In need of assistance and examples to build a Sub-tasks like addon via the SDK/Java

[Garry May]

Hi everyone,

I've been doing a load of work with Script Runner for JIRA & Confluence so I've managed to wrap my head around groovy/java, the libraries and the REST API, but I've hit the limits (as far as I can tell) of its scripting usefulness and now need to build an addon; which is a little more involved than "Hello World!"  I've searched the web and forums and couldn't really find anything that suits (or is current).

In short, I want to build and addon which is similar to the current Sub-tasks panel in Jira, but with a little more visual flexibility and usefulness.

My questions are:

1. is this even possible?
2. if so, does anyone have any code/examples that are even remotely similar which I can work with?
3. Is Java and the SDK the only language & framework we can use for building non-cloud addon's?
4. Is Server and Data Centre the same under the hood or are there coding gotcha's I need to look-out for?

Sorry about the length of this post and much thanks to anyone that can assist me.