This page describes the CXF Distributed OSGi with Declarative Services demo.
The Declarative Services demo uses a DS implementation to create a remoted OSGi service from a DS component. The consumer side uses DS to create a component that consumes the remote OSGi service. By using Declarative Services, you don't need to write code to interact with the OSGi Service Registry. That's all handled through injection which hugely simpliefies the code.
Declarative Services is similar to Spring-DM/OSGi Blueprint in that service dependencies are satisfied through injection. There are a few differences as well. DS is lighter weight than Spring-DM but also has less features. Declarative Services have been part of the OSGi specifications since version 4.0.
This demo can be used with any DOSGi distribution, in this document the single-bundle distribution is used with the Equinox implementation of DS.
This demo is quite similar to the Spring-DM demo and the Greeter demo in structure. It consists of 3 bundles:
- An interface bundle defining the Adder Service interface.
- An Adder Service implementation bundle.
- An Adder Service consumer bundle.
The service implementation and consumer bundle are built using DS.
The Adder Service interface is as follows:
The Adder Service Implementation
The service implementation providers a simplistic implementation of the AdderService interface, which is instantiated as a DS component.
In the OSGI-INF/component.xml file the AdderServiceImpl is instantiated and registered with the OSGi service registry with the distribution properties. These properties instruct. Distributed OSGi into making the service available on http://localhost:9090/adder.
Note that the META-INF/MANIFEST.MF file needs to contain a special DS header that tells the system where to find this file. In case of this demo, this header is added by the Maven build system. The header used by the demo is:
So let's install the server side in Equinox, together with the Equinox DS implementation. You can do this from the Equinox command line, but in this document I'll launch Equinox from within Eclipse (last tried with Eclipse 3.6).
I'm starting off by importing the Single Bundle Distribution as a binary project in Eclipse by going File -> Import | Plug-ins and Fragments and then select the directory that contains the single bundle distribution JAR file. My workspace now looks like this:
Next I'll create an OSGi Framework launch configuration that includes DS. To do this
- deselect the 'Target Platform' tickbox in the Eclipse Launch configuration screen
- select org.eclipse.equinox.ds
- hit the 'Add Required Bundles' button
Now run the OSGi container, you will get a setup like this:
Now I can install the DOSGi DS bundles in the OSGi container directly from the maven repository.
|You can also import the maven projects of the DS demo into Eclipse, this would save you from installing it with a URL as above. To do this, check out the CXF/DOSGi source from SVN (http://svn.apache.org/repos/asf/cxf/dosgi/trunk), run mvn eclipse:eclipse and import all Eclipse projects under the samples/ds directory in Eclipse with File -> Import | Existing Projects into Workspace.|
At this point, the service should be available remotely, you can check this by obtaining the WSDL:
The Adder Service Consumer
The service consumer is also created using DS. DS creates an AdderConsumer component which is injected with a reference to the remote AdderService. Like in Spring, the injection is done by DS, which makes the code nice and simple. When the injection is done, the start() method is called.
The client side bundle contains an OSGI-INF/component.xml which drives the component creation and injection:
As on the service provider side, the client side bundle needs to contain the DS header in the META-INF/MANIFEST.MF:
As in the Greeter demo, the client side needs to be configured to know where the remote service actually is. This is one in the OSGI-INF/remote-service/remote-services.xml file:
Install and run the consumer side of the demo in a separate Equinox instance (tip: you can duplicate the launch configuration used for the server side in the 'Run Configurations' dialog):
The remote adder service has now been invoked. You will see the following line on the server side Equinox window:
Some OSGi Declarative Services implementations don't explicitly register interest in the requested services with the OSGi Framework. They rather user a generic Service Tracker or Service Listener to track all available services. This doesn't provide the CXF-DOSGi implementation with the information about what services the consumer is looking for through the ListenerHook and hence it can't register the remote service on-the-fly. A simple workaround to this problem is to add an Activator to a bundle in the client-side framework (this activator could be in any bundle) which registers an explicit ServiceTracker for the remote service the DS component wants to be injected with. An example of such an Activator can be found here.
In the future a more elegant solution to this problem will hopefully be provided.