Plugin Architecture / Modular Design

Question

I'm trying to find some more information on creating a plugin capable program architecture. I would like to learn how to design a program so that it can later be extended/enhanced by plugins/modules. I would like for a compiled version of my program to be able to access these plugins.
Has anyone completed a similar task with Java? Can anyone point me in the right direction to start learning more?
Thank you for your help!
DesQuite

Answer 1

To be more explicit, the typical method of creating a "plugin architecture" is to use a combination of interfaces and reflection. For example, if I want to allow developers to create a forum plugin that displays a special footer, I might create an interface that looks like this:
* A plugin interface. Developers should implement
* this interface, and register their class.
public interface ForumFooterWriter {
* Writes the footer contents to the stream.
public void writeFooter( PrintStream stream );
Then you provide a means for the developers to 'register' their plugin. This can be as simple as them editing a properties/xml file and creating an entry with their class name. For example,
--- Begin plugin.properties
footerPlugin=com.foo.MyFooterPlugin
--- End plugin.properties
Then, you can read the file, and instantiate the plugins. For example,
/** Your code.
void writeFooter( PrintStream out ) {
Properties p = new Properties();
p.load( "plugin.properties" );
String footerPlugin = p.getProperty( "footerPlugin" );
Class footerPluginClass = Class.forName( "footerPlugin" );
ForumFooterWriter ffw = ( ForumFooterWriter ) footerPluginClass.newInstance();
ffw.writeFooter( out );
Note, that if you want people to be able to change which plugins are registered, or just be able to re-compile a plugin and have the effect take place without restarting your app, you'll have to add some more complexity to the above (for example, using your own custom class loader), but this ought to be enough to get you started.
God bless,
-Toby Reyelts