OSGI Service Lookup in stanbol.commons.web.base

[Rupert Westenthaler <ru...@gmail.com>]

Hi all

The ContextHelper utility class [1] contains the following Method

    public static <T> T getServiceFromContext(Class<T> clazz,
ServletContext context) {
        BundleContext bundleContext = (BundleContext)
context.getAttribute(BundleContext.class.getName());
        ServiceReference reference =
bundleContext.getServiceReference(clazz.getName());
        return (T) bundleContext.getService(reference);
    }

This is typically called by JAX-RS Providers and Resources to lookup
required OSGI services via the servlet context. Such resources
(typically) do have a score that is limited to a single request (so
that implementers do not have to worry about threading issues).
Therefore one need to expect a lot of calls to this method.

I am not worried about the frequent lookup of services, but I am
worried about that with this there are no calls to the ungetService
[2] method.

If this is a problem I see two alternatives:
 (1)  add an additional method to WebFragment that allows to specify
the services this Fragment needs. Than it would be possible to add
such services to the Servlet context as soon as the WebFragment starts
and remove it when the fragment is removed. The above method would
than just return the service already present within the servlet
context
 (2) remove this method altogether and replace it with one that just
returns the BundleContext. Callers would than need to lookup, get and
unget the services themselves.

WDYT
Rupert

[1] http://svn.apache.org/repos/asf/incubator/stanbol/trunk/commons/web/base/src/main/java/org/apache/stanbol/commons/web/base/ContextHelper.java
[2] http://www.osgi.org/javadoc/r4v43/org/osgi/framework/BundleContext.html#ungetService(org.osgi.framework.ServiceReference)


-- 
| Rupert Westenthaler             rupert.westenthaler@gmail.com
| Bodenlehenstraße 11                             ++43-699-11108907
| A-5500 Bischofshofen

Re: OSGI Service Lookup in stanbol.commons.web.base

[Olivier Grisel <ol...@ensta.org>]

2011/5/11 Rupert Westenthaler <ru...@gmail.com>:
> Hi all
>
> The ContextHelper utility class [1] contains the following Method
>
>    public static <T> T getServiceFromContext(Class<T> clazz,
> ServletContext context) {
>        BundleContext bundleContext = (BundleContext)
> context.getAttribute(BundleContext.class.getName());
>        ServiceReference reference =
> bundleContext.getServiceReference(clazz.getName());
>        return (T) bundleContext.getService(reference);
>    }
>
> This is typically called by JAX-RS Providers and Resources to lookup
> required OSGI services via the servlet context. Such resources
> (typically) do have a score that is limited to a single request (so
> that implementers do not have to worry about threading issues).
> Therefore one need to expect a lot of calls to this method.
>
> I am not worried about the frequent lookup of services, but I am
> worried about that with this there are no calls to the ungetService
> [2] method.
>
> If this is a problem I see two alternatives:
>  (1)  add an additional method to WebFragment that allows to specify
> the services this Fragment needs. Than it would be possible to add
> such services to the Servlet context as soon as the WebFragment starts
> and remove it when the fragment is removed. The above method would
> than just return the service already present within the servlet
> context

+1.

The endpoint should leverage the JAX-RS @Context annotation to
introspect all the JAX-RS components contributed by WebFragments to
build the list of OSGi service that need to be looked up by the
endpoint once to be used and re-used by the many instances of JAX-RS
components.

We need to have a look at how jersey handles context lookups and
whether this can be extended for our purposes.

-- 
Olivier
http://twitter.com/ogrisel - http://github.com/ogrisel