How to have two different Firebase projects instances in one single app

Issue

Is it possible to have two different firebase projects instances in one single application? As there can be only one google-services.json.

I have an application where I need to access the two different Realtime Database so how is it possible to have different Firebase instances.

Solution

It is possible. When using just a single database there is a single FirebaseApp object that you don't have to explicitly handle. If you want to connect to another database, you would simply configure that object manually, like this:

FirebaseOptions options = new FirebaseOptions.Builder()
   .setApplicationId("YOUR_APP_ID") // Required for Analytics.
   .setApiKey("YOUR_API_KEY") // Required for Auth.
   .setDatabaseUrl("https://YOUR_FIREBASE_URL.firebaseio.com/") // Required for RTDB.
   .build();
FirebaseApp.initializeApp(this /* Context */, options, "secondary");

Then you can access your app by passing in the name you set for the app, then pass that in the request for the database instance:

FirebaseApp app = FirebaseApp.getInstance("secondary");
FirebaseDatabase secondaryDatabase = FirebaseDatabase.getInstance(app);

To be clear, you will still be able to access the other database in the standard way i.e.:

FirebaseDatabase database = FirebaseDatabase.getInstance();

Answered By - sam_c