SslContextBuilder and Private Key
SslContextBuilder and so Netty´s
SslContext implementations only support PKCS8 keys.
If you have a key with another format you need to convert it to PKCS8 first to be able to use it. This can be done easily by using openssl.
For example to convert a non-encrypted PKCS1 key to PKCS8 you would use:
openssl pkcs8 -topk8 -nocrypt -in pkcs1_key_file -out pkcs8_key.pem
For more details please see the OpenSSL docs.