Adds HandlerFunctions.forward()

See gh-2949

spring-cloud-gateway-server-mvc/src/main/java/org/springframework/cloud/gateway/server/mvc/filter/CircuitBreakerFilterFunctions.java

@@ -101,6 +101,7 @@ public abstract class CircuitBreakerFilterFunctions {
 				request.attributes().put(MvcUtils.CIRCUITBREAKER_EXECUTION_EXCEPTION_ATTR, throwable);
 
 				// handle fallback
+				// ok() is wrong, but will be overwritten by the forwarded request
 				return GatewayServerResponse.ok().build((httpServletRequest, httpServletResponse) -> {
 					try {
 						String expandedFallback = MvcUtils.expand(request, config.getFallbackPath());

spring-cloud-gateway-server-mvc/src/main/java/org/springframework/cloud/gateway/server/mvc/handler/HandlerFunctions.java

@@ -16,12 +16,15 @@
 
 package org.springframework.cloud.gateway.server.mvc.handler;
 
+import java.io.IOException;
 import java.lang.reflect.Method;
 import java.net.URI;
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.concurrent.atomic.AtomicReference;
 
+import jakarta.servlet.ServletException;
+
 import org.springframework.cloud.gateway.server.mvc.common.MvcUtils;
 import org.springframework.web.servlet.function.HandlerFunction;
 import org.springframework.web.servlet.function.ServerRequest;
@@ -33,6 +36,21 @@ public abstract class HandlerFunctions {
 
 	}
 
+	public static HandlerFunction<ServerResponse> forward(String path) {
+		// ok() is wrong, but can be overridden by the forwarded request.
+		return request -> GatewayServerResponse.ok().build((httpServletRequest, httpServletResponse) -> {
+			try {
+				String expandedFallback = MvcUtils.expand(request, path);
+				request.servletRequest().getServletContext().getRequestDispatcher(expandedFallback)
+						.forward(httpServletRequest, httpServletResponse);
+				return null;
+			}
+			catch (ServletException | IOException e) {
+				throw new RuntimeException(e);
+			}
+		});
+	}
+
 	// TODO: current discovery only goes by method name
 	// so last one wins, so put parameterless last
 	public static HandlerFunction<ServerResponse> http(URI uri) {

spring-cloud-gateway-server-mvc/src/test/java/org/springframework/cloud/gateway/server/mvc/ServerMvcIntegrationTests.java

@@ -111,6 +111,7 @@ import static org.springframework.cloud.gateway.server.mvc.filter.FilterFunction
 import static org.springframework.cloud.gateway.server.mvc.filter.LoadBalancerFilterFunctions.lb;
 import static org.springframework.cloud.gateway.server.mvc.filter.RetryFilterFunctions.retry;
 import static org.springframework.cloud.gateway.server.mvc.handler.GatewayRouterFunctions.route;
+import static org.springframework.cloud.gateway.server.mvc.handler.HandlerFunctions.forward;
 import static org.springframework.cloud.gateway.server.mvc.handler.HandlerFunctions.http;
 import static org.springframework.cloud.gateway.server.mvc.predicate.GatewayRequestPredicates.cloudFoundryRouteService;
 import static org.springframework.cloud.gateway.server.mvc.predicate.GatewayRequestPredicates.cookie;
@@ -581,6 +582,17 @@ public class ServerMvcIntegrationTests {
 						"{\"message\":\"HELLO WORLD\"}"));
 	}
 
+	@Test
+	public void forwardWorks() {
+		restClient.get().uri("/doforward").exchange().expectStatus().isOk().expectBody(String.class).isEqualTo("Hello");
+	}
+
+	@Test
+	public void forwardNon200StatusWorks() {
+		restClient.get().uri("/doforward2").exchange().expectStatus().isCreated().expectBody(String.class)
+				.isEqualTo("hello2");
+	}
+
 	@SpringBootConfiguration
 	@EnableAutoConfiguration
 	@LoadBalancerClient(name = "httpbin", configuration = TestLoadBalancerConfig.Httpbin.class)
@@ -612,6 +624,12 @@ public class ServerMvcIntegrationTests {
 			return route(GET("/hello"), testHandler).withAttribute(MvcUtils.GATEWAY_ROUTE_ID_ATTR, "hello");
 		}
 
+		@Bean
+		public RouterFunction<ServerResponse> nonGatewayRouterFunctions2() {
+			return route(GET("/hello2"), request -> ServerResponse.status(HttpStatus.CREATED).body("hello2"))
+					.withAttribute(MvcUtils.GATEWAY_ROUTE_ID_ATTR, "hello2");
+		}
+
 		@Bean
 		public RouterFunction<ServerResponse> gatewayRouterFunctionsAddReqHeader() {
 			// @formatter:off
@@ -1063,6 +1081,26 @@ public class ServerMvcIntegrationTests {
 			// @formatter:on
 		}
 
+		@Bean
+		public RouterFunction<ServerResponse> gatewayRouterFunctionsForward() {
+			// @formatter:off
+			return route("testforward")
+					.GET("/doforward", forward("/hello"))
+					.before(new LocalServerPortUriResolver())
+					.build();
+			// @formatter:on
+		}
+
+		@Bean
+		public RouterFunction<ServerResponse> gatewayRouterFunctionsForwardNon200Status() {
+			// @formatter:off
+			return route("testforwardnon200status")
+					.GET("/doforward2", forward("/hello2"))
+					.before(new LocalServerPortUriResolver())
+					.build();
+			// @formatter:on
+		}
+
 		private Predicate<Event> eventPredicate(String foo) {
 			return new Predicate<>() {
 				@Override