UMLR — Page: 107

I'm attempting to implement a JMI repository for the UML2 superstructure. This is made more difficult by the use of package merge in the model. The following issues should be clarified in the definition of package merge: 1. Do classifiers in the resulting package extend classifiers in the merged package? If they do: The meta-model must contain all infrastructure packages and extension packages defined in the superstructure spec e.g. 'ProtocolStateMachines', 'PackagingComponents' etc., in order to facilitate reflective access to instances of types defined in these packages. However, these packages are not collected in one or more sensible enclosing namespaces (e.g. root 'infrastructure' and 'superstructure' packages), so the root namespace will be cluttered. Root packages for these elements should therefore be introduced in the infrastructure and superstructure specifications. If they do not: Some infrastructure packages will still be needed in the resulting meta-model, because the infrastructure specification uses import + generalisation rather than merge to establish the relationships between packages, e.g. in core::abstractions. For conformance level 0, instances of types in package UML would therefore not be instances of types in core::basic, because this package is merged into UML. However, they would be instances of types in other infrastructure packages, e.g. core::abstractions::elements::Element, because core::basic imports this type, rather than merging it. This seems illogical. Why should core::abstractions::elements have instances in this case, but not core::basic? This could be corrected by using merge in the infrastructure spec, hence avoiding dragging imports into the UML package. However, using merge in this way means redefining the semantics for each copy of the same type, which is why there is so much unnecessary duplication in the infrastructure and superstructure specs. In both interpretations of merge, infrastructure classes and packages penetrate the UML meta-model, leading to a lack of good namespace organisation. This has an impact on the ease with which reflection can be used in the repository. 2. On page 107 the specification states that the use of explicit merges and pre-applied merges in a meta-model is equivalent with regards to the semantics of the meta-model. This is false when reflection is considered, as the meta-model retreived by reflective methods will be different depending on the approach taken. This is significant: To retrieve all features of a classifier where merge has been used in the meta-model, you need not only to recurse the generalisation hierarchy, but also the package containment hierarchy to determine if any containing package is the recipient of a merge that could modify the features of the classifier. This is highly inconvenient. My recommendation to address these problems is that classifiers in a package that is the recipient of a merge should be defined to generalise matching types in the merged package. The package structure of the infrastructure and superstructure specifications should be revised to reflect the fact that most packages and types defined in the specification will therefore be retained in any UML2 specification. Alternatively, use merge rather than import in the infrastructure specification, with the prescription that meta-models accessed by reflection must have the merges rolled out. Also therefore provide explicit documentation of the rolled out meta-models for the various conformance levels for the superstructure.