diff --git a/loader/spring-boot-loader/src/main/java/org/springframework/boot/loader/net/protocol/jar/JarFileUrlKey.java b/loader/spring-boot-loader/src/main/java/org/springframework/boot/loader/net/protocol/jar/JarFileUrlKey.java
index cae978d133c8..45932d35f91e 100644
--- a/loader/spring-boot-loader/src/main/java/org/springframework/boot/loader/net/protocol/jar/JarFileUrlKey.java
+++ b/loader/spring-boot-loader/src/main/java/org/springframework/boot/loader/net/protocol/jar/JarFileUrlKey.java
@@ -44,6 +44,21 @@ final class JarFileUrlKey {
 		this.runtimeRef = "runtime".equals(url.getRef());
 	}
 
+	/**
+	 * Return a string representation of this key suitable for use as a
+	 * {@code HashMap<String, ...>} key. Unlike {@code URL.toString()}, this
+	 * method does not trigger DNS lookups.
+	 * @return a unique string key for this URL
+	 */
+	String toKeyString() {
+		StringBuilder sb = new StringBuilder();
+		sb.append(this.protocol).append("://").append(this.host).append(':').append(this.port).append(this.file);
+		if (this.runtimeRef) {
+			sb.append("#runtime");
+		}
+		return sb.toString();
+	}
+
 	@Override
 	public boolean equals(Object obj) {
 		if (this == obj) {
@@ -68,4 +83,4 @@ private boolean equalsIgnoringCase(String s1, String s2) {
 		return (s1 == s2) || (s1 != null && s1.equalsIgnoreCase(s2));
 	}
 
-}
+}
\ No newline at end of file
diff --git a/loader/spring-boot-loader/src/main/java/org/springframework/boot/loader/net/protocol/jar/UrlJarFiles.java b/loader/spring-boot-loader/src/main/java/org/springframework/boot/loader/net/protocol/jar/UrlJarFiles.java
index e2dbb3a29e95..591220d347fc 100644
--- a/loader/spring-boot-loader/src/main/java/org/springframework/boot/loader/net/protocol/jar/UrlJarFiles.java
+++ b/loader/spring-boot-loader/src/main/java/org/springframework/boot/loader/net/protocol/jar/UrlJarFiles.java
@@ -140,11 +140,13 @@ void clearCache() {
 	}
 
 	/**
-	 * Internal cache.
+	 * Internal cache. Uses {@code String} keys in the underlying map to avoid
+	 * polluting the JVM's shared {@code HashMap.getNode} type profile with a
+	 * custom key type. See gh-51141.
 	 */
 	private static final class Cache {
 
-		private final Map<JarFileUrlKey, JarFile> jarFileUrlToJarFile = new HashMap<>();
+		private final Map<String, JarFile> jarFileUrlToJarFile = new HashMap<>();
 
 		private final Map<JarFile, URL> jarFileToJarFileUrl = new HashMap<>();
 
@@ -154,9 +156,8 @@ private static final class Cache {
 		 * @return the cached {@link JarFile} or {@code null}
 		 */
 		JarFile get(URL jarFileUrl) {
-			JarFileUrlKey urlKey = new JarFileUrlKey(jarFileUrl);
 			synchronized (this) {
-				return this.jarFileUrlToJarFile.get(urlKey);
+				return this.jarFileUrlToJarFile.get(new JarFileUrlKey(jarFileUrl).toKeyString());
 			}
 		}
 
@@ -180,11 +181,11 @@ URL get(JarFile jarFile) {
 		 * they were already there
 		 */
 		boolean putIfAbsent(URL jarFileUrl, JarFile jarFile) {
-			JarFileUrlKey urlKey = new JarFileUrlKey(jarFileUrl);
 			synchronized (this) {
-				JarFile cached = this.jarFileUrlToJarFile.get(urlKey);
+				String key = new JarFileUrlKey(jarFileUrl).toKeyString();
+				JarFile cached = this.jarFileUrlToJarFile.get(key);
 				if (cached == null) {
-					this.jarFileUrlToJarFile.put(urlKey, jarFile);
+					this.jarFileUrlToJarFile.put(key, jarFile);
 					this.jarFileToJarFileUrl.put(jarFile, jarFileUrl);
 					return true;
 				}
@@ -200,7 +201,7 @@ void remove(JarFile jarFile) {
 			synchronized (this) {
 				URL removedUrl = this.jarFileToJarFileUrl.remove(jarFile);
 				if (removedUrl != null) {
-					this.jarFileUrlToJarFile.remove(new JarFileUrlKey(removedUrl));
+					this.jarFileUrlToJarFile.remove(new JarFileUrlKey(removedUrl).toKeyString());
 				}
 			}
 		}
@@ -214,4 +215,4 @@ void clear() {
 
 	}
 
-}
+}
\ No newline at end of file