Only cache JarFile URL keys that are cheap to lookup

Update `JarFileUrlKey` so that only URLs that have a cheap `equals()`
method call are cached. This should prevent expensive DNS lookups from
being performed.

Fixes gh-46015

spring-boot-project/spring-boot-tools/spring-boot-loader/src/main/java/org/springframework/boot/loader/net/protocol/jar/JarFileUrlKey.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2024 the original author or authors.
+ * Copyright 2012-2025 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.
@@ -41,6 +41,9 @@ final class JarFileUrlKey {
 	 * @return a {@link JarFileUrlKey} instance
 	 */
 	static String get(URL url) {
+		if (!isCachableUrl(url)) {
+			return create(url);
+		}
 		Map<URL, String> cache = (JarFileUrlKey.cache != null) ? JarFileUrlKey.cache.get() : null;
 		if (cache == null) {
 			cache = new ConcurrentHashMap<>();
@@ -49,6 +52,11 @@ final class JarFileUrlKey {
 		return cache.computeIfAbsent(url, JarFileUrlKey::create);
 	}
 
+	private static boolean isCachableUrl(URL url) {
+		// Don't cache URL that have a host since equals() will perform DNS lookup
+		return url.getHost() == null || url.getHost().isEmpty();
+	}
+
 	private static String create(URL url) {
 		StringBuilder value = new StringBuilder();
 		String protocol = url.getProtocol();