Question on M-Levels
Legacy Issue Number: 7749
Source: SAP SE ( Axel Uhl)
section 7.2 of the MOF 2.0 specification (similar to previous versions of the MOF spec) still contains this bold sentence: "The MOF 1.4 Reflection interfaces allow traversal across any number of metalayers recursively." I'm wondering how I would create a MOF-based system with more than three layers that really "live" inside a MOF-based repository. Let's take a look at UML, for example. If we forget for a while that UML 2.0 and MOF 2.0 share a common core, I could consider the UML metamodel as just another modeling language defined using MOF.
Now the UML metamodel defines zillions of classes, all of them being instances of MOF::Class. One of them happens to be UML::Class, a classifying model element which allows me to create model elements in the UML modeling languages that themselves conceptually act as meta-objects that I may wish to instantiate.
However, apart from the "shared core trick," the MOF doesn't have any knowledge about the MOF::Class object UML::Class being special, and in particular being a classifying meta-object.
The sentence from 7.2 suggests that I can have the following:
myDog.getClass() == Dog
Dog.getClass() == UML::Class
UML::Class.getClass() == MOF::Class
However, creation of a model element with the reflective capabilities as found in the MOF 2.0 specification seems to allow me only to create the UML::Class instance named "Dog" as I can use UML::Class as an argument to Factory::createObject(...). The resulting object, however, is no longer an instance of MOF::Class and can therefore not be used for a factory call in order to create an instance.
Currently available language bindings for MOF 1.4 as I understand them correspondingly don't allow for this flexible multi-layering idea. Take JMI, for example. While the standard defines how to derive the class proxy for UML::Class, I cannot simply instantiate the UML::Class instance Dog because it's not a MOF::Class instance.
Would I have to import the MOF core into my metamodel and have UML::Class specialize MOF::Class? (Tricky enough, MOF 2.0 doesn't even have to create a specialization relation because the two are identical. But I think you get the picture; I could have chosen to use my own metamodel, different from UML, but still introducing a classifying class.) If this was the case, then probably there would be something missing from the language bindings and reflective capabilities. For the JMI example, I'd then expect to be able to obtain a class proxy for an instance of my UML::Class so that I can create instances of that particular UML class, e.g., the "myDog" instance used in the example above.
Reported: MOF2I 2.0b1 — Thu, 9 Sep 2004 04:00 GMT
Disposition: Resolved — MOF2I 2.0
No Data Available
Updated: Sun, 8 Mar 2015 22:06 GMT