2.5 Advanced: UML Metamodel of Structural Elements (for the Advanced Modeler)

Note: Here be dragons....^[7]

^[7] These particular dragons are the most likely thing in this book to be wrong as the internals of the metamodel are reworked during the finalization process of the UML 2.0. However, their semantic meaning will be stable even if the details of the metarelations change, so for the average UML modeler, such changes will be largely irrelevant.

We've talked about the UML concepts quite a bit in this chapter. In this section, we discuss the UML internals in just a bit more detail. Models done in UML are done in what is called a four-layer metamodel architecture, as shown in Table 2-1.

Table 2-1. Metamodel Language Architecture

Layer	Description	Examples
M3 Meta-metamodel	Provides a language for specifying metamodels, such as UML	Metaclass, meta-attribute, meta-association, meta-operation
M2 Metamodel	An instance of a modeling language, such as UML	Class, interface, attribute, association, operation, state, component
M1 User Model	Model of the user application or system done in the metamodel language	Sensor, DC motor, Gaussian filter, window, direction, speed, inertia, waveform, text display
M0 User Objects	Instances of the specification elements of the user model	Sensor [1], myMotor:Motor, MAF:Filter, mainWindow:Window

The idea is that each layer is built using the elements specified in the layer above. The UML at the M2 level is constructed from a basis defined by the meta-metamodel at level M3, while user model elements (M1) are constructed using UML elements, such as classes, associations, and so on. The runtime model (M0) executes instances of the classes defined in the user model.

That having been said, we've thrown around some UML metamodel elements such as classifier, class, component, subsystem, interface, and so on. The UML 2.0 specification is the ultimate arbitrator of what is or is not UML, and is a maintained (i.e., managed, updated, and revised) standard, so the particulars at any point in time might differ slightly from what is presented here. Nevertheless, it is useful to understand the metarelations between the meta-elements in the UML. Figure 2-19 shows the relation of the primary structural elements in the UML that we have discussed in this chapter.

Figure 2-19. Subset of UML Metamodel for Structural Elements

The UML metamodel is, of course, (far) more complex than this. I have not shown all the relations among the elements, nor all the more abstract metasuperclasses, the most central metaclass being Classifier. Common Classifiers include Class, DataType, Association, StructuredClassifier, Component, Subsystem, Node, and Use Case. The fact that they have a common ancestry allows the common aspects to be captured in a single place. Classifers associate with Features, which may be either BehavioralFeatures (such as operations) or StructuralFeatures. In addition, BehavioredClassifiers may have Behaviors, such as Statemachines or Activities.