Introduce RouterFunction attributes

This commit introduces support for router function attributes, a
way to associate meta-data with a route.

Closes: gh-25938

spring-webmvc/src/test/java/org/springframework/web/servlet/function/AttributesTestVisitor.java

@@ -0,0 +1,72 @@
+/*
+ * Copyright 2002-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.web.servlet.function;
+
+import java.util.Map;
+import java.util.Optional;
+import java.util.function.Function;
+
+import org.springframework.core.io.Resource;
+import org.springframework.lang.Nullable;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.entry;
+
+/**
+ * @author Arjen Poutsma
+ */
+class AttributesTestVisitor implements RouterFunctions.Visitor {
+
+	@Nullable
+	private Map<String, Object> attributes;
+
+	private int visitCount;
+
+	public int visitCount() {
+		return this.visitCount;
+	}
+
+	@Override
+	public void startNested(RequestPredicate predicate) {
+	}
+
+	@Override
+	public void endNested(RequestPredicate predicate) {
+	}
+
+	@Override
+	public void route(RequestPredicate predicate, HandlerFunction<?> handlerFunction) {
+		assertThat(this.attributes).isNotNull();
+		this.attributes = null;
+	}
+
+	@Override
+	public void resources(Function<ServerRequest, Optional<Resource>> lookupFunction) {
+	}
+
+	@Override
+	public void attributes(Map<String, Object> attributes) {
+		assertThat(attributes).containsExactly(entry("foo", "bar"), entry("baz", "qux"));
+		this.attributes = attributes;
+		this.visitCount++;
+	}
+
+	@Override
+	public void unknown(RouterFunction<?> routerFunction) {
+
+	}
+}

spring-webmvc/src/test/java/org/springframework/web/servlet/function/RouterFunctionBuilderTests.java

@@ -214,4 +214,23 @@ class RouterFunctionBuilderTests {
 				PathPatternsTestUtils.initRequest(httpMethod, null, requestUri, true, consumer), emptyList());
 	}
 
+	@Test
+	public void attributes() {
+		RouterFunction<ServerResponse> route = RouterFunctions.route()
+				.GET("/atts/1", request -> ServerResponse.ok().build())
+				.withAttribute("foo", "bar")
+				.withAttribute("baz", "qux")
+				.GET("/atts/2", request -> ServerResponse.ok().build())
+				.withAttributes(atts -> {
+					atts.put("foo", "bar");
+					atts.put("baz", "qux");
+				})
+				.build();
+
+		AttributesTestVisitor visitor = new AttributesTestVisitor();
+		route.accept(visitor);
+		assertThat(visitor.visitCount()).isEqualTo(2);
+	}
+
+
 }

spring-webmvc/src/test/java/org/springframework/web/servlet/function/RouterFunctionTests.java

@@ -24,6 +24,7 @@ import org.junit.jupiter.api.Test;
 import org.springframework.web.servlet.handler.PathPatternsTestUtils;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.springframework.web.servlet.function.RequestPredicates.GET;
 
 /**
  * @author Arjen Poutsma
@@ -109,7 +110,29 @@ class RouterFunctionTests {
 	}
 
 
+	@Test
+	public void attributes() {
+		RouterFunction<ServerResponse> route = RouterFunctions.route(
+				GET("/atts/1"), request -> ServerResponse.ok().build())
+				.withAttribute("foo", "bar")
+				.withAttribute("baz", "qux")
+				.and(RouterFunctions.route(GET("/atts/2"), request -> ServerResponse.ok().build())
+				.withAttributes(atts -> {
+					atts.put("foo", "bar");
+					atts.put("baz", "qux");
+				}));
+
+		AttributesTestVisitor visitor = new AttributesTestVisitor();
+		route.accept(visitor);
+		assertThat(visitor.visitCount()).isEqualTo(2);
+	}
+
+
+
 	private ServerResponse handlerMethod(ServerRequest request) {
 		return ServerResponse.ok().body("42");
 	}
+
+
+
 }