Most frameworks rise out of a specific set of practices and an effort to automate those practices. Which means that they are usually built bottom up rather than top down. Rails is a primary example where 37 Signals built a framework that automated away the all the plumbing it was doing job after job building applications.

The tendency toward bottom up development is probably a reason there are so many frameworks...everyone has different practices that need automating.

The best way would be to learn from one of the existing open source projects.

You can start with sparkjava: https://github.com/perwendel/spark/

Also, multi-threading, I/O and networking are areas that you would need to understand inside out.