diff --git a/spring-test/src/main/java/org/springframework/test/context/junit/jupiter/AbstractExpressionEvaluatingCondition.java b/spring-test/src/main/java/org/springframework/test/context/junit/jupiter/AbstractExpressionEvaluatingCondition.java
index 7ee63916923a..58ed0b9f4de0 100644
--- a/spring-test/src/main/java/org/springframework/test/context/junit/jupiter/AbstractExpressionEvaluatingCondition.java
+++ b/spring-test/src/main/java/org/springframework/test/context/junit/jupiter/AbstractExpressionEvaluatingCondition.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2023 the original author or authors.
+ * Copyright 2002-2024 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.
@@ -18,6 +18,7 @@
 
 import java.lang.annotation.Annotation;
 import java.lang.reflect.AnnotatedElement;
+import java.util.Locale;
 import java.util.Optional;
 import java.util.function.Function;
 
@@ -192,7 +193,7 @@ private <A extends Annotation> boolean evaluateExpression(String expression, boo
 			return b;
 		}
 		else if (result instanceof String str) {
-			str = str.trim().toLowerCase();
+			str = str.trim().toLowerCase(Locale.ROOT);
 			if ("true".equals(str)) {
 				return true;
 			}
diff --git a/spring-websocket/src/main/java/org/springframework/web/socket/client/AbstractWebSocketClient.java b/spring-websocket/src/main/java/org/springframework/web/socket/client/AbstractWebSocketClient.java
index 5a15fd0eb941..8a5a3905fab3 100644
--- a/spring-websocket/src/main/java/org/springframework/web/socket/client/AbstractWebSocketClient.java
+++ b/spring-websocket/src/main/java/org/springframework/web/socket/client/AbstractWebSocketClient.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2024 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 @@
 import java.net.URI;
 import java.util.Collections;
 import java.util.List;
+import java.util.Locale;
 import java.util.Map;
 import java.util.Set;
 import java.util.concurrent.CompletableFuture;
@@ -81,7 +82,7 @@ public final CompletableFuture<WebSocketSession> execute(WebSocketHandler webSoc
 		HttpHeaders headersToUse = new HttpHeaders();
 		if (headers != null) {
 			headers.forEach((header, values) -> {
-				if (values != null && !specialHeaders.contains(header.toLowerCase())) {
+				if (values != null && !specialHeaders.contains(header.toLowerCase(Locale.ROOT))) {
 					headersToUse.put(header, values);
 				}
 			});
diff --git a/spring-websocket/src/main/java/org/springframework/web/socket/server/support/AbstractHandshakeHandler.java b/spring-websocket/src/main/java/org/springframework/web/socket/server/support/AbstractHandshakeHandler.java
index 9fcd10e40b0a..fe43f674eef0 100644
--- a/spring-websocket/src/main/java/org/springframework/web/socket/server/support/AbstractHandshakeHandler.java
+++ b/spring-websocket/src/main/java/org/springframework/web/socket/server/support/AbstractHandshakeHandler.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2023 the original author or authors.
+ * Copyright 2002-2024 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.
@@ -23,6 +23,7 @@
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
+import java.util.Locale;
 import java.util.Map;
 
 import org.apache.commons.logging.Log;
@@ -154,7 +155,7 @@ public RequestUpgradeStrategy getRequestUpgradeStrategy() {
 	public void setSupportedProtocols(String... protocols) {
 		this.supportedProtocols.clear();
 		for (String protocol : protocols) {
-			this.supportedProtocols.add(protocol.toLowerCase());
+			this.supportedProtocols.add(protocol.toLowerCase(Locale.ROOT));
 		}
 	}
 
@@ -329,10 +330,10 @@ protected boolean isValidOrigin(ServerHttpRequest request) {
 	protected String selectProtocol(List<String> requestedProtocols, WebSocketHandler webSocketHandler) {
 		List<String> handlerProtocols = determineHandlerSupportedProtocols(webSocketHandler);
 		for (String protocol : requestedProtocols) {
-			if (handlerProtocols.contains(protocol.toLowerCase())) {
+			if (handlerProtocols.contains(protocol.toLowerCase(Locale.ROOT))) {
 				return protocol;
 			}
-			if (this.supportedProtocols.contains(protocol.toLowerCase())) {
+			if (this.supportedProtocols.contains(protocol.toLowerCase(Locale.ROOT))) {
 				return protocol;
 			}
 		}
diff --git a/spring-websocket/src/main/java/org/springframework/web/socket/sockjs/frame/AbstractSockJsMessageCodec.java b/spring-websocket/src/main/java/org/springframework/web/socket/sockjs/frame/AbstractSockJsMessageCodec.java
index 044a4a7bce84..05438ff71866 100644
--- a/spring-websocket/src/main/java/org/springframework/web/socket/sockjs/frame/AbstractSockJsMessageCodec.java
+++ b/spring-websocket/src/main/java/org/springframework/web/socket/sockjs/frame/AbstractSockJsMessageCodec.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2023 the original author or authors.
+ * Copyright 2002-2024 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.
@@ -16,6 +16,8 @@
 
 package org.springframework.web.socket.sockjs.frame;
 
+import java.util.Locale;
+
 import org.springframework.util.Assert;
 
 /**
@@ -58,7 +60,7 @@ private String escapeSockJsSpecialChars(char[] characters) {
 		for (char c : characters) {
 			if (isSockJsSpecialChar(c)) {
 				result.append('\\').append('u');
-				String hex = Integer.toHexString(c).toLowerCase();
+				String hex = Integer.toHexString(c).toLowerCase(Locale.ROOT);
 				result.append("0".repeat(Math.max(0, (4 - hex.length()))));
 				result.append(hex);
 			}