Can I extend the application with my own code for commercial use?

I am interested in using your rest and I would like to adapt it and add functionalities and take away some others, my questions,
1.- Can I download the source code and modify it or use parts of code in a new project using spring or similar?
2.- Do I have to pay something especially for commercial with my modified version, (license or something like that), or just by making a donation?

1. Yes.
2. You don't have to pay, but donations are always welcome.