Tutorials : Plug-in to Reusability in Java :

Step 4: Make the driver

The driver has three things to do:

1. parse the parameter file
2. create and initialize instances of the classes specified
3. call the receive, transform and send methods

Parse the parameter file

We'll use JDOM for this task, since it transforms the XML document into a tree-structure that is easy to navigate within. Here's a method that does the parsing:

private Element parseFile(String fileName) 
    throws JDOMException, IOException {
  File parameters = new File(fileName);
  SAXBuilder builder = new SAXBuilder();
  Document doc = builder.build(parameters);
  return doc.getRootElement();
}

Create and initialize instances of the three classes specified

IReceive reader = 
  (IReceive) initClass("Read", IReceive.class, 
	root.getChild("Read"));
ITransform transformer = 
  (ITransform) initClass("Transform", ITransform.class,
	 root.getChild("Transform"));
ISend writer = 
  (ISend) initClass("Write", ISend.class, 
	root.getChild("Write"));

 I've made a helper method--initClass--that pulls the name of a class from the JDOM tree, creates an instance, and calls the initialize method on the instance:

/* 
* Initialize a plug-in.
* elementName: The name used in the JDOM tree
* clas: The Interface class
* child: The JDOM element holding the plug-in data 
*/ 
private Object initClass(String elementName, Class clas, Element child)
     throws Exception {
  if (child == null)
    throw new Exception("No '" + elementName 
       + "' element in parameter file.");

  // Get 'class' attribute
  Attribute classAtt = child.getAttribute("class");
  if (classAtt == null)
    throw new Exception("No 'class' attribute for the '" 
       + elementName + "' element.");

  // Make an instance of this plug-in
  String className = classAtt.getValue();
  Object o = Class.forName(className).newInstance();

  // Check for correct interface 
  if (!clas.isInstance(o))
    throw new Exception("Not an " 
       + clas.getName() + " class: " + o.getClass().getName());
  ((IPlugin)o).initialize(child);

  return o;
}

Call the receive, transform and send methods

Now it's a simple thing to call these methods:

List list = reader.receive();
List out  = transformer.transform(list);
writer.send(out);

How to Add Java Applets to Your Site

New on the Java Boutique:

New Review:

Time Management Made Easy with the Quartz Enterprise Job Scheduler
Why not just use the Java timer API? This open source scheduling API boasts simplicity, ease-of-integration, a well-rounded feature set, and it's free!

New Applet:

Reverse Complement
Reverse Complement is a simple applet that converts DNA or RNA sequences into three useful formats.

Elsewhere on internet.com:

WebDeveloper Java
Lots of Java information on webdeveloper.com

WDVL Java
Thorough Java resource at the Web Developer's Virtual Library.

ScriptSearch Java
Hundreds of free Java code files to download.

jGuru: Your View of the Java Universe
Customizable portal with online training, FAQs, regular news updates, and tutorials.