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How to push notification with Cloud Messaging Firebase from the server

I already have an app and I want to start sending notification to the users. I already set up everything in the app(using react native) and I checked manually that I can send notification to the devices and it works.
Now I want to run a job in the server who will push the message (with the device token) to the cloud messaging in firebase.
I can't find a lot of details about how to do it. I would like if someone can give me any guide I can use with. my server is in Kotlin(java can be good too) and I m working with gradle.
Thank you so much for the help

From a Java server you can use the Firebase Admin SDK to send messages. From that documentation comes this minimal example:
// This registration token comes from the client FCM SDKs.
String registrationToken = "YOUR_REGISTRATION_TOKEN";
// See documentation on defining a message payload.
Message message = Message.builder()
.putData("score", "850")
.putData("time", "2:45")
.setToken(registrationToken)
.build();
// Send a message to the device corresponding to the provided
// registration token.
String response = FirebaseMessaging.getInstance().send(message);
// Response is a message ID string.
System.out.println("Successfully sent message: " + response);
Note that this sends a data message, so that will always be delivered to your code, where you can decide to display a notification or not. To send a notification message, which is what the Firebase console does, you'd use:
Message message = Message.builder()
.setNotification(new Notification("This is the title", "This is the body"))
.setToken(registrationToken)
.build();
Both of these send the message to a specific registration token, so only to a single device/app instance. This means you will need to maintain a list of these tokens, in a way that allows you to send the messages to fit your needs. E.g. a common way is to store the tokens per user. For an example of that, see the functions-samples repo. While this example is in Node.js, the same logic could be applied to a Java server.
Finally: you can also send message to topics. For an example of that (again: using a Node.js server), have a look at this blog post Sending notifications between Android devices with Firebase Database and Cloud Messaging.

When do you need an app server for Firebase Cloud Messaging?

I am new to using FCM notifications for Android Application at https://firebase.google.com/docs/cloud-messaging/server. I was reading up on it and found that in the About FCM Server page requirements, it says the following:
An app server that you must implement in your environment. This app
server sends data to a client app via the chosen FCM connection
server, using appropriate XMPP or HTTP protocol
However, I am sorely confused about this. As I read more into the article, I see that there is an API that looks like this:
POST http://fcm.googleapis.com/fcm/send
If I invoke this API using something like OkHttpClient and build my request like so, (provided that I have authentication headers and a POST body included)
private void sendRegistrationToServer(String token) {
OkHttpClient client = new OkHttpClient();
RequestBody body = new FormBody.Builder().add(“Body", "").build();
//Assuming I have authentication and body put in
Request request = new Request.Builder().url("http://fcm.googleapis.com/fcm/send”).post(body).build();
try {
client.newCall(request).execute();
} catch (IOException e) {
e.printStackTrace();
}
}
Would I in theory, be able to send a notification with whatever information I want to that device? I can receive the message through the following class:
public class NotificationService extends FirebaseMessagingService {
...
// [START receive_message]
#Override
public void onMessageReceived(RemoteMessage remoteMessage) {
// TODO(developer): Handle FCM messages here.
Log.d(TAG, "From: " + remoteMessage.getFrom());
// Check if message contains a data payload.
if (remoteMessage.getData().size() > 0) {
Log.d(TAG, "Message data payload: " + remoteMessage.getData());
}
// Check if message contains a notification payload.
if (remoteMessage.getNotification() != null) {
Log.d(TAG, "Message Notification Body: " + remoteMessage.getNotification().getBody());
}
}
I’m sure my understanding is incorrect somewhere as the documentation does say we need an application server, but if someone could please point out where I am misunderstanding how to implement FCM notifications, that would be great. If someone could give an example of where or when we would need an app server, or how it should ACTUALLY be implemented, that would also be much appreciated. Thanks!

When the documentation says that you need an app server is mainly because you need an application that store the tokens of the devices to which you would like to send the notifications and this application should update the tokens if any of your client devices change its token. However, you could use the OkHttpClient to send request to the FCM service and therefore send notification to other devices if you have, off course, the token ID of those devices. It depends on what you want to do and it depends on how you want to manage the notifications.
If you want an example on how to implement the server app in java here is a good example example 1 that was posted or here is another post with an implementation on PHP. If you want an example on how to implement the client application and how to test it from the firebase console here is another good example.

If you use the XMPP protocol. You should implement a connection server that manages the connection to FCM to handle upstream and downstream messages.
This is a sample java project to showcase the Firebase Cloud Messaging (FCM) XMPP Connection Server. This project is a very simple standalone server that I developed as a base of a larger project. It is an application server that we must implement in our environment. This server sends data to a client app via the FCM CCS Server using the XMPP protocol.
https://github.com/carlosCharz/fcmxmppserver
And also I've created a video in youtube where I explain what it does.
https://www.youtube.com/watch?v=PA91bVq5sHw
Hope you find it useful.

No need to do any extra work just follow below link:
https://github.com/firebase/quickstart-android/tree/master/messaging

How to send direct message via Firebase from user to user?

I am really rookie and need an advice.
I have read documentation, and as far as i understood if you need send direct message, follow next steps:
Make authentification, eventually you get Firebase TokenId and
userId
Send them to your server side and store it in DB
When you are going to send a message you need create json and put
inside topic text and resipent userId so on...
Send this json via HTTP to your server side
When server retrive this json, it should use Firebase API to
create new message bloc child with random name in firebase
Eventually server have to find recipent user in DB by userId that we get from message.
After server will find current recipent user by userId , next we should take firebase tokenId In order to sent notification .
And send recipent user notification with such data - name of new
message bloc child
Recipent will connect to this current bloc and retrive data
It is as i understood this consept, fix me please if smth wrong?

Your suggested approach sounds good. The most important thing to realize is that you require an app server to send a downstream message to a device. Using the database as the communication mechanism between the app and the app server is a popular approach.
You could also use Cloud Messaging's upstream capabilities. But I've never tried that approach, because the database works fine for me and I had little interest in learning yet another protocol (XMPP).
You can read how I implemented it in this Firebase blog post Sending notifications between Android devices with Firebase Database and Cloud Messaging.

Sending Push Notification to device using parse4j in java

I am using parse4j library for server side coding and on client side I have iOS device. Now I want to send the push notification from my web browser page I developed in JAVA in which I am using parse4j library to communicate with iOS device through Parse cloud. I am using gwt for coding the server side.
public void sendPushtoIOS() {
Parse.initialize("appId", "restApiId");
ParsePush parsePushObj = new ParsePush();
parsePushObj.sendInBackground("hello from server",null);
}
I am trying to send the notification with the above code, but nothing happens and iOS device doesn't receive any notification. Please could someone guide the code I written is correct or not, If not, how can I send the notification then?

try {
String rawJSON = "{\"aps\":{\"alert\":\""+"your message"+"\"},\"alerts\":{\"others\":\""+others+"\"}}";
PushNotificationPayload payload=PushNotificationPayload.fromJSON(rawJSON);
Push.payload(payload, "path of certificate.p12", password,false, appleDeviceToken);
}
catch (Exception e) {
}

As far as I understand correct, you are using the Parse4j library to send Push Notification. Did you control the current version of parse4j if it can send Push notification? Or it is pending enhancement? One suggestion; in order to send push notification write a cloud code and trigger this cloud code from parse4j. Then cloud code will send the Push notification.
Hope this helps,
Regards.

This library works fine. You can check pushes in parse dashboard, most likely you collected push requests. The only problem was that library works with channels. Maybe you do not set channels.
If you want to work with conditions, add this lines of code in library
JSONObject local = new JSONObject();
local.put("deviceType", "android");
data.put("where", local);
and remove
data.put("channel", "");
data.put("channels", new JSONArray(this.channelSet));
all of these changes must be done in method getJSONData() in class ParsePush
instead of android you can set your devices

Send post notification to specific UUID android device

I'm trying to find some ways to send a post notification to specific android device using its UUID , so I will do the following :
1 : login screen on android and then get UUID number from the device
2 : send this UUID to online database linked to userID
3 : I want to know how to send post notification to a specific UUID which I get from my database ?

Sending notification to android device using java is very easy.
If you want to push the notification directly from REST client, then you can refer this
If you want to send notifications from your java program, then follow these simple steps,
Add gcm-server.jar library file in your application
Create a Sender, that will send the notification
com.google.android.gcm.server.Sender sender = new Sender("pass your gcm api key here");
Then create a message object that will contain the data that you want to send
com.google.android.gcm.server.Message message = new Message.Builder().addData("key","value").build();
addData("key","value") in above code will add the key value in json to be send.
Then you can choose any of send method provided in Sender class
com.google.android.gcm.server.Result result = sender.send(message, device_token, no of retries);
There are total four version of send method available. You can find more detail about Sender class here