Add efficient existence check to ClassPathResource.isReadable()

Includes reduced isReadable() check in PathResourceLookupFunction, aligned with PathResourceResolver.

Closes gh-27538
See gh-21372

spring-core/src/main/java/org/springframework/core/io/AbstractFileResolvingResource.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2021 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.
@@ -88,7 +88,15 @@ public abstract class AbstractFileResolvingResource extends AbstractResource {
 	@Override
 	public boolean isReadable() {
 		try {
-			URL url = getURL();
+			return checkReadable(getURL());
+		}
+		catch (IOException ex) {
+			return false;
+		}
+	}
+
+	boolean checkReadable(URL url) {
+		try {
 			if (ResourceUtils.isFileURL(url)) {
 				// Proceed with file system resolution
 				File file = getFile();

spring-core/src/main/java/org/springframework/core/io/ClassPathResource.java

@@ -142,6 +142,18 @@ public class ClassPathResource extends AbstractFileResolvingResource {
 		return (resolveURL() != null);
 	}
 
+	/**
+	 * This implementation checks for the resolution of a resource URL upfront,
+	 * then proceeding with {@link AbstractFileResolvingResource}'s length check.
+	 * @see java.lang.ClassLoader#getResource(String)
+	 * @see java.lang.Class#getResource(String)
+	 */
+	@Override
+	public boolean isReadable() {
+		URL url = resolveURL();
+		return (url != null && checkReadable(url));
+	}
+
 	/**
 	 * Resolves a URL for the underlying class path resource.
 	 * @return the resolved URL, or {@code null} if not resolvable

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2021 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.
@@ -72,7 +72,7 @@ class PathResourceLookupFunction implements Function<ServerRequest, Mono<Resourc
 
 		try {
 			Resource resource = this.location.createRelative(path);
-			if (resource.exists() && resource.isReadable() && isResourceUnderLocation(resource)) {
+			if (resource.isReadable() && isResourceUnderLocation(resource)) {
 				return Mono.just(resource);
 			}
 			else {

spring-webmvc/src/main/java/org/springframework/web/servlet/function/PathResourceLookupFunction.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2021 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.
@@ -71,7 +71,7 @@ class PathResourceLookupFunction implements Function<ServerRequest, Optional<Res
 
 		try {
 			Resource resource = this.location.createRelative(path);
-			if (resource.exists() && resource.isReadable() && isResourceUnderLocation(resource)) {
+			if (resource.isReadable() && isResourceUnderLocation(resource)) {
 				return Optional.of(resource);
 			}
 			else {