Clients on Android, server on Java. How to implement?

Question

Android

It is necessary to implement the Server-Clients task. The server receives a request from the client (clients), returns it (to them, to each his own, i.e. he server must be able to find out which client has contacted him and send messages only to him). Also, the server itself can initiate the transfer of data to the client (clients). How can such a problem be solved? They say to use sockets on android - this is an energy-consuming solution. What are the "correct" ways to solve this problem?