How do you send a Firebase Notification to all devices via CURL?

The most easiest way I came up with to send the push notification to all the devices is to subscribe them to a topic "all" and then send notification to this topic. Copy this in your main activity

FirebaseMessaging.getInstance().subscribeToTopic("all");

Now send the request as

{
    "to": "/topics/all",
    "data":
    {
        "title":"Your title",
        "message":"Your message"
        "image-url":"your_image_url"
    }
}

This might be inefficient or non-standard way, but as I mentioned above it's the easiest. Please do post if you have any better way to send a push notification to all the devices.

Just checked the FCM documentation, this is the only way to send notifications to all the devices (as of 8th July 2022).

As mentioned in the comments, the notification is not automatically displayed, you have to define a class that is derived from FirebaseMessagingService and then override the function onMessageReceived.

First register your service in app manifest.

<!-- AndroidManifest.xml -->

<service
    android:name=".java.MyFirebaseMessagingService"
    android:exported="false">
    <intent-filter>
        <action android:name="com.google.firebase.MESSAGING_EVENT" />
    </intent-filter>
</service>

Add these lines inside the application tag to set the custom default icon and custom color:

<!-- AndroidManifest.xml -->

<!-- Set custom default icon. This is used when no icon is set 
for incoming notification messages. -->
<meta-data
    android:name="com.google.firebase.messaging.default_notification_icon"
    android:resource="@drawable/ic_stat_ic_notification" />
<!-- Set color used with incoming notification messages. This is used 
when no color is set for the incoming notification message. -->
<meta-data
    android:name="com.google.firebase.messaging.default_notification_color"
    android:resource="@color/colorAccent" />

Now create your service to receive the push notifications.

// MyFirebaseMessagingService.java

package com.google.firebase.example.messaging;

import android.content.Context;
import android.util.Log;

import androidx.annotation.NonNull;
import androidx.work.OneTimeWorkRequest;
import androidx.work.WorkManager;
import androidx.work.Worker;
import androidx.work.WorkerParameters;

import com.google.firebase.messaging.FirebaseMessagingService;
import com.google.firebase.messaging.RemoteMessage;

public class MyFirebaseMessagingService extends FirebaseMessagingService {

    private static final String TAG = "MyFirebaseMsgService";

    // [START receive_message]
    @Override
    public void onMessageReceived(RemoteMessage remoteMessage) {
        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());

            if (/* Check if data needs to be processed by long running job */ true) {
                // For long-running tasks (10 seconds or more) use WorkManager.
                scheduleJob();
            } else {
                // Handle message within 10 seconds
                handleNow();
            }

        }

        // Check if message contains a notification payload.
        if (remoteMessage.getNotification() != null) {
            Log.d(TAG, "Message Notification Body: " + remoteMessage.getNotification().getBody());
        }

    }
    // [END receive_message]

    // [START on_new_token]
    /**
     * There are two scenarios when onNewToken is called:
     * 1) When a new token is generated on initial app startup
     * 2) Whenever an existing token is changed
     * Under #2, there are three scenarios when the existing token is changed:
     * A) App is restored to a new device
     * B) User uninstalls/reinstalls the app
     * C) User clears app data
     */
    @Override
    public void onNewToken(@NonNull String token) {
        Log.d(TAG, "Refreshed token: " + token);

        // If you want to send messages to this application instance or
        // manage this apps subscriptions on the server side, send the
        // FCM registration token to your app server.
        sendRegistrationToServer(token);
    }
    // [END on_new_token]

    private void scheduleJob() {
        // [START dispatch_job]
        OneTimeWorkRequest work = new OneTimeWorkRequest.Builder(MyWorker.class)
                .build();
        WorkManager.getInstance(this).beginWith(work).enqueue();
        // [END dispatch_job]
    }

    private void handleNow() {
        Log.d(TAG, "Short lived task is done.");
    }

    private void sendRegistrationToServer(String token) {
        // TODO: Implement this method to send token to your app server.
    }

    public static class MyWorker extends Worker {

        public MyWorker(@NonNull Context context, @NonNull WorkerParameters workerParams) {
            super(context, workerParams);
        }

        @NonNull
        @Override
        public Result doWork() {
            // TODO(developer): add long running task here.
            return Result.success();
        }
    }
}

_{You can follow this tutorial if you're new to sending push notifications using Firebase Cloud Messaging Tutorial - Push Notifications using FCM and Send messages to multiple devices - Firebase Documentation}

---

To send a message to a combination of topics, specify a condition, which is a boolean expression that specifies the target topics. For example, the following condition will send messages to devices that are subscribed to TopicA and either TopicB or TopicC:

{
   "data":
   {
      "title": "Your title",
      "message": "Your message"
      "image-url": "your_image_url"
   },
   "condition": "'TopicA' in topics && ('TopicB' in topics || 'TopicC' in topics)"
}

Read more about conditions and topics here on FCM documentation

Firebase Notifications doesn't have an API to send messages. Luckily it is built on top of Firebase Cloud Messaging, which has precisely such an API.

With Firebase Notifications and Cloud Messaging, you can send so-called downstream messages to devices in three ways:

1. to specific devices, if you know their device IDs
2. to groups of devices, if you know the registration IDs of the groups
3. to topics, which are just keys that devices can subscribe to

You'll note that there is no way to send to all devices explicitly. You can build such functionality with each of these though, for example: by subscribing the app to a topic when it starts (e.g. /topics/all) or by keeping a list of all device IDs, and then sending the message to all of those.

For sending to a topic you have a syntax error in your command. Topics are identified by starting with /topics/. Since you don't have that in your code, the server interprets allDevices as a device id. Since it is an invalid format for a device registration token, it raises an error.

From the documentation on sending messages to topics:

https://fcm.googleapis.com/fcm/send
Content-Type:application/json
Authorization:key=AIzaSyZ-1u...0GBYzPu7Udno5aA

{
  "to": "/topics/foo-bar",
  "data": {
    "message": "This is a Firebase Cloud Messaging Topic Message!",
   }
}