private byte[] concatLengthPrefixed() {
        ByteArrayOutputStream os = new ByteArrayOutputStream();
        String protocolStr = mSelectedAlpnProtocol.toString().toLowerCase();
        os.write(protocolStr.length());
        os.write(protocolStr.getBytes(Charset.forName("UTF-8")), 0, protocolStr.length());
        return os.toByteArray();
    }
Http request ssl codec enables Application-Layer Protocol Negotiation(ALPN).

See http://tools.ietf.org/html/draft-agl-tls-nextprotoneg-04#page-4

@author Megatron King
@since 2019/1/3 23:01
netbare-core/src/main/java/com/github/megatronking/netbare/http/HttpSSLRequestCodec.java