code: public String getFrameData() {
          if (SockJsFrameType.OPEN == getType() || SockJsFrameType.HEARTBEAT == getType()) {
              return null;
          } else {
              return getContent().substring(1);
          }
      }
docs: Return data contained in a SockJS "message" and "close" frames. Otherwise
  for SockJS "open" and "close" frames, which do not contain data, return
  {@code null}.
  
  @return a {@link java.lang.String} object.