Add warning if Spring MVC is on the classpath.

fixes gh-80

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

@@ -0,0 +1,19 @@
+package org.springframework.cloud.gateway.config;
+
+import org.apache.commons.logging.Log;
+import org.apache.commons.logging.LogFactory;
+import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
+import org.springframework.context.annotation.Configuration;
+
+@Configuration
+@ConditionalOnClass(name = "org.springframework.web.servlet.DispatcherServlet")
+public class GatewayClassPathWarningAutoConfiguration {
+
+	private static final Log log = LogFactory.getLog(GatewayClassPathWarningAutoConfiguration.class);
+	private static final String BORDER = "\n\n**********************************************************\n\n";
+
+	public GatewayClassPathWarningAutoConfiguration() {
+		log.warn(BORDER+"Spring MVC found on classpath, which is incompatible with Spring Cloud Gateway at this time. "+
+			"Please remove spring-boot-starter-web dependency."+BORDER);
+	}
+}

spring-cloud-gateway-core/src/main/resources/META-INF/spring.factories

@@ -1,5 +1,6 @@
 # Auto Configure
 org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
 org.springframework.cloud.gateway.config.GatewayAutoConfiguration,\
+org.springframework.cloud.gateway.config.GatewayClassPathWarningAutoConfiguration,\
 org.springframework.cloud.gateway.config.GatewayLoadBalancerClientAutoConfiguration,\
 org.springframework.cloud.gateway.config.GatewayRedisAutoConfiguration