Expose resource lookup function

This commit exposes the resource lookup function used by
`RouterFunctions.resources(String, Resource)`, so that it can be
composed upon.

Issue: SPR-16788

spring-webflux/src/main/java/org/springframework/web/reactive/function/server/RouterFunctions.java

@@ -123,14 +123,35 @@ public abstract class RouterFunctions {
 	 * For instance
 	 * <pre class="code">
 	 * Resource location = new FileSystemResource("public-resources/");
-	 * RoutingFunction&lt;ServerResponse&gt; resources = RouterFunctions.resources("/resources/**", location);
+	 * RouterFunction&lt;ServerResponse&gt; resources = RouterFunctions.resources("/resources/**", location);
      * </pre>
 	 * @param pattern the pattern to match
 	 * @param location the location directory relative to which resources should be resolved
 	 * @return a router function that routes to resources
+	 * @see #resourceLookupFunction(String, Resource)
 	 */
 	public static RouterFunction<ServerResponse> resources(String pattern, Resource location) {
-		return resources(new PathResourceLookupFunction(pattern, location));
+		return resources(resourceLookupFunction(pattern, location));
+	}
+
+	/**
+	 * Returns the resource lookup function used by {@link #resources(String, Resource)}.
+	 * The returned function can be {@linkplain Function#andThen(Function) composed} on, for
+	 * instance to return a default resource when the lookup function does not match:
+	 * <pre class="code">
+	 * Mono&lt;Resource&gt; defaultResource = Mono.just(new ClassPathResource("index.html"));
+	 * Function&lt;ServerRequest, Mono&lt;Resource&gt;&gt; lookupFunction =
+	 *   RouterFunctions.resourceLookupFunction("/resources/**", new FileSystemResource("public-resources/"))
+	 *     .andThen(resourceMono -&gt; resourceMono.switchIfEmpty(defaultResource));
+	 *
+	 * RouterFunction&lt;ServerResponse&gt; resources = RouterFunctions.resources(lookupFunction);
+     * </pre>
+	 * @param pattern the pattern to match
+	 * @param location the location directory relative to which resources should be resolved
+	 * @return the default resource lookup function for the given parameters.
+	 */
+	public static Function<ServerRequest, Mono<Resource>> resourceLookupFunction(String pattern, Resource location) {
+		return new PathResourceLookupFunction(pattern, location);
 	}
 
 	/**

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 the original author or authors.
+ * 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.
@@ -19,6 +19,7 @@ package org.springframework.web.reactive.function.server;
 import java.io.File;
 import java.io.IOException;
 import java.net.URI;
+import java.util.function.Function;
 
 import org.junit.Test;
 import reactor.core.publisher.Mono;
@@ -95,4 +96,37 @@ public class PathResourceLookupFunctionTests {
 				.verify();
 	}
 
+	@Test
+	public void composeResourceLookupFunction() throws Exception {
+
+		Function<ServerRequest, Mono<Resource>> lookupFunction =
+				new PathResourceLookupFunction("/resources/**",
+						new ClassPathResource("org/springframework/web/reactive/function/server/"));
+
+		ClassPathResource defaultResource = new ClassPathResource("response.txt", getClass());
+
+		Function<ServerRequest, Mono<Resource>> customLookupFunction =
+				lookupFunction.andThen(resourceMono -> resourceMono
+								.switchIfEmpty(Mono.just(defaultResource)));
+
+		MockServerRequest request = MockServerRequest.builder()
+				.uri(new URI("http://localhost/resources/foo"))
+				.build();
+
+		Mono<Resource> result = customLookupFunction.apply(request);
+		StepVerifier.create(result)
+				.expectNextMatches(resource -> {
+					try {
+						return defaultResource.getFile().equals(resource.getFile());
+					}
+					catch (IOException ex) {
+						return false;
+					}
+				})
+				.expectComplete()
+				.verify();
+
+	}
+
+
 }