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Re: [equinox-dev] Declarative Services use case and inconsistencies between old and new (ProSyst-contributed) implementations

I think you should be able to call the locateService(String, 
ServiceReference) method from within the bind method. The call to 
addElement is in the finally, so the seems no valid reason not to call 
addElement before the bind method.

I suggest opening a bug against the incubator component against the new 
ProSyst SCR impl and supply your patch.

BJ Hargrave
Senior Technical Staff Member, IBM
OSGi Fellow and CTO of the OSGi Alliance
hargrave@xxxxxxxxxx

office: +1 386 848 1781
mobile: +1 386 848 3788




"Jeremy Volkman" <jvolkman@xxxxxxxxx> 
Sent by: equinox-dev-bounces@xxxxxxxxxxx
2007-04-13 17:01
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Subject
[equinox-dev] Declarative Services use case and inconsistencies between 
old and new (ProSyst-contributed) implementations






Hi all,

We've recently switched to the ProSyst implementation of SCR in hopes of 
overcoming many of the issues the old one had.  I've encountered an 
implementation detail with the new version that affects one of my standard 
practices. 

We use the whiteboard pattern in a bunch of places to register extensions 
to certain things (similar to the use of Eclipse's Extension registry). 
With many of the extension objects, service properties are used to define 
some metadata. 

My general methodology is to declare a component that has a reference to 
the particular extension interface with cardinality 0..n.  I then create a 
bind method that looks something like this:

public void extensionAdded(ServiceReference extensionRef) { 
  String extensionId = (String) extensionRef.getProperty("extension.id");
  String extensionName = (String) extensionRef.getProperty(" 
extension.name");
  IExtension extension = (IExtension) 
componentContext.locateService("extensions", extensionRef);
  registerExtension(extensionId, extensionName, extension);
}

It's necessary to have a bind method that takes type ServiceReference in 
order to get these service properties.  AFAIK there is no way to get 
service properties after this method has been called other than storing 
them locally (there is no "locateServiceReferences()" method). 

This used to work with the original implementation of SCR, however the 
ProSyst implementation returns null when I call locateService(String, 
ServiceReference) from within the bind method.  I've tracked the issue 
down to the following code at ComponentReference.java:250: 

try {
  bindMethod.invoke(instance.getInstance(), params);
} catch (Throwable t) {
  Activator.log.error(
      "[SCR] Error while trying to bind reference "
       + this, t); 
       // rethrow exception so resolver is eventually notified that
       // this component is bad
       // throw t;
 } finally {
** serviceReferences.addElement(serviceReference);
   SCRUtil.release(params); 
 }

The marked line is the issue.  The bound ServiceReference is being added 
to the ComponentReference's list of tracked ServiceReference after the 
bind method is being invoked.  The locateService(String, ServiceReference) 
method fails to match the passed ServiceReference to this internal list, 
and thus returns null.  I can only assume that the original SCR 
implementation ( org.eclipse.equinox.ds) stores the ServiceReference 
before invoking the bind method.

I couldn't find anything in the DS spec that was clear about which way to 
act in this situation, which is why I'm posting here and not in a bug 
report.  Should I be able to locate the service object for a 
ServiceReference from within the bind method?  If not, how can I ever get 
service properties for DS-controlled references? 

At this point I'm going to patch the code locally to add the 
ServiceReference before invoking the bind method and hope that is the 
correct behavior. :)

Thanks,
Jeremy Volkman_______________________________________________
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