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From David Bosschaert <>
Subject Re: [SPI Fly] Interest for a more dynamic plug-in/service discovery API?
Date Tue, 12 Jun 2012 12:47:01 GMT
Hi Jeremias,

Many thanks for the suggestion. You are highlighting a valid point:
namely that people often have the need for services, like the ones
supported in OSGi, but are unable to (or don't want to) modularize
their project in order to be able to use OSGi.
There is currently some work underway in the OSGi expert groups that
relates to this.

The PojoSR project ( is a relatively
mature project that implements most of the OSGi Service Registry and
also supports things like the BundleActivator without doing the
modularity part of OSGi. So in a way it is similar to what you have
written, if I'm not mistaken.

In the OSGi Core Platform Expert Group there is currently an effort
underway to create a specification inspired by PojoSR. I will ensure
that during the specification work your work is also considered as
input, if you wish [1].

So in short - there will be a specification around this in the not too
distant future, your work can be considered a stepping stone to this
specification and it could even possibly become a compliant
implementation in the future. I assume that PojoSR will also become
compliant but I think that there is always room for multiple
implementations of a spec.

If you are interested in updating your implementation to comply with
the spec in the future then I think that Apache Aries would be a good
place to put your implementation right now from where it can mature. I
think that it should not be a subcomponent of SPI-Fly but rather a
top-level component of its own, within Aries.

Best regards,


[1] or, if you'd rather do that yourself, let me know :)

On 8 June 2012 10:42, Jeremias Maerki <> wrote:
> Hi there,
> due to my involvement with the Apache XML Graphics project (FOP/Batik)
> where we make heavy use of META-INF/services, I've got a big interest in
> how best to bring FOP and Batik into OSGi. My particular requirement
> here is that FOP and Batik have to continue working in plain Java but
> profit from the OSGi service registry for plug-in discovery when
> possible.
> So, two years ago I came up with this:
> That works half-way good enough although I've never felt comfortable
> enough to push it back to the XML Graphics project as I've had a few
> flaws in the above implementation. So, by now SPI Fly does a lot of
> things much better than my approach and it is following an RFC that I
> find very useful.
> However, on the client side, RFC 167 is a bit heavy on J2SE-1.6's
> ServiceLoader. Apache XML Graphics is still on J2SE-1.5 (yes, I know, I
> know) so it contains its own to find plug-ins via
> META-INF/services. Furthermore, with ServiceLoader you basically have to
> poll for changes.
> Finally getting to my point: I'd like to offer a little API that brings
> some of the service dynamics you get with OSGi services. This can
> already be seen on the page indicated above although I've slightly
> modified the API. Essentially, to get notified about new or disappearing
> plug-ins/services, you can register a service listener. And that would
> look like this (client code):
>        ServiceTracker<ImageWriter> tracker = Plugins.getServiceTracker(ImageWriter.class);
>        tracker.addServiceListener(new ServiceListener<ImageWriter>() {
>            public void added(ImageWriter writer) {
>                register(writer);
>            }
>            public void removed(ImageWriter writer) {
>                unregister(writer);
>            }
>        });
> Where Plugins is:
> public class Plugins {
>    /**
>     * This is the services singleton.
>     */
>    private static final Services SERVICES = new Services();
>    public static void setServicesBackend(ServicesBackend backend) {
>        SERVICES.setServicesBackend(backend);
>    }
>    public static <T> ServiceTracker<T> getServiceTracker(Class<T>
providerIntf) {
>        return SERVICES.getServiceTracker(providerIntf);
>    }
> }
> And the BundleActivator for the client looks like this:
> public class Activator implements BundleActivator {
>    private volatile ServicesOSGi services;
>    /** {@inheritDoc} */
>    public void start(BundleContext context) throws Exception {
> = new ServicesOSGi(context);
>        Plugins.setServicesBackend(;
>    }
>    /** {@inheritDoc} */
>    public void stop(BundleContext context) throws Exception {
>        Plugins.setServicesBackend(null);
>        if ( != null) {
>  ;
>        }
>    }
> }
> What happens is this: by default "Services" starts up with a plain-Java
> backend that simply looks up services through classic means. When the
> BundleActivator is called, the backend is replaced by ServicesOSGi that,
> instead, gets the plug-ins from the service registry. In the background,
> any previously discovered plug-ins are "removed()" and the new ones from
> the registry "added()".
> In the case of classic Java, the "removed()" method is never called
> (probably doesn't ever need to be), i.e. no dynamics there.
> To conclude: the goal is to profit from OSGi service dynamics when
> discovering plug-ins while preserving the possibility to run without
> OSGi API runtime dependencies and still in J2SE-1.5. Furthermore, the
> OSGi-specific code shall be as minimal as possible to shield those with
> no OSGi knowledge in the team from the learning curve.
> And now, I'm wondering if you're interested to adopt this as a new part
> in SPI Fly as a useful alternative to using ServiceLoader. If you don't
> want it, I'm going to publish it under Apache Extras.
> Thanks,
> Jeremias Maerki

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