Know your patterns, so you don't re-invent the wheel. Patterns are like LEGO: You can combine them to larger patterns. One of those is the Hexagonal Achitecture officially called:

Ports and Adapters

proposed by Alistair Cockburn, one of the co-signers of the Agile Manifesto.

I've written about his work before and can highly recommend his latest book "Hexagonal Architecture Explained" (or for that matter any of his writings).

In a nushell, Ports and Adapters proposes an approach that leads to loosely coupled systems, that separates business logic from the software environment (think I/O, databses, API and UI) to facilitate clarity and ease of testing.

This post doesn't aim to explain it, for that you can refer to the Wikipedia article or, better, read the book but trying to describe how to apply it to Java in a Maven project.