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From Gianny Damour <>
Subject Re: Whence the geronimo kernel?
Date Tue, 17 Mar 2009 11:24:39 GMT

On 13/03/2009, at 6:44 PM, David Jencks wrote:

> On Mar 12, 2009, at 10:02 AM, David Jencks wrote:
>> On Mar 12, 2009, at 3:25 AM, Gianny Damour wrote:
>>> On 12/03/2009, at 4:29 AM, David Jencks wrote:
>>>> I think I probably have the most experience with classloading  
>>>> problems in geronimo and the only real problem that arises is  
>>>> loading a jar in two different classloaders.   This can be  
>>>> solved by a classloader-per-jar model which offers no  
>>>> theoretical problems to set up in geronimo but practically would  
>>>> take a lot of work (and maven projects to build a plugin per  
>>>> jar).  So I think we'll have to see what kind of problems we get  
>>>> with trying to actually use OSGI.
>>> Hi,
>>> Thinking more about this, I believe we can expedite the  
>>> implementation of a classloader-per-jar model. Under the hood of  
>>> a MultiParentClassLoader we can replace the current  
>>> implementation of find class and resources contracts by an  
>>> implementation which delegates to a bunch of URLClassLoaders (one  
>>> per jar). These bunch of URLClassLoaders are global classloaders,  
>>> i.e. shared across all the configs/MultiParentClassLoaders. The  
>>> core challenge is to create them in a hierarchy respecting the  
>>> maven dependency declarations. So, we could install the pom of  
>>> the dependencies in the repo and lazily parse them when  
>>> MultiParentClassLoader are created to build this global and share  
>>> tree of URLClassLoaders.
>> IMO the danger here is that the maven pom may not exist or may be  
>> wrong.  OSGI has the same problem in that the vast majority of  
>> released jars don't have osgi manifests.  I think I saw a rumor  
>> that spring spent a lot of effort osgi-ifying a lot of commonly  
>> used jars to try to solve this problem.
>> I also don't know if there are situations in which a small number  
>> of closely related jars need to be loaded in a single classloader,  
>> perhaps because one of the jars is "optional" but if present the  
>> "main" jar needs access to its classes.  I think there was an osgi  
>> feature that looked sort of like this.
>>> I just started to work on it and I will post back my findings (i  
>>> should be able to complete this over the week-end). Even if we  
>>> switch to an OSGi kernel, part of this work may hopefully still  
>>> be useful.
>> Unless you are pretty sure we won't have the kind of problems with  
>> bad community metadata suggested above it might be a good idea to  
>> do this in a sandbox branch?
> Thinking about this more I really expect the code will be easy  
> compared to straightening out the dependencies :-(
> As a concrete example, the JACC spec needs the servlet and ejb  
> specs but in our maven poms we've excluded them as dependencies so  
> as to make it easier to upgrade the jars independently.  While  
> (especially with maven 3) we can probably put in the dependencies  
> with version ranges, they aren't there now.  Getting the server to  
> work again may be time consuming.  I really hope I'm wrong :-D

Hi David,

I just created a patch

which provides a preview of the code changes necessary to move to a  
one classloader per jar model. It is not yet good enough to create a  
branch as only 22 configurations out of the 27 of the jetty-jee5  
assemblies can successfully start (failing config is certainly a  
maven dependency problem).

The design is rather simple:
1. when a configuraton classloader is created, configuration  
dependencies and classPath entries are used to generate a bunch of  
GlobalClassLoaders - one per dependency or one for all the classPath  
2. this set of GlobalClassLoaders are put in a hierarchy according to  
maven dependency declarations (note that dependency version  
declarations are not honored even if present). Maven dependency  
declarations can be augmented by dropping an XML file with maven like  
<dependencies> declarations.
3. the classloaders of the parent configuration of the configuration  
being created are added to this set of GlobalClassLoaders.
4. this set of GlobalClassLoaders is used under the hood of the  
configuration's MultiParentClassLoader to implement find class and  
resources contracts.

Four maven dependency declarations had to be supplemented to start 22  
configurations. Assuming the same ratio, about 14 dependency  
declarations may have to be supplemented to start the jetty-jee5  
assembly. This is not too involving.

Could you please have a cursory review of this preview patch and let  
me know your thoughts?


> thanks
> david jencks
>> thanks
>> david jencks
>>> Thanks,
>>> Gianny
>>>> One thing I'd really like actual user data on is how people  
>>>> actually specify osgi classloading info in real life.  I'm very  
>>>> aware that in theory you are supposed to specify the package  
>>>> imports and exports for your bundle but I've been told that in  
>>>> real life everyone with a serious osgi project actually  
>>>> specifies the jar dependencies they want using require-bundle.
>>>> thanks
>>>> david jencks
>>>>> Thanks,
>>>>> Gianny
>>>>> On 11/03/2009, at 7:11 PM, Guillaume Nodet wrote:
>>>>>> On Wed, Mar 11, 2009 at 08:57, Gianny Damour  
>>>>>> <> wrote:
>>>>>> Hi,
>>>>>> FWIW, I believe that improving the configuration style to  
>>>>>> simplify the means of creating a bunch of objects in the  
>>>>>> kernel has more benefits than swapping the classloading infra.  
>>>>>> On paper OSGi may appear as superior from a classloading  
>>>>>> isolation perspective; however, I believe the current CLing  
>>>>>> design is nearly up to par with the OSGi one and that the main  
>>>>>> challenge is to properly tune export/import dependency  
>>>>>> declarations.
>>>>>> I have to disagree with that.  The CLing mechanism is very  
>>>>>> different in Geronimo (from what I recall) and OSGi.  Geronimo  
>>>>>> uses a multi-parent classloader style with some nice features  
>>>>>> to be able to hide / never override + parent or self-first  
>>>>>> delegation.
>>>>>> OSGi CLind is very different: the first one is that you don't  
>>>>>> really have parent classloaders: the classloader for a given  
>>>>>> OSGi bundle is calculated wrt to the constraints expressed in  
>>>>>> the OSGi manifest using imported packages or required bundles.
>>>>>> Let's take an example:
>>>>>> bundle A needs api packages from bundles B and C
>>>>>> implementation classes from bundle B and C needs something  
>>>>>> from bundle D but with different versions
>>>>>> OSGi will be able to handle that because of non tree-like  
>>>>>> CLind mechanism: if bundle A is wired to bundle B, it does not  
>>>>>> have to see all the requirements from bundle B, and same for  
>>>>>> C.  Therefore, bundle A can be wired to both B and C without  
>>>>>> problems because it will not see bundle D at all (so there's  
>>>>>> no conflicts between the two versions of bundle D).
>>>>>> OSGi has a much more powerful CLing mechanism where you can  
>>>>>> express lots of different constraints.  The drawback is that  
>>>>>> establishing the classloader can take a bit of time, so going  
>>>>>> to OSGi most certainly leads to a big slowdown at startup  
>>>>>> while creating the classloaders.
>>>>>> Also, OSGi does not really play nicely with the usual JEE way  
>>>>>> to discover implementations through the MANIFEST/services  
>>>>>> entries.  That's kinda what we've tried to solve in servicemix  
>>>>>> specs, though I'm not sure if that really applies everywhere  
>>>>>> because I would imagine the classloaders for EARs are not  
>>>>>> really OSGi classloaders ...
>>>>>> I certainly don't want to say OSGi is not the way to go, just  
>>>>>> want to make the point that there are benefits but also  
>>>>>> drawbacks.
>>>>>> The JAXB approach to turn xml plans to a bunch of objects is  
>>>>>> certainly interesting. I believe it is still a technology  
>>>>>> limiting decision whereby a lot of custom code will have to be  
>>>>>> implemented to support various style (factory methods or beans  
>>>>>> et cetera) of configurations. I have been bouncing around this  
>>>>>> idea a while back and here it is again. Why do we want to  
>>>>>> define a XML language to create a bunch of objects when  
>>>>>> scripting can do that for us?
>>>>>> I believe that xbean-spring is still unnecessary noisy when  
>>>>>> compared to something like the Spring Bean Builder (http:// 
>>>>>> If there is an interest in a scripting approach, then I can  
>>>>>> investigate further.
>>>>>> Thoughts?
>>>>>> Thanks,
>>>>>> Gianny
>>>>>> On 11/03/2009, at 6:54 AM, David Jencks wrote:
>>>>>> 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.
>>>>>> thanks
>>>>>> 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
>>>>>> -- 
>>>>>> Cheers,
>>>>>> Guillaume Nodet
>>>>>> ------------------------
>>>>>> Blog:
>>>>>> ------------------------
>>>>>> Open Source SOA

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