Hi Andrus,
Sorry for such a tardy reply but I'm just getting going on the JPA
stuff...
Jeff and I were at TSS Symposium last week and we had a chance to
talk through the work for JPA. I wanted to get our thoughts into an
email and off to the list so we are not duplicating work. We are
planning on doing the mapping work. Jeff is going to tackle handling
the annotations and I am going to handle parsing the orm.xml files
and augmenting the mapping info.
Other thoughts etc are inline.
On Mar 7, 2006, at 2:45 AM, Andrus Adamchik wrote:
> I did a little research on bytecode enhancement (CAY-460) and
> wanted to share some thoughts.
>
> It turned out that it is fairly simple to modify bytecode with the
> ASM framework:
>
> http://asm.objectweb.org/
>
> I checked in a basic enhancer that adds objectId and
> persistenceState properties to POJOs already. Coding the rest of
> the DataObject enhancements shouldn't be a problem, however ASM
> code is not something that is easy to maintain by hand (so if say
> CayenneDataObject changes internally, I'd like the enhancer to pick
> up such changes automatically). I guess we may implement some sort
> of template based approach. ASM allows to take an existing class X
> and generate regular Java code that can act as a script for
> creating class X bytcode from scratch. So theoretically an enhancer
> itself can be generated.
>
This looks interesting. I like the idea of using ASM to weave in the
required enhancements. I looked at the code you checked in and I'm
happy to start down the path of getting the rest of the required code
woven in. I will also start looking at the template based approach. I
will probably do this before I get to the XML work so we can start
testing ASAP.
> BTW, ASM visitor API can be used for another purpose - processing
> annotations (CAY-456), thus solving two problems at once.
>
This looks cool (the ASM is really cool for doing this kind of work
AFAICT). Jeff is going to head down this path.
>
> Another issue is integrating the enhancer in the provider. The
> requirement is that enhanced classes must be accessible to the
> application ClassLoader, so my original solution with a specialized
> child ClassLoader won't work. I see three ways to do the
> integration (we may implement all three of them).
>
> 1. "cgen" - by merging all CayenneDataObject code in the class
> generator template. This will work for people who use Cayenne to
> model their ORM classes, but want a superclass that is not a
> DataObject. This is not suitable for persistent classes created
> outside Cayenne and deployed with Cayenne provider.
>
> 2. "cdeploy" - enhancing compiler Ant task ala JDO.
>
> 3. Runtime - this would use JDK 1.5 java.lang.instrument API. On
> the one hand it is the least intrusive option, but on the other it
> requires a special runtime parameter (-javaagent:cayenne.jar).
>
> Ideas, comments?
>
From my reading of the spec it looks like the ClassTransformer is
there so that 3 does not require the special runtime parameter. Is
there something that I'm missing? I am basing my assumptions on the
comment (starting on page 148) on the addTransformer
(ClassTransformer) method on the PersistenceUnitInfo class.
Thoughts welcome. Also if you are doing any of this work please speak
up so we don't duplicate.
TTFN,
Bill Dudney
MyFaces - http://myfaces.apache.org
Cayenne - http://incubator.apache.org/projects/cayenne.html
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