How to build an extensible application

Question

I am currently looking into how to build an application that allows other developers to implement an interface (say DictionaryProvider) and provide a jar file with this implementation and have my application automatically find all classes within a specific directory (under any jar) that implement this interface, instantiate an object and perform some operation on the object. I see in java 1.6 that there is a nice way to do this but I have not seen similar examples in Java 1.5. My basic question is how do I allow users to implement an interface and then at run time find all classes that implement this interface and instantiate an object of that type?

Answer 1

DrClap wrote:
Right. Don't waste your time loading every single class in the jar, there's probably a lot of supporting classes in there. Instead do what the XML people did and use the Services API. For an example, look at how Java loads a DocumentBuilderFactory. The third bullet point there shows you how it's done in that case. You'll find that TransformerFactory has a similar process.Hmm. I think the OP wants people to be able to drop in previously unknown jars that implement some interface, and have that work without that jar creator needing to do anything else. I'd find a property file or manifest entry easy enough, but I can see his angle.
I have heard about another way that I haven't tried, which is to use annotations and spring - but I guess that's just as complicated as writing entries to a property file. So the only plug-and-play method without the external jar producer having to do anything but implement the interface seems to be ClassLoader workarounds...