public static class MyHandler extends AbstractHandshakeHandler {
        public MyHandler() {
            super(new AbstractStandardUpgradeStrategy() {
                @Override
                public String[] getSupportedVersions() {
                    return new String[] {"13"};//copied from the tomcat strategy
                }

                @Override
                protected void upgradeInternal(ServerHttpRequest request, ServerHttpResponse response, String selectedProtocol, List<Extension> selectedExtensions, Endpoint endpoint) throws HandshakeFailureException {
                    var r = getHttpServletResponse(response);
                    r.setStatus(HttpStatus.I_AM_A_TEAPOT.value());  // this works
                    r.addHeader("Content-Type", "application/json");
                    var d = "{error: \"error\"}".getBytes();
                    r.addIntHeader("Content-Length", d.length);  // this doesn't work
                    try {
                        r.getOutputStream().write(d);  // this doesn't work
                        r.flushBuffer();
                    } catch (IOException e) {
                        log.error("Error writing body", e);
                    }
                }
            });
        }
    }

    @Override
    public void registerWebSocketHandlers(WebSocketHandlerRegistry registry) {
        registry.addHandler(new ExceptionWebSocketHandlerDecorator(myWebSocketHandler), "/ws/")
                .setAllowedOrigins("*")
                .addInterceptors(myHandshakeInterceptor)
        .setHandshakeHandler(new MyHandler());
    }