How to send device to device messages using Firebase Cloud Messaging?

Question

After searching the docs I could not find any info on how to send device to device messages using FCM without the use of an external server.

For example, if I was cr

Answer 1

Making a HTTP POST request with the link https://fcm.googleapis.com/fcm/send with required header and data helped me. In the below code snippet Constants.LEGACY_SERVER_KEY is a local class variable, you can find this at your Firebase Project Settings->Cloud Messaging->Legacy Server key. You need to pass device registration token i.e. regToken in below code snippet referenced HERE.

At last you need okhttp library dependency in order to get this snippet work.

public static final MediaType JSON
        = MediaType.parse("application/json; charset=utf-8");
private void sendNotification(final String regToken) {
    new AsyncTask(){
        @Override
        protected Void doInBackground(Void... params) {
            try {
                OkHttpClient client = new OkHttpClient();
                JSONObject json=new JSONObject();
                JSONObject dataJson=new JSONObject();
                dataJson.put("body","Hi this is sent from device to device");
                dataJson.put("title","dummy title");
                json.put("notification",dataJson);
                json.put("to",regToken);
                RequestBody body = RequestBody.create(JSON, json.toString());
                Request request = new Request.Builder()
                        .header("Authorization","key="+Constants.LEGACY_SERVER_KEY)
                        .url("https://fcm.googleapis.com/fcm/send")
                        .post(body)
                        .build();
                Response response = client.newCall(request).execute();
                String finalResponse = response.body().string();
            }catch (Exception e){
                //Log.d(TAG,e+"");
            }
            return null;
        }
    }.execute();

}

further if you want to send message to a particular topic, replace regToken in json like this

json.put("to","/topics/foo-bar")

and don't forget to add INTERNET permission in your AndroidManifest.xml.

IMPORTANT : - Using above code means your server key resides in the client application. That is dangerous as someone can dig into your application and get the server key to send malicious notifications to your users.