Template-Based Code Generation with Apache Velocity, Part 2
Pages: 1, 2, 3, 4

An Example

To provide a real example of code generation, let's consider the class diagram in Figure 6.

Figure 6. Class diagram for Order/OrderItem/Article

We can represent it with XML in the following way:

<?xml version="1.0" encoding="UTF-8"?>

<!-- order.xml -->

<Content>
    
<Class name="Order">
  <Attribute name="number" type="integer"/>
  <Attribute name="date" type="date"/>    
</Class>

<Class name="OrderItem">
  <Attribute name="number" type="integer"/>
  <Attribute name="quantity" type="integer"/>   
</Class>

<Class name="Article">
  <Attribute name="code" type="integer"/>
  <Attribute name="description" type="integer"/>    
</Class>
  
<Association name="">
  <Role class="Order" multiplicity="1" name="" 
    type="start" navigable="true"/>
  <Role class="OrderItem" multiplicity="*" name="" 
    type="end" navigable="true"/>
</Association>

<Association name="">
  <Role class="OrderItem" multiplicity="*" name="" 
    type="start" navigable="false"/>
  <Role class="Article" multiplicity="1" name="" 
    type="end" navigable="true"/>
</Association>
        
</Content>

Then we can create an importer that creates the Internal Object Model starting from that XML information -- inside of the source code for this article (in the "Resources" section below) you can find an implementation of such an importer. Finally, we can create a main program having three command-line parameters: the input file, an alias for the importer implementation, and an alias for the exporter implementation. Here's the code:

import com.codegenerator.velocity.*;

import java.util.*;

public class Generator {

public static void main(String args[]) throws Exception {

  if (args.length!=3) {
    System.out.println(
      "Syntax: Generator <input-file> <importer> <exporter>");
    System.exit(1);
  }

  Importer importer = null;
  if (args[1].equals("XML"))
    importer = new XMLImporter(args[0]);
  else {
    System.out.println("Importer " + args[1] +
      " non found!");
    System.exit(1);
  }

  Exporter exporter = null;
  if (args[2].equals("IOMVelocityExporter"))
    exporter = new IOMVelocityExporter();
  else if (args[2].equals("PSMVelocityExporter"))
    exporter = new PSMVelocityExporter();
  else {
    System.out.println("Exporter " + args[2] +
       " non found!");
    System.exit(1);
  }

  importer.start();
  IOMNavigator navigator = new IOMNavigator(exporter);
  navigator.start();

}

}

Note: We will see later what PSMVelocityExporter is.

Launching the application in this way:

java com.codegenerator.Generator order.xml XML IOMVelocityExporter

creates, by using IOMVelocityExporter, the Java classes indicated by order.xml: Order, OrderItem, and Article. The generator, for example, creates OrderItem.java as the following:

// Generated by IOMVelocityExporter

import java.util.*;

public abstract class OrderItem {

// number
private int number;
public int getNumber() {
  return this.number;
} 
public void setNumber(int number) {
  this.number = number;
}

// quantity
private int quantity;
public int getQuantity() {
  return this.quantity;
} 
public void setQuantity(int quantity) {
  this.quantity = quantity;
}
  
// Association
// OrderItem -- Order
// Navigable   : true
// Multiplicity: 1
   
public abstract Order getOrder();
public abstract void setOrder(Order order);
          
  
// Association
// OrderItem -- Article
// Navigable   : true
// Multiplicity: 1
    
public abstract Article getArticle();
public abstract void setArticle(Article article);
    
}

Note: The classes are defined as abstract because the implementations of the association methods have not been specified.

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