Code Generator Part VI: Exporter Layer

It's been a long time I managed to keep myself busy :) , back again to stretch a little bit on the blog. It's the turn of the Exporter part of my code generator. I would try to put together a rough idea about the design model of the Exporter. This layer is responsible for exporting the object model (IOM) that has been created in the memory by the Importer to a desired code (e.g. Java, C++ etc.). The input for the exporter should be the IOM (stored in the memory) and the output should be the generated code in a desired programming language or any other format (e.g. CSV, RTF), provided the exporter has to be made knowledgeable enogh to transform the IOM into the desired format. I will try put the pieces (Importer, Exporter, IOM) together in a a controlling framework in my next post.


The Exporter Layer

The exporter has the responsibility of translating the IOM created by the importer and generating the desired output document (i.e. Source Codes or Documents in a specific format).
Incase of code generation the exporter translates the IOM created by the importer and generates the desired output source codes from the IOM. The code generator can have different exporters depending on different output types (i.e. output languages like Java, C++ etc.). Currently we are generating the Java source codes by the exporter. The exporter has been designed using a generic interface (Exporter) as the parent. Any class that generates source codes in a specific object-oriented language from the IOM should have the Exporter interface as its parent. In our case we have an exporter class (JavaExporter) that exports Java source codes according to the object-oriented class specification as defined in the input XML model. This class, called JavaExporter implements another interface called OOCodeExporter which already implements the Exporter interface (Fig 7). The reason behind introducing the OOSpecExporter interface is to encapsulate the activities involved in generating source code in all object-oriented languages in a single interface. This way the Exporter interface can be further implemented by other exporters for generating source codes in different object-oriented languages other than Java (e.g. C++, C# etc.). Below are the descriptions for the classes that belong to the exporter layer.
Please refer to Fig 1 for the classes involved in the Exporter layer.

Fig 1

Exporter is designed as a generic interface. The interface contains the start(), initialize() and finalize() methods. The main entry point of the code generator i.e. the Generator class invokes the exporter using the start() method of this interface. To use the code generator for generating source code in a specific object-oriented language (e.g. Java, C++, C# etc.), new exporter classes can be introduced implementing the same Exporter interface.

OOCodeExporter is an interface implementing the Exporter interface. The purpose of this interface is to encapsulate the operations required to generate source codes in object-oriented languages. Currently the JavaExporter implements this interface to generate Java source codes according to class specifications defined in the input XML model. In future new exporters can be introduced as children to this interface for generating source codes in different OO languages other than Java, e.g. C++, C#, VB etc. The implementation of the Exporter interface depends on the type of OO languages (Java in our case) in which the source code will be generated, so different languages can have very different implementations.

JavaExporter implements the OOCodeExporter interface. The architecture of this class is closely bound to the SAX parser philosophy. This class works on the IOM objects (i.e. IOMClass, IOMAttribute, IOMOperation etc.) stored in the IOMOOController by XMLOOImplorter. The start() method begins to navigate through each object in the internal object model (IOM) and calls the startClass() method each time a IOMClass object is processed, and in general the startXxx() method each time an instance of IOMXxx is processed. The exporter then invokes the endXxx() methods when an instance of IOMXxx is finished to be processed. It creates an output file using the name of the class and adding the JAVA extension. Then it writes, by using the information coming from the IOMClass object, Java code for declaring a class. By implementing all the methods, the JavaExporter class generates the Java classes corresponding to XML model in terms of attributes, operations, and associations. The JavaExporter works based on certain assumptions as described below.
The generated Java source files involved in the DTOs will by default
- Do not need to implement a base class.
- Implement the "Serializable" interface.
- Import "java.util.*" and “java.io.Serializable”.
- Have the stereotype as "class".
- Have visibility "private" for attributes.
- Uses public getter/setter operations.
- Has non-final and non-static attributes or operations.
- Use attributes with type "String".
- Use attributes with multiplicity as "1".