Hi Bhagat,
I recommend SAP GW to expose business logic. It allows you to use standard SSO mechanisms or extend generated services with custom ABAP code to do any custom logic. You can expose any RFC or Function in easy way.
If you decide don't use GW it is possible consume directly an RFC with this custom Json adapterJSON Adapter for ABAP Function Modules on Abap side or you could create a service layer with Jersey in Java https://jersey.java.net
Hope this helps you,
Kind regards