Link Multiple Auth Providers to an Account on Android

You can allow users to sign in to your app using multiple authentication providers by linking auth provider credentials to an existing user account. Users are identifiable by the same Firebase user ID regardless of the authentication provider they used to sign in. For example, a user who signed in with a password can link a Google account and sign in with either method in the future. Or, an anonymous user can link a Facebook account and then, later, sign in with Facebook to continue using your app.

Before you begin

Add support for two or more authentication providers (possibly including anonymous authentication) to your app.

Link auth provider credentials to a user account

To link auth provider credentials to an existing user account:

1. Sign in the user using any authentication provider or method.
2. Complete the sign-in flow for the new authentication provider up to, but not including, calling one of the FirebaseAuth.signInWith methods. For example, get the user's Google ID token, Facebook access token, or email and password.

3. Get a AuthCredential for the new authentication provider:

   Google Sign-In

   AuthCredential credential = GoogleAuthProvider.getCredential(googleIdToken, null);
   

   Facebook Login

   AuthCredential credential = FacebookAuthProvider.getCredential(token.getToken());
   

   Email-password sign-in

   AuthCredential credential = EmailAuthProvider.getEmailAuthCredential(email, password);
   

4. Pass the AuthCredential object to the signed-in user's linkWithCredential method:

   mAuth.getCurrentUser().linkWithCredential(credential)
           .addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
               @Override
               public void onComplete(@NonNull Task<AuthResult> task) {
                   Log.d(TAG, "linkWithCredential:onComplete:" + task.isSuccessful());
   
                   // If sign in fails, display a message to the user. If sign in succeeds
                   // the auth state listener will be notified and logic to handle the
                   // signed in user can be handled in the listener.
                   if (!task.isSuccessful()) {
                       Toast.makeText(AnonymousAuthActivity.this, "Authentication failed.",
                               Toast.LENGTH_SHORT).show();
                   }
   
                   // ...
               }
           });
   AnonymousAuthActivity.java
   

   The call to linkWithCredential will fail if the credentials are already linked to another user account. In this situation, you must handle merging the accounts and associated data as appropriate for your app:

   FirebaseUser prevUser = currentUser;
   currentUser = auth.signInWithCredential(credential).await().getUser();
   // Merge prevUser and currentUser accounts and data
   // ...
   

If the call to linkWithCredential succeeds, the user can now sign in using any linked authentication provider and access the same Firebase data.

Unlink an auth provider from a user account

You can unlink an auth provider from an account, so that the user can no longer sign in with that provider.

To unlink an auth provider from a user account, pass the provider ID to the unlink method. You can get the provider IDs of the auth providers linked to a user by calling getProviderData.

FirebaseAuth.getInstance().getCurrentUser().unlink(providerId)
        .addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
            @Override
            public void onComplete(@NonNull Task<AuthResult> task) {
                if (!task.isSuccessful()) {
                    // Auth provider unlinked from account
                }
            }
        });