Throught the Metro runtime there is general concept of separation of model and controller. At the very top-level is the main net.dpml.metro.runtime.CompositionController class which handles component establishment.

A reference to the default controller is resolved using the net.dpml.component.Controller interface:

Controller controller = Controller.STANDARD;

While the operations exposed by a controller are rather limited, the semantics concerning delegation are somewhat deeper and worthy of a brief discussion.

The Controller interface defines operations enabling:

• creation of a component using network accessible part
• creation of a component using a remote component model

Operations dealing with network-accessible parts potentially involve a process of delegation by the default controller to a controller asserted by the part datatype. This delegation process will result in the potential establishment of a different controller supporting a foreign component implementation (or more specifically a controller with an alternative or legacy implementation strategy).The default controller will locate a URI from a part, load the foreign controller (if required) in an isolated classloader, and delegate part loading to that controller.

Operations dealing with the construction of a component from a model are focussed around the requirements for management acess to the component model. The Controller interface allows the construction of a component model and subsequent creation of a Component using a supplied model. This separation allows for potentially different versions of managed models and components to co-exist in the same runtime environment.

Controller controller = Controller.STANDARD;
URI uri = new URI( "artifact:part:acme/acme-widget#1.1" );
Component component = controller.createComponent( uri );
Provider provider = component.getProvider();
Widget widget = (Widget) provider.getValue( true );