Move static methods and constants to support class.

src/main/java/org/springframework/cloud/gateway/filter/GatewayFilter.java

@@ -31,10 +31,6 @@ import reactor.core.publisher.Mono;
  */
 public interface GatewayFilter {
 
-	String GATEWAY_ROUTE_ATTR = "gatewayRoute";
-	String GATEWAY_REQUEST_URL_ATTR = "gatewayRequestUrl";
-	String GATEWAY_HANDLER_MAPPER_ATTR = "gatewayHandlerMapper";
-
 	/**
 	 * Process the Web request and (optionally) delegate to the next
 	 * {@code WebFilter} through the given {@link WebFilterChain}.
@@ -44,14 +40,4 @@ public interface GatewayFilter {
 	 */
 	Mono<Void> filter(ServerWebExchange exchange, WebFilterChain chain);
 
-	static <T> T getAttribute(ServerWebExchange exchange, String attributeName, Class<T> type) {
-		if (exchange.getAttributes().containsKey(attributeName)) {
-			Object attr = exchange.getAttributes().get(attributeName);
-			if (type.isAssignableFrom(attr.getClass())) {
-				return type.cast(attr);
-			}
-			throw new ClassCastException(attributeName + " is not of type " + type);
-		}
-		return null;
-	}
 }

src/main/java/org/springframework/cloud/gateway/filter/RouteToRequestUrlFilter.java

@@ -5,12 +5,13 @@ import java.net.URI;
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
 import org.springframework.cloud.gateway.config.Route;
+import org.springframework.cloud.gateway.support.ServerWebExchangeUtils;
 import org.springframework.core.Ordered;
 import org.springframework.web.server.ServerWebExchange;
 import org.springframework.web.server.WebFilterChain;
 import org.springframework.web.util.UriComponentsBuilder;
 
-import static org.springframework.cloud.gateway.filter.GatewayFilter.getAttribute;
+import static org.springframework.cloud.gateway.support.ServerWebExchangeUtils.getAttribute;
 
 import reactor.core.publisher.Mono;
 
@@ -29,7 +30,7 @@ public class RouteToRequestUrlFilter implements GatewayFilter, Ordered {
 
 	@Override
 	public Mono<Void> filter(ServerWebExchange exchange, WebFilterChain chain) {
-		Route route = getAttribute(exchange, GATEWAY_ROUTE_ATTR, Route.class);
+		Route route = getAttribute(exchange, ServerWebExchangeUtils.GATEWAY_ROUTE_ATTR, Route.class);
 		if (route == null) {
 			return chain.filter(exchange);
 		}
@@ -38,7 +39,7 @@ public class RouteToRequestUrlFilter implements GatewayFilter, Ordered {
 				.uri(route.getUri())
 				.build(true)
 				.toUri();
-		exchange.getAttributes().put(GATEWAY_REQUEST_URL_ATTR, requestUrl);
+		exchange.getAttributes().put(ServerWebExchangeUtils.GATEWAY_REQUEST_URL_ATTR, requestUrl);
 		return chain.filter(exchange);
 	}
 

src/main/java/org/springframework/cloud/gateway/filter/WriteResponseFilter.java

@@ -9,8 +9,9 @@ import org.springframework.web.reactive.function.client.ClientResponse;
 import org.springframework.web.server.ServerWebExchange;
 import org.springframework.web.server.WebFilterChain;
 
-import static org.springframework.cloud.gateway.filter.GatewayFilter.getAttribute;
-import static org.springframework.cloud.gateway.handler.GatewayWebHandler.CLIENT_RESPONSE_ATTR;
+import static org.springframework.cloud.gateway.support.ServerWebExchangeUtils.CLIENT_RESPONSE_ATTR;
+import static org.springframework.cloud.gateway.support.ServerWebExchangeUtils.RESPONSE_COMMITTED_ATTR;
+import static org.springframework.cloud.gateway.support.ServerWebExchangeUtils.getAttribute;
 
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
@@ -21,6 +22,7 @@ import reactor.core.publisher.Mono;
 public class WriteResponseFilter implements GatewayFilter, Ordered {
 
 	private static final Log log = LogFactory.getLog(WriteResponseFilter.class);
+
 	public static final int WRITE_RESPONSE_FILTER_ORDER = -1;
 
 	@Override
@@ -39,6 +41,13 @@ public class WriteResponseFilter implements GatewayFilter, Ordered {
 			}
 			log.trace("WriteResponseFilter start");
 			ServerHttpResponse response = exchange.getResponse();
+
+			//make other filters aware that the response has been committed
+			response.beforeCommit(() -> {
+				exchange.getAttributes().put(RESPONSE_COMMITTED_ATTR, true);
+				return Mono.empty();
+			});
+
 			Flux<DataBuffer> body = clientResponse.body((inputMessage, context) -> inputMessage.getBody());
 			return response.writeWith(body);
 		});

src/main/java/org/springframework/cloud/gateway/filter/route/SetPathRouteFilter.java

@@ -7,7 +7,7 @@ import org.springframework.web.server.WebFilter;
 import org.springframework.http.server.reactive.ServerHttpRequest;
 import org.springframework.web.util.UriTemplate;
 
-import static org.springframework.cloud.gateway.filter.GatewayFilter.getAttribute;
+import static org.springframework.cloud.gateway.support.ServerWebExchangeUtils.getAttribute;
 import static org.springframework.cloud.gateway.handler.predicate.UrlRoutePredicate.URL_PREDICATE_VARS_ATTR;
 
 /**

src/main/java/org/springframework/cloud/gateway/filter/route/SetStatusRouteFilter.java

@@ -1,14 +1,19 @@
 package org.springframework.cloud.gateway.filter.route;
 
+import org.apache.commons.logging.Log;
+import org.apache.commons.logging.LogFactory;
 import org.springframework.http.HttpStatus;
 import org.springframework.web.server.WebFilter;
 
+import static org.springframework.cloud.gateway.support.ServerWebExchangeUtils.isResponseCommitted;
+
 import reactor.core.publisher.Mono;
 
 /**
  * @author Spencer Gibb
  */
 public class SetStatusRouteFilter implements RouteFilter {
+	protected final Log logger = LogFactory.getLog(getClass());
 
 	@Override
 	public WebFilter apply(String statusString, String[] args) {
@@ -25,10 +30,27 @@ public class SetStatusRouteFilter implements RouteFilter {
 		final HttpStatus finalStatus = httpStatus;
 
 		//TODO: caching can happen here
-		return (exchange, chain) ->
-			chain.filter(exchange).then(() -> {
+		return (exchange, chain) -> {
+
+			// option 1 (runs in filter order)
+			/*exchange.getResponse().beforeCommit(() -> {
 				exchange.getResponse().setStatusCode(finalStatus);
 				return Mono.empty();
 			});
+			return chain.filter(exchange);*/
+
+			// option 2 (runs in reverse filter order)
+			return chain.filter(exchange).then(() -> {
+				// check not really needed, since it is guarded in setStatusCode, but it's a good example
+				if (!isResponseCommitted(exchange)) {
+					boolean response = exchange.getResponse().setStatusCode(finalStatus);
+					if (!response && logger.isWarnEnabled()) {
+						logger.warn("Unable to set status code to "+ finalStatus + ". Response already committed.");
+					}
+				}
+				return Mono.empty();
+			});
+		};
 	}
+
 }

src/main/java/org/springframework/cloud/gateway/handler/GatewayFilteringWebHandler.java

@@ -40,7 +40,7 @@ import org.springframework.web.server.WebHandler;
 import org.springframework.web.server.handler.WebHandlerDecorator;
 
 import static java.util.Collections.emptyList;
-import static org.springframework.cloud.gateway.filter.GatewayFilter.GATEWAY_ROUTE_ATTR;
+import static org.springframework.cloud.gateway.support.ServerWebExchangeUtils.GATEWAY_ROUTE_ATTR;
 
 import reactor.core.publisher.Mono;
 

src/main/java/org/springframework/cloud/gateway/handler/GatewayPredicateHandlerMapping.java

@@ -18,8 +18,8 @@ import org.springframework.web.server.WebHandler;
 
 import reactor.core.publisher.Mono;
 
-import static org.springframework.cloud.gateway.filter.GatewayFilter.GATEWAY_HANDLER_MAPPER_ATTR;
-import static org.springframework.cloud.gateway.filter.GatewayFilter.GATEWAY_ROUTE_ATTR;
+import static org.springframework.cloud.gateway.support.ServerWebExchangeUtils.GATEWAY_HANDLER_MAPPER_ATTR;
+import static org.springframework.cloud.gateway.support.ServerWebExchangeUtils.GATEWAY_ROUTE_ATTR;
 
 /**
  * @author Spencer Gibb

src/main/java/org/springframework/cloud/gateway/handler/GatewayWebHandler.java

@@ -10,7 +10,8 @@ import org.springframework.web.reactive.function.client.WebClient;
 import org.springframework.web.server.ServerWebExchange;
 import org.springframework.web.server.WebHandler;
 
-import static org.springframework.cloud.gateway.filter.GatewayFilter.GATEWAY_REQUEST_URL_ATTR;
+import static org.springframework.cloud.gateway.support.ServerWebExchangeUtils.CLIENT_RESPONSE_ATTR;
+import static org.springframework.cloud.gateway.support.ServerWebExchangeUtils.GATEWAY_REQUEST_URL_ATTR;
 
 import reactor.core.publisher.Mono;
 
@@ -19,8 +20,6 @@ import reactor.core.publisher.Mono;
  */
 public class GatewayWebHandler implements WebHandler {
 
-	public static final String CLIENT_RESPONSE_ATTR = "webHandlerClientResponse";
-
 	private final WebClient webClient;
 
 	public GatewayWebHandler(WebClient webClient) {

src/main/java/org/springframework/cloud/gateway/support/ServerWebExchangeUtils.java

@@ -0,0 +1,31 @@
+package org.springframework.cloud.gateway.support;
+
+import org.springframework.web.server.ServerWebExchange;
+
+/**
+ * @author Spencer Gibb
+ */
+public class ServerWebExchangeUtils {
+
+	public static final String CLIENT_RESPONSE_ATTR = "webHandlerClientResponse";
+	public static final String GATEWAY_ROUTE_ATTR = "gatewayRoute";
+	public static final String GATEWAY_REQUEST_URL_ATTR = "gatewayRequestUrl";
+	public static final String GATEWAY_HANDLER_MAPPER_ATTR = "gatewayHandlerMapper";
+	public static final String RESPONSE_COMMITTED_ATTR = "responseCommitted";
+
+	public static <T> T getAttribute(ServerWebExchange exchange, String attributeName, Class<T> type) {
+		if (exchange.getAttributes().containsKey(attributeName)) {
+			Object attr = exchange.getAttributes().get(attributeName);
+			if (type.isAssignableFrom(attr.getClass())) {
+				return type.cast(attr);
+			}
+			throw new ClassCastException(attributeName + " is not of type " + type);
+		}
+		return null;
+	}
+
+	public static boolean isResponseCommitted(ServerWebExchange exchange) {
+		Boolean responseCommitted = getAttribute(exchange, RESPONSE_COMMITTED_ATTR, Boolean.class);
+		return responseCommitted != null && responseCommitted;
+	}
+}

src/test/java/org/springframework/cloud/gateway/test/GatewayIntegrationTests.java

@@ -29,6 +29,8 @@ import org.springframework.web.server.ServerWebExchange;
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.springframework.boot.test.context.SpringBootTest.WebEnvironment.RANDOM_PORT;
+import static org.springframework.cloud.gateway.support.ServerWebExchangeUtils.GATEWAY_HANDLER_MAPPER_ATTR;
+import static org.springframework.cloud.gateway.support.ServerWebExchangeUtils.GATEWAY_ROUTE_ATTR;
 import static org.springframework.web.reactive.function.BodyExtractors.toMono;
 import static org.springframework.web.reactive.function.client.ClientRequest.GET;
 import static org.springframework.web.reactive.function.client.ClientRequest.POST;
@@ -41,8 +43,8 @@ import reactor.test.StepVerifier;
 @SuppressWarnings("unchecked")
 public class GatewayIntegrationTests {
 
-	public static final String HANDLER_MAPPER_HEADER = "X-Gateway-Handler-Mapper-Class";
-	public static final String ROUTE_ID_HEADER = "X-Gateway-Route-Id";
+	private static final String HANDLER_MAPPER_HEADER = "X-Gateway-Handler-Mapper-Class";
+	private static final String ROUTE_ID_HEADER = "X-Gateway-Route-Id";
 
 	@LocalServerPort
 	private int port;
@@ -361,9 +363,9 @@ public class GatewayIntegrationTests {
 		public GatewayFilter modifyResponseFilter() {
 			return (exchange, chain) -> {
 				log.info("modifyResponseFilter start");
-				String value = (String) exchange.getAttribute(GatewayFilter.GATEWAY_HANDLER_MAPPER_ATTR).orElse("N/A");
+				String value = (String) exchange.getAttribute(GATEWAY_HANDLER_MAPPER_ATTR).orElse("N/A");
 				exchange.getResponse().getHeaders().add(HANDLER_MAPPER_HEADER, value);
-				Route route = (Route) exchange.getAttribute(GatewayFilter.GATEWAY_ROUTE_ATTR).orElse(null);
+				Route route = (Route) exchange.getAttribute(GATEWAY_ROUTE_ATTR).orElse(null);
 				if (route != null) {
 					exchange.getResponse().getHeaders().add(ROUTE_ID_HEADER, route.getId());
 				}