Validate URI has host unless scheme is forward

Fixes gh-2919

spring-cloud-gateway-server/src/main/java/org/springframework/cloud/gateway/route/Route.java

@@ -187,6 +187,10 @@ public B uri(URI uri) {
 			this.uri = uri;
 			String scheme = this.uri.getScheme();
 			Assert.hasText(scheme, "The parameter [" + this.uri + "] format is incorrect, scheme can not be empty");
+			if (!scheme.equalsIgnoreCase("forward")) {
+				Assert.hasText(this.uri.getHost(),
+						"The parameter [" + this.uri + "] format is incorrect, host can not be empty");
+			}
 			if (this.uri.getPort() < 0 && scheme.startsWith("http")) {
 				// default known http ports
 				int port = this.uri.getScheme().equals("https") ? 443 : 80;

spring-cloud-gateway-server/src/test/java/org/springframework/cloud/gateway/route/RouteTests.java

@@ -54,6 +54,26 @@ public void nullScheme() {
 				.isInstanceOf(IllegalArgumentException.class);
 	}
 
+	@Test
+	public void emptyHostFails() {
+		assertThatThrownBy(() -> Route.async().id("1").predicate(exchange -> true).uri("localhost:8080"))
+				.isInstanceOf(IllegalArgumentException.class);
+	}
+
+	@Test
+	public void noOpWorks() {
+		Route route = Route.async().id("1").predicate(exchange -> true).uri("no://op").build();
+
+		assertThat(route.getUri()).hasScheme("no").hasHost("op");
+	}
+
+	@Test
+	public void forwardWorks() {
+		Route route = Route.async().id("1").predicate(exchange -> true).uri("forward:/some/path").build();
+
+		assertThat(route.getUri()).hasScheme("forward").hasPath("/some/path");
+	}
+
 	@Test
 	public void defaultMetadataToEmpty() {
 		Route route = Route.async().id("1").predicate(exchange -> true).uri("http://acme.com:8080").build();