pritaeas is correct, the app always needs to initiate the connection to the server before the server can send data to it. The only exception to this would be using a Push Notification system in the Android platform similar to iOS. This would be where your server sends a message to a service on the device/handset and not the client app. Example: https://parse.com/tutorials/android-push-notifications
Implementations such as that result in the background service listening for messages from the remote gateway/api which your server is communicating with also.