An easy way to do OSGi services

MFS is a simple system for providing a service-orientated architecture with an "inversion of control" methodology. MFS builds on the OSGi framework, but provides a simple way to use this framework and will function also without an OSGi runtime. Further, MFS allows a service framework to be created without the need for special coding patterns or clunky libraries. MFS was developed under the Monnet Project to meet the challenge of aggregating multiple Natural Language Processsing toolkits for many languages under a single architecture.

A service in MFS is simply a (POJO) class with a single constructor and implements some interface. For example:

package com.mycompany;

public class TokenizerImpl implements Tokenizer {
   public TokenizerImpl() { }
}

Services are declared by including a file in the JAR under the path META-INF/components whose name is exactly the interface and whose contains are all implementations in this JAR (bundle). For example for the above example we would have a file called META-INF/components/com.mycompany.Tokenizer and may be as follows

com.mycompany.TokenizerImpl
com.mycompany.AnotherTokenizerImpl

The dependencies of a service are given by the arguments of this single constructor. For example we may define a part-of-speech tagger that is dependent on a Tokenizer as follows:

 package com.mycompany;

 public class POSTaggerImpl implements POSTagger {
     public POSTaggerImpl(Tokenizer tokenizer) { }
 }

This POSTaggerImpl service will be available in the OSGi framework only when an implementation of the Tokenizer interface is also available.

Multiple dependencies may be indicated by using the java.util.Collection class or any of its parents, e.g.,

 package com.mycompany;

 public class POSTaggerImpl implements POSTagger {
     public POSTaggerImpl(Collection<Tokenizer> tokenizers) { }
 }

The type of service is given by the generic parameter. Note that the contents of this collection object may change as services become (un)available.

Services can finally be obtained through the OSGi service registry as usual. This should generally be done at the application level (for example in a Servlet or Bundle Activator), e.g.,

public class MyActivator extends BundleActivator {
   @Override
   public void start(BundleContext bundleContext) throws Exception {
      final ServiceReference serviceRef = bundleContext.getServiceReference(
                POSTagger.class.getName());
      if(serviceRef != null) {
           final POSTagger posTagger = (POSTagger)bundleContext.getService(serviceRef);
           // Do something with the posTagger
      }
   }
   // etc
}

For testing and agile development purposes MFS can also be used without an OSGi runtime. This is done through the class eu.monnetproject.framework.services.Services. A single instance of a service can thus be obtained as follows:

final POSTagger posTagger = Services.get(POSTagger.class);

The dependencies are resolved recursively so the above statement with the classes defined as above are equivalent to

final POSTagger posTagger = new POSTaggerImpl(Services.get(Tokenizer.class));

And then

final POSTagger posTagger = new POSTaggerImpl(new TokenizerImpl());

However as MFS does not require that the name of the implementations be specified this allows for implementations to be changed without affecting dependent code.

All instances of a service may be obtained through the method getAll

final Collection<POSTagger> allTaggers = Services.getAll(POSTagger.class);

MFS in addition provides a small number of annotations for customizing the way that MFS resolves services.

The @Inject annotation can be used to indicate which constructor is to be used for dependency injection. This can be useful for integrating MFS with other frameworks

public class MyPOSTagger implements POSTagger {
   // Do not use for injection
   public MyPOSTagger() {}

   // Use for injection
   @Inject public MyPOSTagger(Tokenizer tokenizer) { }
}

This annotation must be used if there are more than one constructor

This annotation when applied to a constructor argument of a Collection will ensure the service is only created when there is at least one satisfying dependency

public class MyPOSTagger implements POSTagger {
   public MyPOSTagger(@NonEmpty Collection<Tokenizer> tokenizers) { }
}

This annotation returns a dependency as an aggregate factory. Calling a method on this factory results in all known implementations of the class being queried, and the first implementation that did not return null or throw an exception being returned.

Consider for example we have an interface

public interface POSTaggerFactory {
   POSTagger getTagger(Language language);
}

And two associated implementations:

public class ENPOSTaggerFactory implements POSTaggerFactory {
   public POSTagger getTagger(Language language) {
       if(language == ENGLISH) {
         return new ENPOSTagger();
       } else {
         return null;
       }
   }
}

public class DEPOSTaggerFactory implements POSTaggerFactory {
   public POSTagger getTagger(Language language) {
       if(language == GERMAN) {
         return new DEPOSTagger();
       } else {
         return null;
       }
   }
}

Then by obtain an @Factory reference the call to getFactory() will return a result for either English or German.

Dependencies can be returned as aggregate factories without an OSGi runtime using the method getFactory

final POSTaggerFactory posTaggerFactory = Services.getFactory(POSTaggerFactory.class);

This annotation means that MFS will only return one instance of the class

@Singleton public class MyPOSTagger implements POSTagger {
   public MyPOSTagger(Collection<Tokenizer> tokenizers) { }
}

Note MFS does not guarantee that the class will only be constructed once, so this should not be used to perform tasks that can only be once (e.g., connecting to a database). This annotation is intended to save on resources by avoiding duplicate instances of similar classes. Similarly, if dependent services become unavailable and then new dependencies become available the constructor will be called again.