Tutorials : Plug-in to Reusability in Java :

Plug-in to Reusability in Java

by Keld H. Hansen

Introduction

As a Java programmer with an object oriented brain you're used to handling classes in your programs by instantiating them and using their methods:

. . .
Employee e = new Employee();
e.setBirthday(someDay);
. . .

To make your programs more flexible it's often a good idea to start by defining Interfaces for the classes you use. By referring to the Interfaces instead of specific classes, you can make your program more general:

. . .
Person p = getPerson();
p.setBirthday(someDay);
. . .

If Person is an Interface and Employee implements it, then getPerson() could return an Employee object, but also many other objects, as long as they implement the Person Interface. So with a small modification we've made our two-line program more general.

getPerson() could be implemented in many ways, it could for example get the name of the class implementing Person from some source, maybe a properties file:

public Person getPerson() {
  Properties props = new Properties();
  . . .
  String className = props.getProperty("class_name");
  Person p = (Person)Class.forName(className).newInstance();
  return p;
}

In this article I'll try to push this idea further, by first defining Interfaces for several important classes used in an application, and then, giving the actual class names in a parameter file read by the application on start-up. This application design is often referred to as a plug-in architecture, where the parameterized classes are the plug- ins.

A sample application

To show you how a plug-in architecture could be implemented we'll consider a simple case. Assume that we must develop an application which reads temperatures for some cities from some source, and delivers some of the temperatures to one or more targets. The design criteria we'd like to meet are these:

• it must be simple to use a new module that reads the temperatures from some source
• it must be simple to use a new module that delivers the temperatures to new targets
• we must be able to handle temperatures in various formats, both on input and output
• we'd like to have a core module that won't change if the modules handling source or targets change  

As we will see shortly it's possible to meet these goals by using a plug-in architecture.

The core module -- which we'll call the driver -- is the user of the plug-ins. As said above the idea is that each plug- in implements a specific Java Interface, which the driver knows. The driver therefore may call methods from the Interfaces, without having to know which specific plug-in is being used. The plug-ins used are defined in a parameter file, which the driver has access to. It's the drivers task to instantiate the plug-ins and use their methods.

Step 1: Define the tasks in your application

We'll work with three tasks:

1. read temperatures
2. transform temperatures (from input to output format)
3. send temperatures 

These three tasks will correspond to our three plug-in types. Each plug-in will output data that is used as input to the next plug-in.

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