Creating an extensible Java library (JAR)

Question

I'm looking to create a Java 'library' (JAR) that can be used within other projects, but I'd like my library to be extensible. I'm after something OSGi-esque, where my library has 'extension points' that other JARs can hook into. The thinking is that my library will have a core set of methods that will be called by the projects it's used in, but the exact implementation of these methods might vary based on the project/context.

I've looked into OSGi (e.g. Equinox), but I'm not sure it can be used in the way I'm after. It seems to be geared towards standalone apps rather than creating a library.

What would be the best way of achieving this? Can OSGi be used in this way, and if not are there frameworks out there that will do this?

I hope all that's clear - I have a clear idea of what I want, but it's hard to articulate.

Answer 1

OSGi is great, but I don't think that this is what you need. By using OSGi (-Services), you force the user of your library to use an OSGi environment, too.

I think as @Peter stated, you can do this by simply extending classes of your library in the specific project / context.

But in case you want to use OSGi, there is a simple way to achieve this. It's called Bundle Fragments. This way you can create a bundle and extend a so-called Host-Bundle", i.e. your library, without altering the original library. A popular use case for this is if you have platform specific code in your bundles.