Common backend for web and mobile apps

I am planning to roll out a service which can be accessed through website and mobile applications (android,ios, may be more..).

The website and the mobile applications are all to be designed.

I would like to write my own backend service, not tightly coupling to services like parse.

What is the best design for the backend which will be common for all the front ends (web,mobile) a web-service?

I would also like to know how easy it would be to integrate mobile specific features like push notifications into this design.

I am willing to do this in Java.