Code Generator Part II: Architecture of a typical Code Generator

A code generator is a program that takes a model as an input and produces an output source code, by implementing that model. The model consists of a set of metadata containing information about the code that will be generated. Based on the requirement a model can be a Unified Modeling Language (UML) design, a proprietary descriptor file, a database schema or even another source code. The model format can be XML, plain text, CSV, or other kinds of sources such as directories, databases, or repositories. Starting from the information contained in the model, a code generator creates source code, which belongs to a programming language (C, C++, Java, C#, VB, and so on) or to another type of output: documentation, descriptors, configuration files, SQL code, and so on (Fig 1).

Fig 1

Depending on the type of input, code generators can be classified in two types – code driven and model driven. A code-driven generator takes as input a file containing source code and special tags, which drive the code generation process e.g. JavaDoc. Model-driven generators can also be sub-classified into two types – custom and MDA. A custom generator takes as input a proprietary model representing the information that must be converted in source code e.g. Apache Velocity. When the model as input is a representation of UML, then the code generator follows the Model-Driven Architecture (MDA). An MDA code generator takes a platform-independent model (usually XMI, an XML representation of UML) as input and turns it into a platform-specific model, which can be converted easily in source code by means of templates.

In this article and the following articles I will try to explain a code generator that converts a set of object definitions (expressed in an object-oriented paradigm i.e. UML) of simple Data Transfer Objects (DTO) from a XML text to a Java program. The same code generator can be enhanced to generate C++ programs, SQL scripts, PHP scripts from the same XML input. In the real world scenario, a programmer might not possess the skills in all the languages but he/she can be well conversant with the concepts of the object-oriented (OO) paradigm and is able to design an OO system. The concepts or design of a system can be stored in a widely popular common format e.g. XML. The utility of this tool is to facilitate the transformation of the concepts and design from the XML to a desired language. Most of Computer-aided Software Engineering (CASE) tools like Rational Rose etc. support this sort of code generation.