sw_in_vaadin.java

public static class MyUIServlet extends VaadinServlet {

  @Override
  protected void service(HttpServletRequest request, HttpServletResponse response)
      throws ServletException, IOException {
    String pathInfo = request.getPathInfo();

    if (pathInfo.endsWith("sw.js")) {
      try (InputStream in = getClass().getResourceAsStream("/sw.js")) {
        if (in == null) {
          response.sendError(404);
          return;
        }
        response.setContentType("application/javascript");
        OutputStream out = response.getOutputStream();
        IOUtils.copy(in, out);
        in.close();
        out.close();
      }
    } else {
      super.service(request, response);
    }
  }
}