docs: synchronized
    public
    void close() {
      closed = true;
      if (sqw != null) {
          try {
              if (layoutHeaderChecked && layout != null && layout.getFooter() != null) {
                  sendLayoutMessage(layout.getFooter());
              }
              sqw.close();
              sqw = null;
          } catch(java.io.IOException ex) {
              sqw = null;
          }
      }
    }