FCMHelper.java

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.net.HttpURLConnection;
import java.net.URL;

import org.json.JSONException;
import org.json.JSONObject;

public class FCMHelper {
	 private final static String AUTH_KEY_FCM = "xxxxxx" // Provide your FCM key here;
	 private final static String API_URL_FCM = "https://fcm.googleapis.com/fcm/send";
	
	 public static String sendPushNotification(String deviceToken, String notificationTitle, String messageBody) throws IOException, JSONException {
        String result = "";
        URL url = new URL(API_URL_FCM);
        HttpURLConnection conn = (HttpURLConnection) url.openConnection();

        conn.setUseCaches(false);
        conn.setDoInput(true);
        conn.setDoOutput(true);

        conn.setRequestMethod("POST");
        conn.setRequestProperty("Authorization", "key=" + AUTH_KEY_FCM);
        conn.setRequestProperty("Content-Type", "application/json");

        JSONObject json = new JSONObject();

        json.put("to", deviceToken.trim());
        
        JSONObject oData = new JSONObject();
        JSONObject oNotif = new JSONObject();
                
        oNotif.put("title", notificationTitle); 
        oNotif.put("body", messageBody); 
                
        oData.put("data1", "AAAA");
        oData.put("data2", "BBBBB");
        oData.put("data3", "CCC");
        
        json.put("notification", oNotif);
        json.put("data", oData);
        
        try {
            OutputStreamWriter wr = new OutputStreamWriter(conn.getOutputStream());
            wr.write(json.toString());
            wr.flush();

            BufferedReader br = new BufferedReader(new InputStreamReader((conn.getInputStream())));

            String output;
            System.out.println("Output from server: \n");
            while ((output = br.readLine()) != null) {
                System.out.println(output);
            }
            result = "OK";
        } 
        catch (Exception e) {
            e.printStackTrace();
            result = "BAD";
        }
        
        System.out.println("GCM Notification is sent successfully");

        return result;

    }
}