The code of Pi4J is based on a layered approach, visualized in the picture below.
Since Pi4J is a low-level library, it tries to avoid inheriting third-party libraries at all costs. More complex dependency chains make it more difficult for users, especially novice users. Therefor, the only dependency Pi4J V.2 has is SLF4J to provide a standardized and extensible logging framework.
The dark grey blocks "Annotation Engine", "@Register" and "@Inject" are here as a future idea but are not included in the current V2.