default Mono<Void> fallback(ServerWebExchange exchange, URI uri, Throwable t) {
        // client HttpStatusCodeException, return the client response directly
        if (t instanceof HttpStatusCodeException || Objects.isNull(uri)) {
            return withoutFallback(exchange, t);
        }
        if (uri.toString().startsWith(PREFIX)) {
            String fallbackPath = uri.toString().substring(PREFIX.length());
            DispatcherHandler dispatcherHandler =
                    SpringBeanUtils.getInstance().getBean(DispatcherHandler.class);
            URI previsouUri = exchange.getRequest().getURI();
            // avoid redirect loop, return error.
            if (UriUtils.getPathWithParams(previsouUri).equals(fallbackPath)) {
                return withoutFallback(exchange, t);
            }
            URI fallbackUri = UriUtils.createUri(previsouUri.getScheme(), previsouUri.getAuthority(), fallbackPath);
            ServerHttpRequest request = exchange.getRequest().mutate().uri(fallbackUri).build();
            ServerWebExchange mutated = exchange.mutate().request(request).build();
            return dispatcherHandler.handle(mutated);
        }
        ServerHttpResponse response = exchange.getResponse();
        ServerHttpRequest request = exchange.getRequest();
        // avoid redirect loop, return error.
        boolean isSameUri;
        if (Objects.nonNull(uri.getScheme())) {
            isSameUri = request.getURI().toString().equals(uri.toString());
        } else {
            String uriStr = UriUtils.repairData(uri.toString());
            isSameUri = uriStr.equals(UriUtils.getPathWithParams(request.getURI()));
        }
        if (isSameUri) {
            return withoutFallback(exchange, t);
        }
        // redirect to fallback uri.
        response.setStatusCode(HttpStatus.FOUND);
        response.getHeaders().setLocation(uri);
        return Mono.empty();
    }
do fallback.

@param exchange the web exchange
@param uri      the uri
@param t        the throwable
@return Mono
shenyu-plugin/shenyu-plugin-base/src/main/java/org/apache/shenyu/plugin/base/fallback/FallbackHandler.java