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From Andrus Adamchik <and...@objectstyle.org>
Subject Exploring DataObject mixins
Date Mon, 01 Nov 2010 23:27:20 GMT
Been thinking about work-related design issues, and came up with an idea of DataObject "mixins".
So what are the problems:

1. Often you'd like to associate a certain set of common properties and/or behavior with a
group of persistent objects that are unrelated and not a part of the same inheritance hierarchy.
Here is an abstracted real-life example:

* entities A, B, C are "referenceable" (they have a public UUID)
* entities A, C, D are "auditable" (when a user changes their data in some way, a system must
record the change history)
* entities A, C and E are "access-controlled" (a certain permission level is required for
a user to view or edit them).

2. In a layered architecture, DataObject itself may not be the right place to implement complex
behavior. All non-data methods should ideally be pluggable (normally meaning must be IoC-driven),
but still attached to a specific DataObject.

In other words there's no multiple inheritance in Java, and anyways we want to push most of
the logic to the higher app layers. So... I am experimenting with something I called "mixins",
based on DataObject dynamic nature + listener capabilities. I checked in to github a simple
mixin extension for Cayenne:

http://github.com/andrus/cayenne-mixin/tree/master/other/cayenne-mixin/

and a proto-CMS using it:

http://github.com/andrus/cayenne-mixin/tree/master/oc/

I may actually keep developing the CMS piece for my own needs, but here a very basic version
of it is used for a mixin demo. A mixin in the simplest form is just a custom annotation placed
on a DataObject:

  @ReferenceableMixin
  @AuditableMixin
  public class Article extends _Article { }

It is up to the application to attach lifecycle handlers for a given type of mixin. MixinHandlerManager
class from cayenne-mixin module provides API to bind such handlers:

  MixinHandlerManager handlerManager = new MixinHandlerManager(entityResolver);
  handlerManager.addMixinHandler(handler);

A handler implements MixinHandler interface and does whatever is needed to the object during
its lifecycle. Below is a mixin handler that calculates and injects a UUID property in a referenceable
object:

  class ReferenceableMixinHandler implements MixinHandler<ReferenceableMixin> {

	@Override
	public Class<ReferenceableMixin> getMixinType() {
		return ReferenceableMixin.class;
	}

	@Override
	public void setupListeners(LifecycleCallbackRegistry registry,
				Class<? extends DataObject> entityType) {

		registry.addListener(LifecycleEvent.POST_PERSIST, entityType, this,
				"initUuidCallback");
		registry.addListener(LifecycleEvent.POST_LOAD, entityType, this,
				"initUuidCallback");
	}

	void initUuidCallback(DataObject object) {
		int id = DataObjectUtils.intPKForObject(object);
		String uuid = object.getObjectId().getEntityName() + ":" + id;
		object.writePropertyDirectly(Referenceable.UUID_PROPERTY, uuid);
	}
  }

Of course any DataObject can store any transient property, so we are taking advantage of that.
Similarly AuditableMixinHandler is used to create ContentVersion records when an object changes.
I am still exploring various uses of mixins and ways to extract common handler patterns in
cayenne-mixin. A few early observations:

* Mixins provide a better way to organize and understand listeners. From experience, ad-hoc
mapping of listeners quickly results in a mess - each listener maps to more than 1 entity
and more than 1 type of events. Very hard to remember and manage that stuff. I'd rather not
think about event types at all, and just think that e.g. a listener manages 'uuid' property
for a set of referenceable objects.

* Mixins may be a better way for inheritance mapping. I am still to explore this idea, but
I suspect in many cases mixins may be more scaleable than e.g. vertical inheritance. Besides
of course they allow for many "superclasses" at once.

* Mixins may provide a facility for custom relationship faulting. E.g. an injected property
can be a lazy collection backed by a custom query (maybe try overriding standard Cayenne relationships
this way?)

* There are some limitations, most obviously all mixin properties have to be accessed via
DataObject.readProperty(..) which may not fly well with scripting (e.g. "article.uuid"). This
can be solved by an optional mixin interface and manual cover methods. Not ideal, but works:

  public class Article extends _Article implements Referenceable {

	@Override
	public String getUuid() {
		return (String) readProperty(Referenceable.UUID_PROPERTY);
	}
  }


Anyways, this is something to explore. Mixins solve a whole class of design problems for me,
and I am sure we can find other uses.

Cheers,
Andrus




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