sendToTopic.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 static String sendToTopic(String topicName, 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 jsObj = new JSONObject();
	        JSONObject oNotif = new JSONObject();

	        jsObj.put("to", "/topics/"+ topicName);
	        
	        JSONObject oData = new JSONObject();
	        oData.put("message", messageBody);
	        jsObj.put("data", oData);
	        
	        try {
	            OutputStreamWriter wr = new OutputStreamWriter(conn.getOutputStream());
	            wr.write(jsObj.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;

	    }