Restores ability to just check for the presence of a header.

fixes gh-715

spring-cloud-gateway-core/src/main/java/org/springframework/cloud/gateway/handler/predicate/HeaderRoutePredicateFactory.java

@@ -18,11 +18,13 @@
 package org.springframework.cloud.gateway.handler.predicate;
 
 import java.util.Arrays;
+import java.util.Collections;
 import java.util.List;
 import java.util.function.Predicate;
 
 import javax.validation.constraints.NotEmpty;
 
+import org.springframework.util.StringUtils;
 import org.springframework.validation.annotation.Validated;
 import org.springframework.web.server.ServerWebExchange;
 
@@ -45,16 +47,21 @@ public class HeaderRoutePredicateFactory extends AbstractRoutePredicateFactory<H
 
 	@Override
 	public Predicate<ServerWebExchange> apply(Config config) {
+		boolean hasRegex = !StringUtils.isEmpty(config.regexp);
+
 		return exchange -> {
-			List<String> values = exchange.getRequest().getHeaders().get(config.header);
-			if (values != null) {
-				for (String value : values) {
-					if (value.matches(config.regexp)) {
-						return true;
-					}
-				}
+			List<String> values = exchange.getRequest().getHeaders().getOrDefault(config.header, Collections.emptyList());
+			if (values.isEmpty()) {
+				return false;
 			}
-			return false;
+			// values is now guaranteed to not be empty
+			if (hasRegex) {
+				// check if a header value matches
+				return values.stream().anyMatch(value -> value.matches(config.regexp));
+			}
+
+			// there is a value and since regexp is empty, we only check existence.
+			return true;
 		};
 	}
 

spring-cloud-gateway-core/src/test/java/org/springframework/cloud/gateway/handler/predicate/HeaderRoutePredicateFactoryTests.java

@@ -19,11 +19,15 @@ package org.springframework.cloud.gateway.handler.predicate;
 
 import org.junit.Test;
 import org.junit.runner.RunWith;
+import org.springframework.beans.factory.annotation.Value;
 import org.springframework.boot.SpringBootConfiguration;
 import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
 import org.springframework.boot.test.context.SpringBootTest;
 import org.springframework.cloud.gateway.handler.RoutePredicateHandlerMapping;
+import org.springframework.cloud.gateway.route.RouteLocator;
+import org.springframework.cloud.gateway.route.builder.RouteLocatorBuilder;
 import org.springframework.cloud.gateway.test.BaseWebClientTests;
+import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Import;
 import org.springframework.test.annotation.DirtiesContext;
 import org.springframework.test.context.junit4.SpringRunner;
@@ -60,9 +64,35 @@ public class HeaderRoutePredicateFactoryTests extends BaseWebClientTests {
 				.expectHeader().valueEquals(ROUTE_ID_HEADER, "default_path_to_httpbin");
 	}
 
+	@Test
+	public void headerExistsWorksWithDsl() {
+		testClient.get()
+				.uri("/get")
+				.header("X-Foo", "bar")
+				.exchange()
+				.expectStatus().isOk()
+				.expectHeader().valueEquals(HANDLER_MAPPER_HEADER,
+				RoutePredicateHandlerMapping.class.getSimpleName())
+				.expectHeader().valueEquals(ROUTE_ID_HEADER, "header_exists_dsl");
+	}
+
 	@EnableAutoConfiguration
 	@SpringBootConfiguration
 	@Import(DefaultTestConfig.class)
-	public static class TestConfig { }
+	public static class TestConfig {
+
+		@Value("${test.uri}")
+		private String uri;
+
+		@Bean
+		RouteLocator queryRouteLocator(RouteLocatorBuilder builder) {
+			return builder.routes()
+					.route("header_exists_dsl", r ->
+							r.header("X-Foo")
+							.filters(f -> f.prefixPath("/httpbin"))
+							.uri(uri))
+					.build();
+		}
+	}
 
 }