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From Guillaume Nodet <>
Subject Re: OSGi support for libraries that use the META-INF/services
Date Mon, 18 Jan 2010 17:35:36 GMT
FWIW, the javamail spec do use the META-INF/services discovery
mechanism, but to detect and instantiate classes using constructors
that have arguments.
We need to investigate if it could cause any real problem with the
design ... or find a specific way for javamail maybe ...

On Mon, Jan 18, 2010 at 15:20,  <> wrote:
> Very well, then.
> For the moment, I've nicknamed my component 'SPI Fly', but I'm open to
> any other name too.
> Currently the implementation does the following:
> * For every new bundle being installed it checkes the
> META-INF/services directory for any files. If found it registers a
> service in the Service Registry with the following property
>  spi.provider.url = {the URL to the associated META-INF/services file}
> * I think it would be better to do an opt-in for the bundle. So only
> do this when the bundle has a specific heading in its manifest (like
> SPI-Provider or something). I will enhance the code with this shortly.
> So what's there now works fine for bundles that are aware of this
> mechanism, clients simply look in the service registry for their SPI
> implementations.
> It doesn't fix anything for clients that aren't aware of this feature.
> So we need more ideas in that area...
> I would propose using the Wiki to collaborate further on the design. I've added:
> I'll commit my code + tests to
> soon,
> unless someone objects...
> Best regards,
> David
> 2010/1/14 Ian Robinson <>:
>> One of the goals of Aries is to develop implementation to inform new OSGi
>> specs so ironing out the thinking through implementation in Aries is a good
>> thing.  I'd be interested in seeing this contribution.
>> Ian
>> On 14/01/2010 09:04, wrote:
>>> Hi David,
>>> I'm familiar with the work that Guillaume did for this. It shadows the
>>> Factory Finder of a particular implementation in the wrapping bundle
>>> and makes it OSGi-friendly (unless I'm wildly mistaken).
>>> I'm not aware of it being a separate component of itself, the SMX work
>>> I know is embedded in the SMX-bundles components.
>>> What I was thinking of is a general 'SPI-support' component that you
>>> could install in an OSGi framework and that would make SPI providers
>>> work. I'm proposing to create and collaborate of a generic component
>>> like this and I think Aries would be a good place to do this. I would
>>> welcome any contributions to it, including anything that we could
>>> reuse from the SMX work that Guillaume did.
>>> As mentioned, I'm working in parallel on a proposed standard in the
>>> OSGi Alliance around this and would like to use this work as input.
>>> I hope this makes sense...
>>> Best regards,
>>> David
>>> 2010/1/14 David Jencks <>:
>>>> I think Guillaume Nodet has implemented something like this for
>>>> servicemix.
>>>>  I'm not sure where the code is currently but I hope we don't need more
>>>> than
>>>> one implementation of this idea at apache.
>>>> thanks
>>>> david jencks
>>>> On Jan 13, 2010, at 2:46 PM, wrote:
>>>>> Hi all,
>>>>> One of the things that seems to be coming up regularly is the support
>>>>> in OSGi for libraries that make use of what is often referred to as
>>>>> the JRE SPI model. This model is used in many pluggable technologies
>>>>> coming out of the JCP and typically involves a FactoryFinder or the
>>>>> ServiceLoader class. Some technologies that use this mechanism are
>>>>> JAXP, JAXB, JAXWS, JAXRS, media codecs and many more...
>>>>> Generally this model causes problems in OSGi, mostly because in OSGi
>>>>> it's impossible to export-package META-INF/services from multiple
>>>>> bundles (well you can export it multiple times, but a consumer will
>>>>> only ever get linked to a single one) and also because the mechanism
>>>>> doesn't take the OSGi Classloaders into account.
>>>>> There are a number FactoryFinder implementations in the JRE, they vary
>>>>> slightly but in general they try to do two things:
>>>>> A. Find the class name of a factory implementation of technology X.
>>>>> The factory implements/subclasses an interface/class called x.y.Z.
>>>>> They roughly take the following steps:
>>>>>  1. Look up a system property (x.y.Z) - if set use this class name
>>>>>  2. Look up a well known file in ${java.home}/lib (this step isn't
>>>>> always there) - if found read the class name from it
>>>>>  3. Load a resource called META-INF/services/x.y.Z using
>>>>> ClassLoader.getResource(). Typically the ThreadContext classloader is
>>>>> tried as well as the system classloader. Sometimes a classloader is
>>>>> passed in to the Finder. If the resource can be loaded, read class
>>>>> name from it.
>>>>>  4. If all of the above fails, use a default hardcoded classname
>>>>> B. Once the classname has been obtained the FactoryFinder will try to
>>>>> load that class using either a provided classloader, the Thread
>>>>> Context Classloader or the system classloader.
>>>>> OSGi has solutions for a few individual cases of this problem. E.g.
>>>>> the XML Parser Specification handles this for JAXP, but rather than
>>>>> having to write a spec for every single use of this I'd rather like to
>>>>> see if we can come up with a generic solution to this problem.
>>>>> Therefore I'd like to propose an Aries component to address this issue
>>>>> in a generic manner. I've written a small extender that scans bundles
>>>>> for META-INF/services files, instantiating any services found and
>>>>> registering them with the OSGi Service Registry.
>>>>> While this doesn't cover use-cases yet for clients that use the
>>>>> JRE-style lookup (generally through a static method), but it does
>>>>> provide some value to OSGi aware clients as they can look up their
>>>>> services in the Service Registry.
>>>>> What I have is just a starting point, I'd like to see how far we can
>>>>> get building this out, hopefully also supporting the traditional
>>>>> client lookup scenarios.
>>>>> Ultimately once we have something that works this can be brought into
>>>>> the OSGi Alliance as input in the standardization process, so that
>>>>> hopefully at some point we'll have a standard covering this issue
>>>>> nicely.
>>>>> So... does the Aries community think this would be a useful component
>>>>> to work on? If so I can commit my code and tests, and we can
>>>>> collaborate on it from there...
>>>>> Best regards,
>>>>> David

Guillaume Nodet
Open Source SOA

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