2.2.1 Implementation based on a specific language

Figure 2: Model design based on specific language and compilation

The most straightforward approach to executable UML models is to create a direct link (or dependency) between the meta-model and a specific language. In the meta-model, users are allowed to write statements and expressions in the language (called an ``action language''). The libraries of the language are also available for immediate use in the model. When the design of the model is finished, it can be converted into source code (again, in the same language), and can be executed after compilation (Figure 2).

The language chosen is arbitrary. It can be any of the contemporary programming languages such as C++, Java, Python [Fen02] and so on.

This gives model designers much power to express software systems. In a sense, to design a model is much like programming, because they have to write real code in appropriate places of the model ⁴. The code has unlimited power to access and modify any object or variable in the run-time system.

Because almost all the modelling environments implemented in this scheme naively place the language-specific expressions and statements unchanged in the source code generated from the model ⁵, the disadvantages are very obvious:

• No programming language contains all the useful software concepts. Though different programming languages may be equivalent in power and are able to produce all kinds of software, they differ in the difficulty or ease with which different problems are solved.
• UML concepts are not directly supported by any contemporary programming language [MTAL98].
• Using any of the programming languages in the models enforces modelers to focus on verbose implementation details too early.
• The expressiveness of the code embedded in the model makes automatic model analysis and verification difficult. This also makes the generated software error-prone. It is difficult to tell whether the error is caused by the model structure or by the embedded code.
• The requirement for compilation delays the change to the system and increases time-to-market [RFBLO01]. The system must be re-installed.