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From David Jencks <david_jen...@yahoo.com>
Subject Re: Problem loading classes with Class.forName() in StandardContext.createWrapper()
Date Tue, 05 Jan 2010 17:41:33 GMT

On Jan 4, 2010, at 2:56 AM, Mark Thomas wrote:

> On 04/01/2010 02:46, Tim Whittington wrote:
>>
> No need to re-post. A ping would have sufficed.
>
>> We've experienced similar issues integrating lots of third party  
>> libraries (Tomcat being one of them) into our OSGi runtime.
>
> Thanks for your input. I can't speak for the other Tomcat committers  
> but
> I know very little about OSGI so it is useful to have input from those
> more experienced.
>
>> Essentially this boils down to OSGi liking extension functionality  
>> to be provided by instantiation in the providing bundles, and  
>> publication using OSGi services.
>> (i.e. pure OSGi services or declarative services etc.). Put another  
>> way, the concepts of global application classloaders where  
>> Class.forName works (ala Java EE etc.) break down in OSGi.
>>
>> Libraries that use the TCCL in preference to Class.forName help,  
>> but in cases like the one you describe where there's no direct  
>> invocation from the bundle that has access to the classes it  
>> doesn't help. Fragment bundles (and Eclipse specific buddy  
>> classloaders) also help, and we've had to use them with tools like  
>> Hibernate, but they're less than ideal.
>>
>> I think the conclusion you've reached is correct - either fragment  
>> bundles (which is a sub-optimal solution) or a pluggable extension  
>> loading framework could be the solution.
>
> OK. The consensus amongst those that know OSGI seems to be some form  
> for
> pluggable extension point.
>
>> I believe we ran into issues like this when integrating Tomcat 5.5  
>> into our OSGi runtime - we had to patch up the web app ClassLoaders  
>> at runtime to make taglib loading work.
>> In that case we were able to wrap the ClassLoaders with the help of  
>> some declarative metadata in bundles containing taglibs.
>>
>> If you can tell which bundles can contain implementions of whatever  
>> it is you're trying to instantiate, you can construct a ClassLoader  
>> spanning those bundles yourself and use that (you'd only want to  
>> use it for loading these extensions, as it defeats the purpose/ 
>> nature of OSGi to some extent to do this).
>> It might be that the web application bundles would be all you need  
>> (and the upcoming OSGi Enterprise Spec will give you a standard way  
>> of locating these), and that'd probably be a reasonable limitation,  
>> or you could accomodate applications partitioned to a finer degree  
>> by some additional marker to include other bundles.
>>
>> The other, more OSGi approach would be for listeners to be  
>> published as OSGi services with target properties, that are then  
>> just looked up by name by the OSGi version of the extension loader  
>> (as opposed to instantiating them).
>> i.e. an instance of the Listener interface is published by a bundle  
>> as an OSGi service with a property  
>> tomcatClassName=org.myproject.impl.MyListener. The extension loader  
>> then looks up the service with a property filter on  
>> 'tomcatClassName' to find the available extension.
>> OSGi apps using Tomcat would simply publish these using Declarative  
>> Services or similar, and this would be a very natural approach for  
>> an OSGi app.
>
> Using Services does seem more in the spirit of OSGI.
>
>> With this latter approach you have delightful lifecycle management  
>> issues because of the dynamic nature of OSGi (extension bundles  
>> starting after the Tomcat bundles for instance). We solve some of  
>> these with a combination of declarative only metadata (using the  
>> Eclipse Extension Registry) to advertise extension existence on  
>> bundle resolution, and Declarative Services to instantiate and  
>> publish the actual extension, and some by having the framework  
>> accept dynamic injection of extensions (Listeners come and go).
>
> Glad I don't have to worry about those issues :)
>
> The main purpose of the InstanceManager is meet some of the  
> requirements
> for annotations support. As such, it is only used for instances that  
> may
> have annotations. There is one InstanceManager instance per web  
> application.
>
> One thing that isn't clear to me is whether the requirement is for an
> extension point for web application related instances (ie things  
> that in
> a J2EE environment would be bundled in the WAR) or for container  
> related
> instances such as LifecycleListeners. The current patch in bug 48414
> seems to focussed on Tomcat internals and I don't understand how the
> line was drawn between what to access via the InstanceManager and what
> not to.

Thanks for taking another look at this subject. I've been thinking of  
the InstanceManager as the extension point for creating objects by  
reflection rather than as the annotation handler, perhaps because of  
how the Geronimo InstanceManager happens to be implemented.  So, it  
seems to me that adding a newSystemInstance method to it for creating  
objects that are expected to come from the system rather than  
application classes is reasonable.  I'll try to come up with a patch  
using this additional method in the next day or two.

thanks again!
david jencks


>
> Mark
>
>
>
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