code: protected AptControlInterface initControlInterface()
      {
          TypeMirror fieldType = _fieldDecl.getType();        
          if (! (fieldType instanceof InterfaceType))
          {
              _ap.printError( _fieldDecl, "context.field.badinterface" );
              return null;
          }
  
          //
          // For contextual services, the declared type of the field is always the public
          // interface for the contextual service.
          //
          return new AptControlInterface(((InterfaceType)_fieldDecl.getType()).getDeclaration(), 
                                         _ap);
      }
docs: Initializes a ControlInterface associated with this context field.  Because
  contextual services can expose both APIs and events, they are similar to controls.