public static Key importRsaKey(String pathname, KeyType type) throws IOException {
    try {
        byte[] readBytes = Files.readAllBytes(Paths.get(pathname));
        switch (type) {
            case PRIVATE:
                return KeyFactory.getInstance("RSA").generatePrivate(new PKCS8EncodedKeySpec(readBytes));
            case PUBLIC:
                return KeyFactory.getInstance("RSA").generatePublic(new X509EncodedKeySpec(readBytes));
        }
    } catch (InvalidKeySpecException e) {
        System.out.println("Invalid key file");
        e.printStackTrace();
    } catch (NoSuchAlgorithmException e) {
        //Should not happen.
        e.printStackTrace();
    }
    return null;
}

private enum KeyType {
    PRIVATE, PUBLIC
}