聊聊spring cloud gateway的PreserveHostHeaderGatewayFilter

@Configuration
@ConditionalOnProperty(name = "spring.cloud.gateway.enabled", matchIfMissing = true)
@EnableConfigurationProperties
@AutoConfigureBefore(HttpHandlerAutoConfiguration.class)
@AutoConfigureAfter({GatewayLoadBalancerClientAutoConfiguration.class, GatewayClassPathWarningAutoConfiguration.class})
@ConditionalOnClass(DispatcherHandler.class)
public class GatewayAutoConfiguration {
    //......
    @Bean
    public PreserveHostHeaderGatewayFilterFactory preserveHostHeaderGatewayFilterFactory() {
        return new PreserveHostHeaderGatewayFilterFactory();
    }
    //......
}

public class PreserveHostHeaderGatewayFilterFactory extends AbstractGatewayFilterFactory {

    public GatewayFilter apply() {
        return apply(o -> {});
    }

    public GatewayFilter apply(Object config) {
        return (exchange, chain) -> {
            exchange.getAttributes().put(PRESERVE_HOST_HEADER_ATTRIBUTE, true);
            return chain.filter(exchange);
        };
    }
}

public class NettyRoutingFilter implements GlobalFilter, Ordered {

    private final HttpClient httpClient;
    private final ObjectProvider<List<HttpHeadersFilter>> headersFilters;

    public NettyRoutingFilter(HttpClient httpClient,
            ObjectProvider<List<HttpHeadersFilter>> headersFilters) {
        this.httpClient = httpClient;
        this.headersFilters = headersFilters;
    }

    @Override
    public int getOrder() {
        return Ordered.LOWEST_PRECEDENCE;
    }

    @Override
    public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
        URI requestUrl = exchange.getRequiredAttribute(GATEWAY_REQUEST_URL_ATTR);

        String scheme = requestUrl.getScheme();
        if (isAlreadyRouted(exchange) || (!"http".equals(scheme) && !"https".equals(scheme))) {
            return chain.filter(exchange);
        }
        setAlreadyRouted(exchange);

        ServerHttpRequest request = exchange.getRequest();

        final HttpMethod method = HttpMethod.valueOf(request.getMethod().toString());
        final String url = requestUrl.toString();

        HttpHeaders filtered = filterRequest(this.headersFilters.getIfAvailable(),
                exchange);

        final DefaultHttpHeaders httpHeaders = new DefaultHttpHeaders();
        filtered.forEach(httpHeaders::set);

        String transferEncoding = request.getHeaders().getFirst(HttpHeaders.TRANSFER_ENCODING);
        boolean chunkedTransfer = "chunked".equalsIgnoreCase(transferEncoding);

        boolean preserveHost = exchange.getAttributeOrDefault(PRESERVE_HOST_HEADER_ATTRIBUTE, false);

        return this.httpClient.request(method, url, req -> {
            final HttpClientRequest proxyRequest = req.options(NettyPipeline.SendOptions::flushOnEach)
                    .headers(httpHeaders)
                    .chunkedTransfer(chunkedTransfer)
                    .failOnServerError(false)
                    .failOnClientError(false);

            if (preserveHost) {
                String host = request.getHeaders().getFirst(HttpHeaders.HOST);
                proxyRequest.header(HttpHeaders.HOST, host);
            }

            return proxyRequest.sendHeaders() //I shouldn't need this
                    .send(request.getBody().map(dataBuffer ->
                            ((NettyDataBuffer)dataBuffer).getNativeBuffer()));
        }).doOnNext(res -> {
            ServerHttpResponse response = exchange.getResponse();
            // put headers and status so filters can modify the response
            HttpHeaders headers = new HttpHeaders();

            res.responseHeaders().forEach(entry -> headers.add(entry.getKey(), entry.getValue()));

            HttpHeaders filteredResponseHeaders = HttpHeadersFilter.filter(
                    this.headersFilters.getIfAvailable(), headers, exchange, Type.RESPONSE);

            response.getHeaders().putAll(filteredResponseHeaders);
            HttpStatus status = HttpStatus.resolve(res.status().code());
            if (status != null) {
                response.setStatusCode(status);
            } else if (response instanceof AbstractServerHttpResponse) {
                // https://jira.spring.io/browse/SPR-16748
                ((AbstractServerHttpResponse) response).setStatusCodeValue(res.status().code());
            } else {
                throw new IllegalStateException("Unable to set status code on response: " +res.status().code()+", "+response.getClass());
            }

            // Defer committing the response until all route filters have run
            // Put client response as ServerWebExchange attribute and write response later NettyWriteResponseFilter
            exchange.getAttributes().put(CLIENT_RESPONSE_ATTR, res);
        }).then(chain.filter(exchange));
    }
}