Firebase FCM token - When to send to server?

Note that you can always retrieve the token with:

FirebaseInstanceID.getInstance().getToken();

This will return null if the token has not yet been generated or the token if it has been generated. In your case it is very likely that the token will be generated by the time the user has signed in. So you should be able to send it to your app server as soon as the user has signed in. If it is not available then you would send it in the onTokenRefresh callback as Chintan Soni mentioned.

Edit

Using the new Firebase SDK (21.0.0) , you will get your token this way :

 FirebaseInstallations.getInstance().getToken(false).addOnCompleteListener(new OnCompleteListener<InstallationTokenResult>() {
          @Override
          public void onComplete(@NonNull Task<InstallationTokenResult> task) {
              if(!task.isSuccessful()){
                  return;
              }
              // Get new Instance ID token
              String token = task.getResult().getToken();

          }
      });

You better add a listener for more handling on the response .

Yes FCM token is generated automatically. But try to see this in a different angle.

This is how I handled it.

Let FCM generate token as soon as your app starts. OnTokenRefresh will be called and you just save it in your preferences as:

@Override
public void onTokenRefresh() {
    // Get updated InstanceID token.
    String refreshedToken = FirebaseInstanceId.getInstance().getToken();
    Log.d(TAG, "Refreshed token: " + refreshedToken);

    sendRegistrationToServer(refreshedToken);
}

private void sendRegistrationToServer(String token) {
    // Add custom implementation, as needed.
    SharedPreferenceUtils.getInstance(this).setValue(getString(R.string.firebase_cloud_messaging_token), token);
    
   // To implement: Only if user is registered, i.e. UserId is available in preference, update token on server.
   int userId = SharedPreferenceUtils.getInstance(this).getIntValue(getString(R.string.user_id), 0);
   if(userId != 0){
       // Implement code to update registration token to server
   }
}

Hope you are clear with the way. Ask if you need more clearance on it.

Edit

Using the new Firebase SDK (21.0.0) , you need to override onNewToken() method instead of onTokenRefresh()

 @Override
public void onNewToken(@NonNull String s) {
    super.onNewToken(s);
    sendRegistrationToServer(s);
}