Suggest setting spring.main.web-application-type=reactive if mvc on classpath.

See gh-2176K

spring-cloud-gateway-integration-tests/mvc-failure-analyzer/pom.xml

@@ -29,6 +29,10 @@
 			<groupId>org.springframework.cloud</groupId>
 			<artifactId>spring-cloud-starter-gateway</artifactId>
 		</dependency>
+		<dependency>
+			<groupId>org.springframework.cloud</groupId>
+			<artifactId>spring-cloud-starter-loadbalancer</artifactId>
+		</dependency>
 		<dependency>
 			<groupId>org.springframework.boot</groupId>
 			<artifactId>spring-boot-starter-test</artifactId>

spring-cloud-gateway-integration-tests/mvc-failure-analyzer/src/main/java/org/springframework/cloud/gateway/sample/MvcFailureAnalyzerApplication.java

@@ -18,12 +18,50 @@ package org.springframework.cloud.gateway.sample;
 
 import org.springframework.boot.SpringBootConfiguration;
 import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
+import org.springframework.boot.autoconfigure.condition.ConditionalOnWebApplication;
+import org.springframework.boot.web.server.LocalServerPort;
+import org.springframework.cloud.client.DefaultServiceInstance;
+import org.springframework.cloud.gateway.route.RouteLocator;
+import org.springframework.cloud.gateway.route.builder.RouteLocatorBuilder;
+import org.springframework.cloud.loadbalancer.annotation.LoadBalancerClient;
+import org.springframework.cloud.loadbalancer.core.ServiceInstanceListSupplier;
+import org.springframework.cloud.loadbalancer.support.ServiceInstanceListSuppliers;
+import org.springframework.context.annotation.Bean;
+import org.springframework.web.bind.annotation.GetMapping;
+import org.springframework.web.bind.annotation.RestController;
 
 /**
  * @author Spencer Gibb
  */
 @SpringBootConfiguration
 @EnableAutoConfiguration
+@RestController
+@LoadBalancerClient(name = "myservice", configuration = MyServiceConf.class)
 public class MvcFailureAnalyzerApplication {
 
+	@GetMapping("hello")
+	public String hello() {
+		return "Hello";
+	}
+
+	@Bean
+	@ConditionalOnWebApplication(type = ConditionalOnWebApplication.Type.REACTIVE)
+	public RouteLocator myRouteLocator(RouteLocatorBuilder builder) {
+		return builder.routes().route(r -> r.path("/myprefix/**")
+				.filters(f -> f.stripPrefix(1)).uri("lb://myservice")).build();
+	}
+
+}
+
+class MyServiceConf {
+
+	@LocalServerPort
+	private int port = 0;
+
+	@Bean
+	public ServiceInstanceListSupplier staticServiceInstanceListSupplier() {
+		return ServiceInstanceListSuppliers.from("myservice", new DefaultServiceInstance(
+				"myservice-1", "myservice", "localhost", port, false));
+	}
+
 }

spring-cloud-gateway-integration-tests/mvc-failure-analyzer/src/test/java/org/springframework/cloud/gateway/sample/MvcFailureAnalyzerApplicationTests.java

@@ -23,6 +23,9 @@ import org.springframework.boot.SpringApplication;
 import org.springframework.boot.test.system.CapturedOutput;
 import org.springframework.boot.test.system.OutputCaptureExtension;
 import org.springframework.cloud.gateway.support.MvcFoundOnClasspathException;
+import org.springframework.cloud.gateway.support.MvcFoundOnClasspathFailureAnalyzer;
+import org.springframework.context.ConfigurableApplicationContext;
+import org.springframework.test.web.reactive.server.WebTestClient;
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatCode;
@@ -40,8 +43,8 @@ public class MvcFailureAnalyzerApplicationTests {
 				() -> new SpringApplication(MvcFailureAnalyzerApplication.class)
 						.run("--server.port=0")).hasRootCauseInstanceOf(
 								MvcFoundOnClasspathException.class);
-		assertThat(output).contains("Spring MVC found on classpath",
-				"Please remove spring-boot-starter-web dependency");
+		assertThat(output).contains(MvcFoundOnClasspathFailureAnalyzer.MESSAGE,
+				MvcFoundOnClasspathFailureAnalyzer.ACTION);
 	}
 
 	@Test
@@ -49,8 +52,28 @@ public class MvcFailureAnalyzerApplicationTests {
 		assertThatCode(() -> new SpringApplication(MvcFailureAnalyzerApplication.class)
 				.run("--spring.cloud.gateway.enabled=false", "--server.port=0"))
 						.doesNotThrowAnyException();
-		assertThat(output).doesNotContain("Spring MVC found on classpath",
-				"Please remove spring-boot-starter-web dependency");
+		assertThat(output).doesNotContain(MvcFoundOnClasspathFailureAnalyzer.MESSAGE,
+				MvcFoundOnClasspathFailureAnalyzer.ACTION);
+	}
+
+	@Test
+	public void exceptionNotThrownWhenReactiveTypeSet(CapturedOutput output) {
+		assertThatCode(() -> {
+			ConfigurableApplicationContext context = new SpringApplication(
+					MvcFailureAnalyzerApplication.class).run(
+							"--spring.main.web-application-type=reactive",
+							"--server.port=0", "--debug=true");
+			Integer port = context.getEnvironment().getProperty("local.server.port",
+					Integer.class);
+			WebTestClient client = WebTestClient.bindToServer()
+					.baseUrl("http://localhost:" + port).build();
+			client.get().uri("/myprefix/hello").exchange().expectStatus().isOk()
+					.expectBody(String.class).isEqualTo("Hello");
+			context.close();
+		}).doesNotThrowAnyException();
+		assertThat(output).doesNotContain(MvcFoundOnClasspathFailureAnalyzer.MESSAGE,
+				MvcFoundOnClasspathFailureAnalyzer.ACTION);
+
 	}
 
 }

spring-cloud-gateway-server/src/main/java/org/springframework/cloud/gateway/config/GatewayClassPathWarningAutoConfiguration.java

@@ -23,6 +23,7 @@ import org.springframework.boot.autoconfigure.AutoConfigureBefore;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingClass;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
+import org.springframework.boot.autoconfigure.condition.ConditionalOnWebApplication;
 import org.springframework.cloud.gateway.support.MvcFoundOnClasspathException;
 import org.springframework.context.annotation.Configuration;
 
@@ -38,6 +39,7 @@ public class GatewayClassPathWarningAutoConfiguration {
 
 	@Configuration(proxyBeanMethods = false)
 	@ConditionalOnClass(name = "org.springframework.web.servlet.DispatcherServlet")
+	@ConditionalOnWebApplication(type = ConditionalOnWebApplication.Type.SERVLET)
 	protected static class SpringMvcFoundOnClasspathConfiguration {
 
 		public SpringMvcFoundOnClasspathConfiguration() {

spring-cloud-gateway-server/src/main/java/org/springframework/cloud/gateway/support/MvcFoundOnClasspathFailureAnalyzer.java

@@ -22,12 +22,20 @@ import org.springframework.boot.diagnostics.FailureAnalysis;
 public class MvcFoundOnClasspathFailureAnalyzer
 		extends AbstractFailureAnalyzer<MvcFoundOnClasspathException> {
 
+	/**
+	 * Message for MvcFoundOnClasspathException.
+	 */
+	public static final String MESSAGE = "Spring MVC found on classpath, which is incompatible with Spring Cloud Gateway.";
+
+	/**
+	 * Action for MvcFoundOnClasspathException.
+	 */
+	public static final String ACTION = "Please set spring.main.web-application-type=reactive or remove spring-boot-starter-web dependency.";
+
 	@Override
 	protected FailureAnalysis analyze(Throwable rootFailure,
 			MvcFoundOnClasspathException cause) {
-		String message = "Spring MVC found on classpath, which is incompatible with Spring Cloud Gateway.";
-		String action = "Please remove spring-boot-starter-web dependency.";
-		return new FailureAnalysis(message, action, cause);
+		return new FailureAnalysis(MESSAGE, ACTION, cause);
 	}
 
 }