Thanks for the clarification now I got it.
Yes whiteboard model should work nicely. Especially as this component might not be the only subscriber.


Christian

On 14.10.2016 08:58, Peter Kriens wrote:
@Christian: The MQTT client is optional and dynamic. So the activate method 
cannot be used. You need to subscribe/unsubscribe based on the availability of 
the mqtt server.

@Daghan:

Divide and conquer! You’re trying to do multiple responsibilities in one 
component and that is the antithesis of modularity, also called lack of 
cohesion. This is a perfect example of how things could get simpler by choosing 
the right decomposition.

The best solution imho is to introduce a second,component. Let the Device 
component just be a Device, it should not have to worry about Mqtt. After all, 
you could be connected to other event queues. (It also makes it easier to 
test.) You made mqtt optional to reflect this. This is exactly the reason the 
Whiteboard Pattern was invented!

This would look like:


        @Component( property = “subscriptionChannel=” )
        public class LT100HDeviceImpl implements Device, TtnMqttMessageListener 
{
                // remove the mqtt subscription code but
                // implement the TtnMqttMessageListener
        }

And the whiteboard component. Notice the _ in the _mqtt field. This ensures it 
is set before the event method that has a name that will appear in a sorted 
list later. (References are sorted by name.) (I vaguely recall field references 
are done before bind methods but this makes it certain.)

        @Component(immediate=true)
        public class MQTTWhiteboard {

                @Reference
                TtnMqttProtocol _mqtt;

                @Reference( cardinality = ReferenceCardinality.OPTIONAL, policy 
= ReferencePolicy.DYNAMIC )
                void addMqttListener( TtnMqttMessageListener l, 
Map<String,Object> props ) {
                        String channel = props.get( “subscriptionChannel” );
                        if ( channel != null && !channel.isEmpty() ) {
                                _mqtt. subscribe( channel, l );
                }

                void removeMqttListener( TtnMqttMessageListener l, 
Map<String,Object> props ) {
                        String channel = props.get( “subscriptionChannel” );
                        if ( channel != null && !channel.isEmpty() ) {
                                _mqtt.unsubscribe( channel );

                }
        }

You now created a whiteboard service that can also be used by other Device 
implementations while significantly reducing the complexity of your 
implementation.

This is why after all those years I still love OSGi … you can only do this when 
you have dynamic components. As your struggle showed, trying to manage this is 
quickly becoming quite complex. Whenever you enter in such a struggle, think, 
lack of cohesion is often your problem.

Kind regards,

        Peter Kriens


On 14 okt. 2016, at 08:09, Christian Schneider <ch...@die-schneider.net> wrote:

In your case simply inject the ttnMqttClient in the @Reference and do the 
subscribe in @Activate when you get the config and the unsubscribe in 
@Deactivate.

Christian

2016-10-13 23:00 GMT+02:00 Daghan ACAY <daghana...@hotmail.com>:
Hi all,

I am trying to create a component that is instantiated by ConfigAdmin and uses 
multiple references to operate. Basically the component should instantiate 
through a factory configuration and use that configuration to set up its own 
@Reference s. You can see the code here:

https://github.com/daghanacay/com.easyiot.device/blob/master/com.easyiot.LT100H.device.provider/src/com/easyiot/LT100H/device/provider/LT100HDeviceImpl.java

All the mentioned @Reference ed components are instantiated by configuration as 
well, so at a given time the @Reference might not be available but my own 
component should still work. yet should the Reference available then it should 
be injected, basic 0-1 strategy.

Problem I am facing with the current form of the code is that, the @Reference 
injection is happening before the @Activate method is called. This leads to NPE 
in the @Reference method due to null configuration. Is it possible to make this 
code work such that config is provided to the component before the dependency 
injection?

I have tried annotating the class fields and set them "volatile". I even make 
them a list and use the class fields in the activate method this time the class fields 
were null due to 0-1 strategy. so I end up with annotating the methods.

I might have designed this all wrong, so any help simple or fundamental is 
appreciated.

Regards

-Daghan

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Christian Schneider
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Open Source Architect
http://www.talend.com
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--
Christian Schneider
http://www.liquid-reality.de

Open Source Architect
http://www.talend.com

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