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iPOJO is a component model on the top of OSGi? OSGi™ aiming to simplify the development of OSGi? OSGi™ applications. Moreover, iPOJO provide an extensibility mechanism allowing developers to extend the component model.

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A composite can be represented as a service registry. Moreover, composite can be contained in other composite. The OSGi? OSGi™ service registry is the root composite.

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To illustrate the composition we will design a Hello World application named HelloComposition. This application offers a service writing a Hello message for each imported Person. Each person is published as a service and is used by the composition to write the message.Image RemovedImage Added

The application provides the HelloDispatcher service :

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• an Hello Service : returning a Hello message
• a Directory service : aggregating Person services to create a Directory
• a Dispatch Service : requiring an Hello Service and a Directory Service to write Hello Service to person contained in the Directory.
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An "instantiable" service is a service that we can instantiate through a Factory. The following code snippets show the different service specification:

public interface Hello {
              public void hello(String name);
	      public String getLanguage();
}

public interface Directory {
                public Person getPerson(String name);
                public List<Person> getPersons();
                public void addPerson(String name, String adress);
                public void removePerson(String name);
}

public interface Person {
       public String getName();
       public String getAddress();
}

public interface Dispatch{
                public void say();
}

So to design our application we will use these services to obtain the following applications.

Application Description

To describe our application, we will use the iPOJO ADL:

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• Imports all available Person service in the parent composite.
• Provides the Hello Dispatch to the parent composite.
• Instantiates inside the composition a Hello service, a Dispatcher Service and a Directory Service.

The providing is based on a method delagation on available specifications as depict in the following image :Image Added

Packaging

A composite is described in the metadata.xml file of an iPOJO bundle. It is possible to create an instance of the composition in this file:

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