Global Filters
GlobalFilter接口方法和GatewayFilter是一样的，GlobalFilter特别之处在于它的作用是全局的。
1. Combined Global Filter and GatewayFilter Ordering
当请求到来时，Filtering Web Handler处理器会添加所有GlobalFilter实例和匹配的GatewayFilter实例到过滤器链中，通过对filterbean配置注解@Order，则过滤器链会对这些过滤器实例bean进行排序。
Spring Cloud Gateway将过滤器的逻辑按请求执行点分为”pre”和”post”的一前一后处理，如果是高优先级的过滤器，则在”pre”逻辑中最先执行，在”post”逻辑中最后执行。
ExampleConfiguration.java.
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@Bean
@Order(-1)
public GlobalFilter a() {
return (exchange, chain) -> {
log.info("first pre filter");
return chain.filter(exchange).then(Mono.fromRunnable(() -> {
log.info("third post filter");
}));
};
}
@Bean
@Order(0)
public GlobalFilter b() {
return (exchange, chain) -> {
log.info("second pre filter");
return chain.filter(exchange).then(Mono.fromRunnable(() -> {
log.info("second post filter");
}));
};
}
@Bean
@Order(1)
public GlobalFilter c() {
return (exchange, chain) -> {
log.info("third pre filter");
return chain.filter(exchange).then(Mono.fromRunnable(() -> {
log.info("first post filter");
}));
};
}
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上面例子陆续会打印的是：
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first pre filter
second pre filter
third pre filter
first post filter
second post filter
third post filter
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2. Forward Routing Filter
ForwardRoutingFilter会查看exchange的属性ServerWebExchangeUtils.GATEWAY_REQUEST_URL_ATTR的URI内容，如果url的scheme是forward，比如：forward://localendpoint，则它会使用Spirng的DispatcherHandler来处理这个请求。
源码实现：
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@Override
public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
URI requestUrl = exchange.getRequiredAttribute(GATEWAY_REQUEST_URL_ATTR);
String scheme = requestUrl.getScheme();
if (isAlreadyRouted(exchange) || !"forward".equals(scheme)) {
return chain.filter(exchange);
}
setAlreadyRouted(exchange);
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3. LoadBalancerClient Filter
LoadBalancerClientFilter会查看exchange的属性ServerWebExchangeUtils.GATEWAY_REQUEST_URL_ATTR的URI内容，如果url的scheme是lb，比如：lb://myservice，或者是ServerWebExchangeUtils.GATEWAY_SCHEME_PREFIX_ATTR属性的内容是lb，则它会使用Spring Cloud的LoadBalancerClient来将host转化为实际的host和port，并以此替换属性ServerWebExchangeUtils.GATEWAY_REQUEST_URL_ATTR的内容，原来的URL则会被添加到ServerWebExchangeUtils.GATEWAY_ORIGINAL_REQUEST_URL_ATTR属性的列表中。
源码实现：
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@Override
public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
URI url = exchange.getAttribute(GATEWAY_REQUEST_URL_ATTR);
String schemePrefix = exchange.getAttribute(GATEWAY_SCHEME_PREFIX_ATTR);
if (url == null || (!"lb".equals(url.getScheme()) && !"lb".equals(schemePrefix))) {
return chain.filter(exchange);
}
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application.yml.
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spring:
cloud:
gateway:
routes:
- id: myRoute
uri: lb://service
predicates:
- Path=/service/**
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默认情况下，如果LoadBalancer找不到服务实例，则会返回HTTP状态码503，你也可以通过修改spring.cloud.gateway.loadbalancer.use404=true配置修改为返回状态码404。
4. Netty Routing Filter
如果ServerWebExchangeUtils.GATEWAY_REQUEST_URL_ATTR属性中的url的scheme是http或https，则Netty Routing Filter才会执行，并使用Netty作为http请求客户端对下游进行代理请求。请求的响应会放在exchange的ServerWebExchangeUtils.CLIENT_RESPONSE_ATTR属性中，以便后面的filter做进一步的处理。
5. Netty Write Response Filter
如果NettyWriteResponseFilter发现exchange的ServerWebExchangeUtils.CLIENT_RESPONSE_ATTR属性中存在Netty的HttpClientResponse类型实例，在所有过滤器都执行完毕后，它会将响应写回到gateway客户端的响应中。
源码实现:
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@Override
public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
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6. RouteToRequestUrl Filter
如果exchange的ServerWebExchangeUtils.GATEWAY_ROUTE_ATTR属性存放了Route对象，则RouteToRequestUrlFilter会根据基于请求的URI创建新的URI，新的URI会更新到ServerWebExchangeUtils.GATEWAY_REQUEST_URL_ATTR属性中。
如果URI有scheme前缀，比如：lb:ws://serviceid，lbscheme截取出来放到ServerWebExchangeUtils.GATEWAY_SCHEME_PREFIX_ATTR属性中，方便后面的filter使用。
源码实现:
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@Override
public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
Route route = exchange.getAttribute(GATEWAY_ROUTE_ATTR);
if (route == null) {
return chain.filter(exchange);
}
log.trace("RouteToRequestUrlFilter start");
URI uri = exchange.getRequest().getURI();
boolean encoded = containsEncodedParts(uri);
URI routeUri = route.getUri();
if (hasAnotherScheme(routeUri)) {
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7. Websocket Routing Filter
如果请求URL的scheme是ws或wss的话，那么Websocket Routing Filter就会使用Spring Web Socket底层来处理对下游的请求转发。
如果Websocket也使用了负载均衡，则需要这样配置：lb:ws://serviceid.
源码实现：
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@Override
public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
changeSchemeIfIsWebSocketUpgrade(exchange);
URI requestUrl = exchange.getRequiredAttribute(GATEWAY_REQUEST_URL_ATTR);
String scheme = requestUrl.getScheme();
if (isAlreadyRouted(exchange) || (!"ws".equals(scheme) && !"wss".equals(scheme))) {
return chain.filter(exchange);
}
setAlreadyRouted(exchange);
HttpHeaders headers = exchange.getRequest().getHeaders();
HttpHeaders filtered = filterRequest(getHeadersFilters(),
exchange);
List<String> protocols = headers.get(SEC_WEBSOCKET_PROTOCOL);
if (protocols != null) {
protocols = headers.get(SEC_WEBSOCKET_PROTOCOL).stream()
.flatMap(header -> Arrays.stream(commaDelimitedListToStringArray(header)))
.map(String::trim)
.collect(Collectors.toList());
}
return this.webSocketService.handleRequest(exchange,
new ProxyWebSocketHandler(requestUrl, this.webSocketClient,
filtered, protocols));
}
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8. Making An Exchange As Routed
上面一些像ForwardRoutingFilter、Websocket Routing Filter的源码中，都可以清楚看到gateway通过设置gatewayAlreadyRouted标识这个请求是否已经路由转发出去了，无需其他filter重复路由，这样就可以避免重复错误的路由操作，保证了路由的实现灵活性。
ServerWebExchangeUtils.isAlreadyRouted检查是否已被路由，ServerWebExchangeUtils.setAlreadyRouted标记已被路由状态。
转自：https://www.edjdhbb.com/2019/01/06/spring%20cloud%20gateway%E7%B3%BB%E5%88%97%E6%95%99%E7%A8%8B3%E2%80%94Global%20Filters/