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

UPDATE: It is now possible to use firebase cloud functions as the server for handling push notifications. Check out their documentation here

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According to the docs you must implement a server for handling push notifications in device to device communication.

Before you can write client apps that use Firebase Cloud Messaging, you must have an app server that meets the following criteria:

...

You'll need to decide which FCM connection server protocol(s) you want to use to enable your app server to interact with FCM connection servers. Note that if you want to use upstream messaging from your client applications, you must use XMPP. For a more detailed discussion of this, see Choosing an FCM Connection Server Protocol.

If you only need to send basic notifications to your users from the server. You can use their serverless solution, Firebase Notifications.

See a comparison here between FCM and Firebase Notifications: https://firebase.google.com/support/faq/#messaging-difference

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<Void,Void,Void>(){
        @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.