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Re: [equinox-dev] Bridging the plain Java and OSGi classloading worlds

If the service defined and registered inside the OSGi framework is relatively simple, you might consider proxying your requests using Java reflection.

For example, if you've launched the OSGi framework using the new standard launch APIs, you will have access to the framework's bundle context, through which you can query the service. It will be returned as an object. If you know the expected signatures of the service methods, you can query its methods and invoke them without having code statically linked to it (which won't work because that service comes from a class loader inside OSGi). Reflection in effect bypasses the class loader.

I've done this myself for some simple cases, and the disclaimer is around the "relatively simple." If your service uses only JDK classes, this is pretty easy. It gets really messy really fast when you find yourself proxying multiple calls and queries.

In my case I queried the package admin service and refreshed packages, all without compiling against the PackageAdmin service class in my code. As a matter of fact, this was before the new OSGi framework launch API, and I created a URLClassLoader for the Equinox bundle, and instantiated and invoking the EclipseStarter from the outside. In other words, my code didn't even have access to org.osgi.framework.*...

I hope this sketch gives you some ideas.

/djk

On Aug 19, 2009, at 1:32 PM, Michael Furtak wrote:

Hi Hal,

Thanks for those references. They were helpful. It feels like I’m close, but problems remain.

Also, I’m a bit confused as to whether the noted techniques can help my problem. My understanding of the system packages and boot delegation stuff is that it helps OSGi find classes that are not explicitly exported from any of the OSGi bundles (i.e. java.*, javax.* stuff, etc.)

My dilemma is somewhat the opposite. I’m originating from Java, outside of the OSGi environment, but want to call objects created inside of OSGi. Of course, this leads to a ClassCastException when I get the service object and try to refer to it by its interface as loaded in Java land.

I’ve attached my source w/ some class/package names changed to protect the innocent. Also attached is my config.ini file. Without the org.osgi.framework.system.packages.extra directive, the ClassCastException occurs. With it, I don’t get a callback on my ServiceTrackerCustomizer’s addedService() method.

Do you have any further thoughts?

Thanks in advance,
-Mike





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From: equinox-dev-bounces@xxxxxxxxxxx [mailto:equinox-dev-bounces@xxxxxxxxxxx ] On Behalf Of Hal Hildebrand
Sent: Wednesday, August 19, 2009 3:18 PM
To: Equinox development mailing list
Subject: Re: [equinox-dev] Bridging the plain Java and OSGi classloading worlds


You can also check out: http://felix.apache.org/site/apache-felix-framework-launching-and-embedding.html

note the section "Creating and Configuring the Framework Instance", and the use of the property "org.osgi.framework.system.packages.extra" which makes this a bit easier. Works the same way with Equinox.

On Aug 19, 2009, at 12:13 PM, Hal Hildebrand wrote:


Sure. See http://blog.springsource.com/2009/01/19/exposing-the-boot-classpath-in-osgi/ for an in depth discussion by Costin of SS.


On Aug 19, 2009, at 12:08 PM, Michael Furtak wrote:


Hi Hal,

Thanks very much for your reply. I’m heartened that you’re confident in this possibility. Is there any documentation for the option you mention that I can refer to?

Thanks,
-Mike






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From: equinox-dev-bounces@xxxxxxxxxxx [mailto:equinox-dev-bounces@xxxxxxxxxxx ] On Behalf Of Hal Hildebrand
Sent: Wednesday, August 19, 2009 3:01 PM
To: Equinox development mailing list
Subject: Re: [equinox-dev] Bridging the plain Java and OSGi classloading worlds


sure, you just have to have a class loader in common. The way this would work is to modify the exports of the System bundle (an option when starting the framework in Equinox) to export the classes from the base class path you need. Then there is no class loader issues, as they're both using the same.

On Aug 19, 2009, at 11:56 AM, Michael Furtak wrote:



Hi all,

I am using the EclipseStarter.startup(…) method to bring up the Equninox framework from within a plain ol’ Java application. My question is this:

Is it possible to allow the plain ol’ Java environment to get and call service objects from the OSGi side? I know their respective classloader environments are disjoint, but is there any hope at all of bridging them? Has anyone been dumb/crazy enough to want to do this?

Thanks,
-Mike




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