code: public String applicationProtocol() {
          SSLEngine engine = engine();
          if (!(engine instanceof ApplicationProtocolAccessor)) {
              return null;
          }
  
          return ((ApplicationProtocolAccessor) engine).getNegotiatedApplicationProtocol();
      }
docs: Returns the name of the current application-level protocol.
  
  @return the protocol name or {@code null} if application-level protocol has not been negotiated