code: @Reference(name = KEY_GENERATOR, service = DDLGenerationParticipant.class, policy = ReferencePolicy.DYNAMIC, cardinality = ReferenceCardinality.MULTIPLE)
      protected void setGenerator(ServiceReference<DDLGenerationParticipant> ref) {
          generators.addReference(ref);
      }
docs: Method which registers a DDL generator. All OSGi services providing this
  interface will be set here.