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From Guillaume Nodet <>
Subject Re: [API] Experimenting OSGI fragments to solve the extensibility issue
Date Thu, 20 Oct 2011 09:43:09 GMT
I think you first need to define your constraints and what you want to focus
If you want someone to be able to write a jar to extend the api which will
work in a non osgi environment and (with minimal changes) in an osgi
environment, go for fragments.
If you want to go a cleaner osgi way, go for services, but forget about
class names in the schema directly, and you need to define two different
ways, one for osgi and one in a non osgi environment (as you won't have osgi
services at all in a flat classloader).

It's all about trade-offs.

When talking with Emmanuel this week, it seems to me that for the api,
extending the api was not a very common operation and did not really require
the osgi dynamism.  Fragments are perfect for those simple use cases.   But
for sure, if you ask an OSGi purist, the recommandation will be to not use

On Thu, Oct 20, 2011 at 11:29, Alex Karasulu <> wrote:

> Excuse the pre post but my comment is somewhat general. Overall the Felix
> and general OSGi community considers fragments as a problematic construct in
> the specification: something they recommend you avoid but if stuck you can
> use. It's use is discouraged.
> On Wed, Oct 19, 2011 at 6:36 PM, Pierre-Arnaud Marcelot <>wrote:
>> Hi Dev,
>> Following our efforts on the OSGI side, I took some time to think about
>> (and experiment) how we could solve our extensibility issue on the LDAP API,
>> where we want our users to be able to provide their own custom
>> implementation of various schema objects like comparators, normalizers, etc.
>> After a quick discussion with Emmanuel and Guillaume Nodet, it turned out
>> using OSGI fragments bundles could probably be our best solution.
>> Here's a definition of what an OSGI fragment bundle is (more information
>> available at [1]):
>> "An OSGi fragment is a Java archive file with specific manifest headers
>> that enable it to attach to a specified host bundle or specified host
>> bundles in order to function.
>> Fragments are treated as part of the host bundles. Relevant definitions of
>> the fragment are merged with the host bundles definitions before the host is
>> resolved, as long as the information does not conflict.
>> Fragment dependencies are resolved if possible. If the fragment
>> dependencies can not be resolved, the fragment does not attach to the host
>> bundle.
>> A fragment can not have its own class loader or bundle activator. It can
>> not override the information present in the host bundles. Fragments extend
>> bundles with resources, classes, and permitted headers enabling you to
>> customize your bundles."
>> The great thing about fragments is that they *share* the same class loader
>> as their host bundle. Which was pretty much the kind of issue we were having
>> with classloaders being differents from one bundle to the other.
>> The other great thing I see with this approach, it that it would also work
>> great outside of an OSGI container application (which is a strong
>> requirement for the LDAP API). A fragment bundle is nothing else than a
>> regular bundle with a specific OSGI directive (Fragment-Host) added to its
>> MANIFEST.MF file, and a bundle behaves exactly like a plain jar file when
>> it's not included in an OSGI container. Thus, it would allow us to support
>> third parties extensions.
>> I have done a small experiment in my Eclipse workspace with two bundles
>> (one being the host and the other being the fragment ) and I was able to
>> classload, without any classloader issue, a class defined in the fragment
>> bundle from another class inside the host bundle.
>> I'd like to go a step further and experiment this on the API itself.
>> Ideally, I'd like to use the 'shared-ldap-model' module
>> ('' bundle) as a host for all our
>> schema objects implementations and move these implementations into a
>> specific fragment bundle containing them all.
>> Users would have to do the same to include their extensions. A simple
>> fragment bundle with the '' bundle as
>> host and it works.
>> One other thing that should be done, is to let the 'shared-ldap-model'
>> module do the instanciation of the schema elements. At the moment, two
>> classes from the 'shared-ldap-schema-data' module are responsible for this,
>> '' and
>> ''. We
>> would need to move these classes to the 'shared-ldap-model' module, for them
>> to have access to the right class loader.
>> All in all, I think that's the only things we need to do to get the
>> extensibility we wanted inside and outside an OSGI container.
>> What do you think of this plan?
>> Regards,
>> Pierre-Arnaud
>> [1] -
> --
> Best Regards,
> -- Alex

Guillaume Nodet
Open Source SOA

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