OSGi - How Can OSGi - I create a component that register a different implementation based on a parameter? Factory? - osgi

Question

I've the following bundles:
- GreetingAPI (bundle which defines the greeting() method) (Service)
- GreetingImpl1 (bundle which implements greeting() method for English mode)
- GreetingImpl2 (bundle which implements greeting() method for Italian mode)
- GreetingConsumer (bundle which uses the greeting service)
How Can I create a component (I suppose it's a factory) that based on a given language parameter lets the consumer bundle to use a different implementation of the service.

Answer 1

You're thinking about this the wrong way around. The provider should not register a different service depending on something that the consumer does, because the provider shouldn't know anything about the consumer.
Instead, you can have multiple providers of the same service but annotate them with appropriate metadata. Then the consumer of the service can choose whether or not to filter on specific properties.
For example, when we register a service we can add properties as follows (note that I am using the OSGi Declarative Services annotations, see OSGi Compendium Release 5, section 112.8):
#Component(property = "locale=en_GB")
public class MyGreetingImpl1 implements Greeting {
public String greet() { return "How do you do"; }
}
#Component(property = "locale=en_US")
public class MyGreetingImpl2 implements Greeting {
public String greet() { return "Howdy"; }
}
#Component(property = "locale=fr_FR")
public class MyGreetingImpl3 implements Greeting {
public String greet() { return "Bonjour"; }
}
Now the consumer can choose whichever language it wants using a target filter. Note the use of a wildcard, as the consumer in this case only cares about the language but not the country code:
#Component
public class GreetingConsumer {
#Reference(target = "(language=en*)")
public void setGreeting(Greeting greeting) { ... }
}

Answer 2

One of the possible solutions is to have some kind of a language manager. So your consumer has a language manager and not directly the greetings service
The manager is notified with the registration / deregistration of every language implementation of your GreetingAPI.
Your language manager keeps trace of the diffrent implementations. Your manager provides the right implementation of the target language (using an enum for example)
Example
public class LanguageManagerImpl implements LanguageManager {
//LanguageEnum can be used to distinguish the different languages
private Map<LanguageEnum, GreetingAPI> greetings = new HashMap<LanguageEnum, GreetingAPI>();
public void registerLanguage(GreetingAPI greeting) {
LanguageEnum language = greeting.getLanguageEnum();
//add the greetings to the map
}
public void deregisterLanguage(GreetingAPI greeting){
//remove your greeting from the map
}
public GreetingAPI getGreetingForLanguage(LanguageEnum language) {
return greetings.get(language);
}
}
If you are using blueprint, then you need to add to the language manager blueprint a reference-list with a refence listener on GreetingAPI
Otherwise you use traditional listeners
Example
<bean id="languageManager"
class="your.language.managerimpl.LanguageManagerImpl">
</bean>
<service ref="languageManager" interface="your.language.manager.interface.LanguageManager" />
<reference-list interface="your.greeting.interface.GreetingAPI"
availability="optional">
<reference-listener ref="languageManager"
bind-method="registerLanguage" unbind-method="deregisterLanguage" />
</reference-list>