When Realm handles multiple .realm files, Migration wants to view data from another realm file.

I think the migration process is described in Migration.execute().
If you create a RealmConfiguration for readonly.realm and call Realm.getInstance() in that part, you can get a Realm instance for readonly.realm.

If this method doesn't work, it would be easier to answer if you could add a description of how it doesn't work.

There may be some problems with the old Realm Java version, but when I tried 0.84.1, I was able to get the Realm instance without any problems.