Add RequestPredicate Visitor to WebFlux.fn

This commit introduces RequestPredicates.Visitor, an implementation of
the visitor pattern for RequestPredicates.

Issue: SPR-17322

spring-webflux/src/test/java/org/springframework/web/reactive/function/server/ToStringVisitorTests.java

@@ -0,0 +1,107 @@
+/*
+ * Copyright 2002-2018 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
+ *
+ *      http://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.reactive.function.server;
+
+import org.junit.Test;
+import reactor.core.publisher.Mono;
+
+import org.springframework.http.HttpMethod;
+import org.springframework.http.MediaType;
+
+import static org.junit.Assert.*;
+import static org.springframework.web.reactive.function.server.RequestPredicates.accept;
+import static org.springframework.web.reactive.function.server.RequestPredicates.contentType;
+import static org.springframework.web.reactive.function.server.RequestPredicates.method;
+import static org.springframework.web.reactive.function.server.RequestPredicates.methods;
+import static org.springframework.web.reactive.function.server.RequestPredicates.path;
+import static org.springframework.web.reactive.function.server.RequestPredicates.pathExtension;
+import static org.springframework.web.reactive.function.server.RequestPredicates.queryParam;
+import static org.springframework.web.reactive.function.server.RouterFunctions.route;
+
+/**
+ * @author Arjen Poutsma
+ */
+public class ToStringVisitorTests {
+
+	@Test
+	public void nested() {
+		HandlerFunction<ServerResponse> handler = new SimpleHandlerFunction();
+		RouterFunction<ServerResponse> routerFunction = route()
+				.path("/foo", builder -> {
+					builder.path("/bar", () -> route()
+							.GET("/baz", handler)
+							.build());
+				})
+				.build();
+
+		ToStringVisitor visitor = new ToStringVisitor();
+		routerFunction.accept(visitor);
+		String result =  visitor.toString();
+
+		String expected = "/foo => {\n" +
+				" /bar => {\n" +
+				"  (GET && /baz) -> \n" +
+				" }\n" +
+				"}";
+		assertEquals(expected, result);
+	}
+
+	@Test
+	public void predicates() {
+		testPredicate(methods(HttpMethod.GET), "GET");
+		testPredicate(methods(HttpMethod.GET, HttpMethod.POST), "[GET, POST]");
+
+		testPredicate(path("/foo"), "/foo");
+
+		testPredicate(pathExtension("foo"), "*.foo");
+
+		testPredicate(contentType(MediaType.APPLICATION_JSON), "Content-Type: application/json");
+		testPredicate(contentType(MediaType.APPLICATION_JSON, MediaType.TEXT_PLAIN), "Content-Type: [application/json, text/plain]");
+
+		testPredicate(accept(MediaType.APPLICATION_JSON), "Accept: application/json");
+
+		testPredicate(queryParam("foo", "bar"), "?foo == bar");
+
+		testPredicate(method(HttpMethod.GET).and(path("/foo")), "(GET && /foo)");
+
+		testPredicate(method(HttpMethod.GET).or(path("/foo")), "(GET || /foo)");
+
+		testPredicate(method(HttpMethod.GET).negate(), "!(GET)");
+	}
+
+	private void testPredicate(RequestPredicate predicate, String expected) {
+		ToStringVisitor visitor = new ToStringVisitor();
+		predicate.accept(visitor);
+		String result = visitor.toString();
+
+		assertEquals(expected, result);
+	}
+
+	private static class SimpleHandlerFunction implements HandlerFunction<ServerResponse> {
+
+		@Override
+		public Mono<ServerResponse> handle(ServerRequest request) {
+			return ServerResponse.ok().build();
+		}
+
+		@Override
+		public String toString() {
+			return "";
+		}
+	}
+
+}