We can use single putIfAbsent instead of separate containsKey/get/put calls.
It makes code a bit cleaner and faster.

            synchronized (files) {
                if (files.containsKey(key)) {
                    src.close();
                    src = files.get(key);
                    src.refs++;
                    return src;
                }
                files.put(key, src);
                return src;
            }