Is it possible to run web service on the android phone?

I just want to know is it possible to write an app (maybe a service o开发者_JAVA百科n the background) on the android whose function works like a web service which means that when another app sends a request to the localhost and specific port, the app could give a reply and execute some functionalities? Is it possible? Anybody has some ideas about that?

It's certainly possible. Check this for example: http://code.google.com/p/i-jetty/