Implement Bluetooth Connection into Service or Application Class without losing connection into a Device

Question

i need some help, can you explain to me how can i implement the Bluetooth Connection from my Application into my Mini Thermal Printer Device.

The scenario is like t

Answer 1

Narayan's answer needs a corrections. I believe the onStartCommand function should read.

@Override
public int onStartCommand(Intent intent, int flags, int startId) {
    Log.d("BT SERVICE", "SERVICE STARTED");
    bluetoothIn = new Handler() {
        public void handleMessage(android.os.Message msg) {
            Log.d("DEBUG", "handleMessage");
            if (msg.what == handlerState) { //if message is what we want
                String readMessage = (String) msg.obj; // msg.arg1 = bytes from connect thread
                recDataString.append(readMessage);//`enter code here`
                Log.d("RECORDED", recDataString.toString());
                // Do stuff here with your data, like adding it to the database
            }
            recDataString.delete(0, recDataString.length()); //clear all string data
        }
    };
    btAdapter = BluetoothAdapter.getDefaultAdapter(); // get Bluetooth adapter
    checkBTState();
    return super.onStartCommand(intent, flags, startId);
}