diff --git a/spring-pulsar-docs/src/main/antora/modules/ROOT/pages/reference/pulsar/transactions.adoc b/spring-pulsar-docs/src/main/antora/modules/ROOT/pages/reference/pulsar/transactions.adoc
index d3baa783a..c15ade7ec 100644
--- a/spring-pulsar-docs/src/main/antora/modules/ROOT/pages/reference/pulsar/transactions.adoc
+++ b/spring-pulsar-docs/src/main/antora/modules/ROOT/pages/reference/pulsar/transactions.adoc
@@ -93,7 +93,7 @@ Given the following listener method:
 ----
 @PulsarListener(topics = "my-input-topic") // <1>
 void listen(String msg) { // <2>
-    var transformedMsg = msg.toUpperCase(); // <3>
+    var transformedMsg = msg.toUpperCase(Locale.ROOT); // <3>
     this.transactionalTemplate.send("my-output-topic", transformedMsg); // <4>
 } // <5> <6>
 ----
@@ -215,7 +215,7 @@ The DB transaction is committed first; if the Pulsar transaction fails to commit
 @PulsarListener(topics = "my-input-topic")
 @Transactional("dataSourceTransactionManager")
 void listen(String msg) {
-    var transformedMsg = msg.toUpperCase();
+    var transformedMsg = msg.toUpperCase(Locale.ROOT);
     this.pulsarTemplate.send("my-output-topic", transformedMsg);
     this.jdbcTemplate.execute("insert into my_table (data) values ('%s')".formatted(transformedMsg));
 }
diff --git a/spring-pulsar/src/main/java/org/springframework/pulsar/function/PulsarFunctionAdministration.java b/spring-pulsar/src/main/java/org/springframework/pulsar/function/PulsarFunctionAdministration.java
index de038d61d..9dac885da 100644
--- a/spring-pulsar/src/main/java/org/springframework/pulsar/function/PulsarFunctionAdministration.java
+++ b/spring-pulsar/src/main/java/org/springframework/pulsar/function/PulsarFunctionAdministration.java
@@ -20,6 +20,7 @@
 import java.util.Collections;
 import java.util.LinkedHashMap;
 import java.util.List;
+import java.util.Locale;
 import java.util.Map;
 import java.util.Optional;
 import java.util.stream.Stream;
@@ -314,7 +315,7 @@ private Optional<Exception> safeInvoke(Runnable invocation) {
 	}
 
 	private String functionDesc(PulsarFunctionOperations<?> function) {
-		return "'%s' %s".formatted(function.name(), function.type().toString().toLowerCase());
+		return "'%s' %s".formatted(function.name(), function.type().toString().toLowerCase(Locale.ROOT));
 	}
 
 	/**
diff --git a/spring-pulsar/src/main/java/org/springframework/pulsar/support/header/PulsarHeaderMatcher.java b/spring-pulsar/src/main/java/org/springframework/pulsar/support/header/PulsarHeaderMatcher.java
index 95ee67ba3..629acc1af 100644
--- a/spring-pulsar/src/main/java/org/springframework/pulsar/support/header/PulsarHeaderMatcher.java
+++ b/spring-pulsar/src/main/java/org/springframework/pulsar/support/header/PulsarHeaderMatcher.java
@@ -16,6 +16,7 @@
 
 package org.springframework.pulsar.support.header;
 
+import java.util.Locale;
 import java.util.Set;
 
 import org.springframework.core.log.LogAccessor;
@@ -92,13 +93,13 @@ public static PatternMatch fromPatternString(String pattern) {
 
 		public PatternMatch(String pattern, boolean negate) {
 			Assert.notNull(pattern, "Pattern must not be null");
-			this.pattern = pattern.toLowerCase();
+			this.pattern = pattern.toLowerCase(Locale.ROOT);
 			this.negate = negate;
 		}
 
 		@Override
 		public boolean matchHeader(String headerName) {
-			if (!PatternMatchUtils.simpleMatch(this.pattern, headerName.toLowerCase())) {
+			if (!PatternMatchUtils.simpleMatch(this.pattern, headerName.toLowerCase(Locale.ROOT))) {
 				return false;
 			}
 			LOGGER.debug(() -> "headerName=[%s] WILL %s be mapped, matched pattern=%s".formatted(headerName,
diff --git a/src/checkstyle/checkstyle-suppressions.xml b/src/checkstyle/checkstyle-suppressions.xml
index 83ed763d9..e695ac0fa 100644
--- a/src/checkstyle/checkstyle-suppressions.xml
+++ b/src/checkstyle/checkstyle-suppressions.xml
@@ -9,6 +9,8 @@
 	<suppress files="PulsarFunctionAdministrationIntegrationTests" checks="Regexp" />
 	<suppress files="Proto" checks=".*"/>
 	<suppress files=".*Tests" checks="HideUtilityClassConstructor" />
+	<suppress files=".*Tests" checks="RegexpSinglelineJava" id="toLowerCaseWithoutLocale"/>
+	<suppress files=".*Tests" checks="RegexpSinglelineJava" id="toUpperCaseWithoutLocale"/>
 	<suppress files="[\\/]spring-pulsar-docs[\\/]" checks="JavadocPackage|JavadocType|JavadocVariable|SpringDeprecatedCheck" />
 	<suppress files="[\\/]spring-pulsar-docs[\\/]" checks="SpringJavadoc" message="\@since" />
 	<suppress files="[\\/]spring-pulsar-docs[\\/].*jooq" checks="AvoidStaticImport" />
diff --git a/src/checkstyle/checkstyle.xml b/src/checkstyle/checkstyle.xml
index 06f99f55f..d10e07be1 100644
--- a/src/checkstyle/checkstyle.xml
+++ b/src/checkstyle/checkstyle.xml
@@ -169,6 +169,22 @@
 					  value="Please use AssertJ imports."/>
 			<property name="ignoreComments" value="true"/>
 		</module>
+		<module name="com.puppycrawl.tools.checkstyle.checks.regexp.RegexpSinglelineJavaCheck">
+			<property name="id" value="toLowerCaseWithoutLocale"/>
+			<property name="format" value="\.toLowerCase\(\)"/>
+			<property name="maximum" value="0"/>
+			<property name="message"
+					  value="String.toLowerCase() should be String.toLowerCase(Locale.ROOT)"/>
+			<property name="ignoreComments" value="true"/>
+		</module>
+		<module name="com.puppycrawl.tools.checkstyle.checks.regexp.RegexpSinglelineJavaCheck">
+			<property name="id" value="toUpperCaseWithoutLocale"/>
+			<property name="format" value="\.toUpperCase\(\)"/>
+			<property name="maximum" value="0"/>
+			<property name="message"
+					  value="String.toUpperCase() should be String.toUpperCase(Locale.ROOT)"/>
+			<property name="ignoreComments" value="true"/>
+		</module>
 		<module name="Regexp">
 			<property name="format" value="[ \t]+$"/>
 			<property name="illegalPattern" value="true"/>