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From "Michael Dick (JIRA)" <>
Subject [jira] Commented: (OPENJPA-24) Allow OpenJPA to be extensible
Date Wed, 20 Sep 2006 01:47:23 GMT
    [ ] 
Michael Dick commented on OPENJPA-24:

It looks like creating an extension is fairly straight forward. With the changes Pinaki committed
yesterday I was able to create a simple test extension. All I did was create three classes,
a ProductDerivation, a PersistenceProvider, and a ConfigurationProvider. I extended PersistenceProductDerivation,
PersistenceProviderImpl and ConfigurationImpl respectively, although I suppose one could write
their own if they really wanted/needed to.  

To get a fairly simple wrapper extension working all I had to do was override : ProductDerivation.newConfigurationProvider()
, and

I didn't see any methods in PersistenceProviderImpl that I needed to change. 

The last thing I did was register the ProductDerivation and PersistenceProvider as services,
and update persistence.xml (specifying the new PersistenceProvider for my persistent unit).

After that the new PersistenceProvider, etc. were used and generated the configuration appropriately
(as far as I can tell). Each ProductDerivation is called to load a ConfigurationProvider until
the first non-null ConfigurationProvider is found. The ConfigProvider and PersistenceProvider
need to match and so forth. 

Is there anything glaring that I missed? Admittedly all I did was a proof of concept. For
a real world extension like Kodo 4.1 there will be other bits to change, changing the default
properties in ConfigurationProvider.loadGlobals() for example. I'm sure Pinaki will have a
more information and a more robust example. 

One other thing I wasn't clear on is the type for the extension (TYPE_SPEC, TYPE_PRODUCT,etc).
My first guess is that any extensions would use TYPE_PRODUCT (value 100), but I could see
it being TYPE_PRODUCT_STORE as well. 

PersistenceProductDerivation is TYPE_SPEC (value 0) and the ProductDerivations are sorted
in ascending order. If extensions are of TYPE_PRODUCT then the default Apache derivation will
always be checked first.  If we assume that when an extension is present it will be used more
often than the default we might want to have any extensions' derivations called first. 

That's all I found. I didn't mean to steal Pinaki's thunder, just wanted to show that someone
else has had some success. 

> Allow OpenJPA to be extensible
> ------------------------------
>                 Key: OPENJPA-24
>                 URL:
>             Project: OpenJPA
>          Issue Type: Bug
>          Components: kernel
>            Reporter: Kevin Sutter
>         Assigned To: Kevin Sutter
> The current OpenJPA architecture is not extendable to other implementations.  For example,
if somebody wanted to provide their own PersistenceProvider implementation, simply extending
the org.apache.openjpa.PersistenceProviderImpl would not suffice due to the dependencies in
the ConfigurationProviderImpl.  The discussion for this improvement was started on the dev
mailing list.  Once it was determined that there was more to this request than a simple conditional
or two, we decided to open a JIRA report.
> The complete history of this request can be found in the OpenJPA dev mailing list.  The
first message was posted by me (Kevin Sutter) on August 14, titled "Extending the OpenJPA
Implementation".  I will attempt to paraphrase the current state of the problem...
> We have three main players in this issue.  The PersistenceProvider, the ConfigurationProvider,
and the ProductDerivation (along with the various implementations of these interfaces).  Currently,
the ConfigurationProvider is in the lib and is unaware of any specific persistence requirements.
 The ProductDerivation is in the kernel and, unfortunately, is aware of persistence requirements,
specifically the spec and store types.  Abe's postings have indicated that we need to make
these two interfaces more aware of each other and work with each other.  We need to start
with either making ConfigurationProvider more persistence-aware and move it into kernel, or
make ProductDerivations less persistence-aware and move it into lib.  The latter approach
is preferred.
> After we get this re-organization of the base framework complete, we still have a couple
of other issues ot resolve:
>     *  Still need the ability to extend EMF's through a ProductDerivation.  This should
be doable by adding a new PluginValue to indicate what class of EMF to load.
>     *  There is still a question as to whether we will need to provide a custom PersistenceProviderImpl
and ConfigurationProviderImpl pair.  I still think this will be necessary.   And, one of Abe's
posts indicated that this might help with class loading issues when multiple versions of OpenJPA-based
implementations are available in the same system.
> I also posted these questions last Friday.  (Abe has responded with some answers, but
I wanted to get this JIRA report created before trying to paraphrase his answers.)
>     *  You mention in several places about separating away the notion of specs and stores.
 In a general sense, I understand what these are.  But, can you elaborate on how these types
are used in the ConfigurationProvider and ProductDerivation interfaces?
>     * I've moved the ProductDerivation interface to the lib and added the "load" methods
from the ConfigurationProvider (as described in your previous notes).  And, I've started to
clean up the implementations that depend on these interfaces.  But, concerning the implementation
of the load methods...  Now that we need to return a ConfigurationProvider, would you expect
that we just new up a ConfigurationProviderImpl and then just call across to the "load" methods
on the implementation?  Since we want to keep the ProductDerivations stateless, I'm not sure
how else you were expecting to create a ConfigurationProvider to return on these "load" methods.
>     * Now that ConfigurationProvider is bare, the ConfigurationTestConfigurationProvider
doesn't have much function.  I'll need to take a look to see if this is even required any
>     * Can you shed a bit more light on the Configurations class?  It doesn't implement
nor extend any interfaces or classes, but it seems to provide many of the same methods as
ConfigurationProvider, but as statics.  And, it's dependent on having a Provider.  Can you
explain the relationship of this class in the bigger picture and how you think it might be
affected by thes changes?
> That's enough for the initial JIRA report.  We will now track this problem here instead
of the dev mailing list.  Thanks.
> Kevin

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