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From Marc Prud'hommeaux <mprud...@apache.org>
Subject Re: the pain of post processing bytecode (another beg for a simple reflection/cglib alternative like hibernate)
Date Thu, 17 May 2007 22:25:45 GMT

I think this is a very worthwhile project. James and a few others  
excoriated me about this issue over beers after JavaOne last week,  
and, while the bruises from their rhetorical assault are still  
healing, their observations about the comparative "out of the box"  
ease of use OpenJPA compared to other systems definitely bears  
attention.

As Patrick mentioned, we aren't too far away from being able to use a  
dynamic subclassing approach.

Another option I've been thinking about recently is that in JDK 1.6,  
you can dynamically attach an agent at runtime to your own JVM (using  
an implementation-specific mechanism), and using the provided  
Instrumentation, you can redefine existing methods in classes, even  
after the classes have already been loaded. While you cannot add or  
remove methods or fields, we might be able to re-work our PCEnhancer  
to use a newly-generated inner class and a lookup in some  
IdentityHashMap to perform the same function. E.g., instead of our  
currently enhanced class:

public class SomeEntity {
   private String someField;
   private StateManager stateManager; // generated

   public String getSomeField() {
     return pcgetSomeField(this);
   }

   // generated method
   private static String getSomeField(SomeEntity entity) {
     if (entity.stateManager == null)
       return entity.someField;
     else
       entity.stateManager.getField(1, entity);
   }
}


we would instead do something like:


public class SomeEntity {
   private String someField;

   public String getSomeField() {
     return GeneratedInnerClass.pcgetSomeField(this);
   }

   private static class GeneratedInnerClass {

     private static String getSomeField(SomeEntity entity) {
       StateManager stateManager = GlobalIdentityMap.get(entity);

       if (stateManager == null)
         return entity.someField;
       else
         stateManager.getField(1, entity);
     }
   }
}


 From the brief amount of time I've spent thinking about this, I  
think we can get 99% of the way there with our current approach.

The remaining 1% is a situation where if someone creates an instance  
of SomeEntity *before* we ever initialize the BrokerFactory (and thus  
perform the re-definition of known entity classes), we might get  
passed an entity instance that isn't persist-able (since you cannot  
redfine the methods for an already-created object, only for objects  
that will subsequently be created).


In summary, we have 3 different possibilities for removing the  
requirement for build-time enhancement and launch-time agent  
specification:

1. Use reflection: considerably slower, would require drastic rework  
of all our interaction with PersistenceCapable instances, but  
wouldn't require any fancy class-generation/enhancement

2. Dynamic subclassing: easier to implement, but would require people  
to use property accessors for everything, and wouldn't support  
EntityManager.persist(someInstance)

3. Dynamic agent attachment and class re-definition: more difficult  
to implement than #2, and would require JDK 1.6 + JVM implementation- 
dependent attachment mechanisms, but might provide the most  
functionality


Does anyone have any thoughts about this? Especially any new ideas  
for other ways to do this would be very interesting for all of us to  
hear.


On May 17, 2007, at 8:19 AM, Patrick Linskey wrote:

>> How hard is it to add a reflection/cglib type alternative to the  
>> upfront
>> bytecode generation (like hibernate does) to save us from the
>> development-time pain?
>
> Not particularly hard. There are a few APIs that would break for some
> cases,  but it's even pretty straightforward to do a subclassing
> approach for property-based access type without losing much
> performance -- the only cost in that configuration is with
> persistent-new instances.
>
> -Patrick
>
> On 5/17/07, James.Strachan <james.strachan@gmail.com> wrote:
>>
>> Firstly before I start, openjpa is a great piece of software; I'm
>> particularly fond of the documentation and in particular the query  
>> language
>> parts. The CSS for the site is also awesome :)
>>
>> However compared to hibernate, openjpa is still pretty painful to  
>> use from
>> an end users perspective and I don't think this should be the  
>> case; plus I
>> don't think it'll take much time to fix. While the pain is still  
>> fresh in my
>> mind I thought I'd post on how much more painful openjpa is to use  
>> in a
>> project. If you're short on time, the basic idea is its that  
>> bytecode post
>> processing stuff thats to blame :). Yes I know its probably faster  
>> that way
>> - its just so painful for Java programmers to work with. (And yes  
>> I know one
>> day we'll all have IDE plugins that hide the bytecode stuff etc etc).
>>
>> So the first thing is having to mess with your build (ant or  
>> maven) to get
>> the post processing properly integrated. Depending on if you have  
>> persistent
>> entities in your main or test area this can often trip you up a  
>> little (as
>> it did me). I don't know about folks on this list but the whole  
>> idea of
>> having to mess with my maven build gives me the jitters :). When  
>> you get
>> that far & the maven planets are aligned with openjpa, the next  
>> hurdle you
>> hit is how do you run stuff in your IDE. If like me you use IDEA  
>> and maven
>> 2, the project gets auto-created by default for all projects you  
>> work on.
>> However these don't work when you use openjpa as you hit the  
>> dreaded 'cannot
>> function at all as you've not run the up front bytecode post  
>> processor you
>> dummy!' type error when trying to run stuff in your IDE.
>>
>> So you then add the maven-generated classes to the front of the  
>> classpath in
>> your project. Hooray, after a day or two's work, you can now  
>> actually use
>> openjpa in your IDE and your build. YAY! The downside is that now  
>> when
>> navigating around your Java code, whenever you navigate into an  
>> entity bean,
>> IDEA shows you the bytecode - not the source code as its confused  
>> since the
>> bytecode generated stuff is different to the source code it knows  
>> about. So
>> now you're faced with a dilemma - choose between navigating nicely  
>> around
>> your source code - or being able to actually run/debug your  
>> application. I
>> won't even get into the refactoring pain or having to continuously  
>> run maven
>> builds while developing code to avoid getting code completion/ 
>> compile errors
>> etc. (I prefer to keep in my IDE where possible).
>>
>> Compare this whole malarkey with hibernate. You add hibernate to  
>> your pom,
>> generate your project and you're good to go. No messing with your  
>> project
>> build; no messing with some secret ninja IDE stuff to be able to  
>> actually
>> run & debug your code while still being able to actually navigate the
>> source. It just works. Now it might work in a crappy & slow way  
>> and openjpa
>> might be way way more efficient and powerful and whatnot - but I'd  
>> rather
>> have a cheap car that just works than a ferrari that you can only  
>> drive on a
>> tuesday if its sunny, but not too hot and refuses to even start if  
>> its wet.
>>
>> FWIW I've just given up using openjpa for development; its just  
>> way too
>> painful. (I'm even hacking projects I work on so I use openjpa in  
>> the maven
>> build but explicitly switch to hibernate in development mode; yeah  
>> its more
>> work but at least I can use my IDE properly again).
>>
>> I'm cool with putting post processing into the build system  
>> (though that
>> should really only be an optimisation); but please can we have some
>> inefficient but usable reflection/cglib type approach so folks can  
>> easily
>> switch from hibernate to openjpa (and stay there) without pulling  
>> out our
>> hair & swearing too much - or sneaking back at the first  
>> opportunity to get
>> an easy life?
>>
>> Please don't take this mail the wrong way - I truly want openjpa  
>> to be a
>> success, its a great piece of software. Its just a bit too hard to  
>> use out
>> of the box right now. I'd truly like it to be trivial to switch from
>> hibernate to openjpa and never have to go back.
>>
>> How hard is it to add a reflection/cglib type alternative to the  
>> upfront
>> bytecode generation (like hibernate does) to save us from the
>> development-time pain?
>>
>> --
>> James
>> -------
>> http://macstrac.blogspot.com/
>>
>> --
>> View this message in context: http://www.nabble.com/the-pain-of- 
>> post-processing-bytecode-%28another-beg-for-a-simple-reflection- 
>> cglib-alternative-like-hibernate%29-tf3770760.html#a10660986
>> Sent from the open-jpa-dev mailing list archive at Nabble.com.
>>
>>
>
>
> -- 
> Patrick Linskey
> 202 669 5907


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