For Java async socket programming, I think https://github.com/harleyw/NioSocketLib/ could be the reference for you. It is totally a async socket library. Currently,

1. it has only 1 worker thread for watching all sockets. In future, the workers could be increased.

2. I didn't try it in Android yet. Let me know if anything wrong on Android.

Here is the arch of how the work thread works with the handlers.