Firebase Java Server to send push notification to all devices

I don't believe this is possible. Instead, what I would suggest is register all devices to the same topic and then you can message all the devices at once. Here is the help documentation on this:

Send Topic Messages from the Server

https://firebase.google.com/docs/cloud-messaging/topic-messaging

This solution send Push notification to Firebase using Apache HttpClient:

HttpClient client = HttpClientBuilder.create().build();
HttpPost post = new HttpPost("https://fcm.googleapis.com/fcm/send");
post.setHeader("Content-type", "application/json");
post.setHeader("Authorization", "key=AIzaSyBSxxxxsXevRq0trDbA9mhnY_2jqMoeChA");

JSONObject message = new JSONObject();
message.put("to", "dBbB2BFT-VY:APA91bHrvgfXbZa-K5eg9vVdUkIsHbMxxxxxc8dBAvoH_3ZtaahVVeMXP7Bm0iera5s37ChHmAVh29P8aAVa8HF0I0goZKPYdGT6lNl4MXN0na7xbmvF25c4ZLl0JkCDm_saXb51Vrte");
message.put("priority", "high");

JSONObject notification = new JSONObject();
notification.put("title", "Java");
notification.put("body", "Notificação do Java");

message.put("notification", notification);

post.setEntity(new StringEntity(message.toString(), "UTF-8"));
HttpResponse response = client.execute(post);
System.out.println(response);
System.out.println(message);