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From David Jencks <>
Subject Re: Whence the geronimo kernel?
Date Tue, 10 Mar 2009 19:54:12 GMT
So as mentioned below I'm starting to look into the osgi classloading  
bit, sort of "from the bottom".

Another approach to many of these issues is perhaps "from the top",  
from the point of view of going from a presumably xml plan to a bunch  
of objects.

I've long thought that it must be possible to leverage jaxb to do most  
of the heavy lifting here.  In particular sxc is some code we can  
presumably actually extend to do stuff like constructor dependency  
injection.  So another avenue that could perhaps be approached in  
parallel would be to investigate sxc, jaxb, xbean-spring, xbean- 
reflect, the blueprint service schema, and jsr299 requirements and see  
what we can come up with.

For instance, it might be possible to have a large part of the  
blueprint service functionality in jaxb-enabled objects that jaxb  
instantiates from the xml.  The "init" method could deal with feeding  
the metadata into the blueprint service core.  Maybe we can get sxc to  
use xbean-reflect to create the objects.

So far this is more or less wild speculation in my head...  but I  
think it would be a lot of fun to investigate.

david jencks

On Mar 4, 2009, at 4:56 PM, David Jencks wrote:

> Geronimo has been around for a while and despite the many good  
> features gbeans and the geronimo kernel are not catching on big  
> time.  I think we want to consider taking action now to avoid ending  
> up being dragged down by supporting a dead container.  Here are a  
> few thoughts.
> Actual problems with geronimo:
> - gbeans are too restrictive.  It's too hard to instantiate other  
> peoples components as gbeans.  GBeans don't support common patterns  
> like factory methods, factory beans, etc etc, and require the  
> component to be instantiated directly by the gbean framework.
> - it's too hard to get the classloaders to work.  The most common  
> problem is a class cast exception due to loading the same jar in two  
> plugins.  NoClassDefFound errors from an optional jar in a child  
> classloader are also really annoying.
> Really good things about geronimo I haven't seen elsewhere (at least  
> in one place):
> - gbean dependencies work across plugins.  Dependencies are a  
> unified system, not per-plugin.
> - gbean dependencies are resolved in the ancestors of a plugin, not  
> server wide.  This means that you can't make a partially specified  
> dependency ambiguous by deploying additional plugins.  I consider  
> this an extremely important feature for predictability.
> - plugin dependencies allow assembly of a server from the explicit  
> dependencies which are normally the same as the maven dependencies.
> Other projects and specs that have stuff we should look into:
> maven.  Maven has a lot better infrastructure for dealing with  
> dependency resolution from partial transitive dependency  
> specification than we do.  We should look into using more of their  
> infrastructure.
> osgi. osgi has a lot of similarities to geronimo. The osgi  
> classloading model is getting a lot of people excited.  The import- 
> bundle idea is pretty much the same as our classloader model where  
> every jar is a plugin.  I don't know if people are really using the  
> allegedly recommended method of specifying imports and exports and  
> letting the osgi runtime figure out where they come from; this seems  
> worth investigating to me. Also, we get periodic inquiries about  
> when we are going to support osgi and the was ce folks get even more.
> osgi blueprint service (rfc 124) This appears to be a simple wiring  
> framework for a single plugin.  IIUC it uses the osgi service  
> registry for component dependencies between bundles.
> xbean-spring.  I'd be reluctant to try to implement a blueprint  
> service that didn't provide the xbean-spring capabilities really well
> ee6 dependency injection.  EE6 is going to have a pretty  
> sophisticated dependency injection service which we'll need to  
> support anyway.  We should try to figure out how much of the core we  
> can assemble using it.
> Other great stuff we have:
> xbean-reflect, xbean-finder, xbean-spring
> These ideas have been floating around in my head for a long time and  
> I've chatted with various people about them occasionally.   While  
> more discussion is certainly needed on everything here I need to do  
> some implementation to understand much more.  So, what I'm planning  
> to do:
> Dave's crazy work plan...
> - Try to use the osgi classloader.  I think this involves putting  
> the classloader creation in Configuration into a service.   
> Configurations will turn into osgi bundles.  I'll put the Kernel in  
> the osgi ServiceRegistry so the Configuration bundle activator  
> should be able to use it to resolve cross-plugin dependencies.
> - try to figure out how maven dependency resolution fits into osgi.
> - see if eclipse p2 is relevant for provisioning geronimo repositories
> at this point I think geronimo would be running on osgi, still using  
> gbeans.
> - look into relaxing the gbean framework so it is more plugin-at-a- 
> time rather than gbean-at-a-time
> - see how that differs from the blueprint service, ee DI, and xbean- 
> spring.  Try to support all of these at once.
> Thoughts? Counter proposals?  Anyone interested?
> many thanks
> david jencks

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