation and closing it right after the operation. I also surrounded any updates, inserts, and deletes with transactions and commits. Seems to work now.
>
> I am rather disappointed in the performance of EclipseLink. I have another version of the same application using native SQL (ResultSet, PreparedStatement, etc.). It is much faster. EclipseLink makes the coding easier, but there is a price to pay.
>
> MikeyL
>
> -----Original Message-----
> From: eclipselink-users-bounces@xxxxxxxxxxx
> [mailto:eclipselink-users-bounces@xxxxxxxxxxx] On Behalf Of James Sutherland
> Sent: Monday, January 12, 2009 10:58 AM
> To: eclipselink-users@xxxxxxxxxxx
> Subject: Re: [eclipselink-users] Stale Data Between GUIs Using EclipseLink JPA
>
>
> In JPA an EntityManager represents an extended persistence context, in that any object read into the EntityManager will remain until you either
> close() or clear() the EntityManager.  You can also call refresh() on an object to refresh it explicitly.  An EntityManager normally relates to a single transaction or request.  So to see changes made in another EntityManager, or another machine, you need to either create a new EntityManager, or clear() the existing one, or refresh() each object you want refreshed.
>
> EclipseLink also managed a 2nd level shared cache at the EntityManagerFactory level.  This cache also caches the objects that are read, so if you want fresh data you must also ensure this cache is refreshed.  Calling refresh() will also refresh the cache (unless you have flushed changes), the cache also has invalidation API and configuration, or can be disabled using persistence.xml "eclipselink.cache.shared.default"="false", or the @Cache annotation or orm.xml.
>
> I would recommend you continue working with JPA, and not use the native API or Mapping Workbench.
>
>
> MikeyL wrote:
>>
>> I am playing with a Movie-Actor application to learn about
>> persistence. I have a Derby database with a Movies table with title,
>> rating, release year, etc., and an Actors table with name, birth date,
>
>> etc. I have an associative table joining a many-to-many relationship
>> between Movies and Actors. The associative table declares foreign key
>> fields with references back to the parent tables. I designed Swing
>> GUIs to display the data. The Movie GUI will display actors for a
>> selected movie in a child table. The Actor GUI will display movies for
>
>> a selected actor in a child table. The Entity Manager Factory and
>> Entity Manager are instantiated once by the main GUI at program start.
>
>> The Movie and Actor DAOs get the Entity Manager from the main screen.
>> The problem is that changes made in one GUIs do not get reflected in
>> the other. E.g., if I add the movie 2 Days in the Valley to actress
>> Charlize Theron in the Actor GUI then click on 2 Days in the Valley in
>
>> the Movie GUI, Charlize is not listed as an actor in the child table.
>> Even pressing the Refresh button to reload the Movie table does not
> help. How do I synchronize the data between the two GUIs?
>>
>> In an attempt to solve the above problem, I took a look at EclipseLink
>
>> Sessions. It seems that I could have the main GUI invoke a server
>> session and have separate client sessions for the Movie and Actor
>> GUIs. However, when using Workbench to create entity source files from
>
>> my database tables the many-to-many relationship is not picked up by
>> Workbench although the foreign key reference constraints are on the
>> associative table. I crawled all over the User's Guide and can't find
>> how to manually define these relationships in the mapping XML file. Is
> this a bug in Workbench?
>>
>> Any help and thoughts will be appreciated.
>>
>> MikeyL
>>
>
>
> -----
> ---
> http://wiki.eclipse.org/User:James.sutherland.oracle.com James Sutherland http://www.eclipse.org/eclipselink/
>  EclipseLink ,  http://www.oracle.com/technology/products/ias/toplink/
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> Book:  http://en.wikibooks.org/wiki/Java_Persistence Java Persistence
> --
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