diff --git a/spring-boot-project/spring-boot/src/main/java/org/springframework/boot/logging/CorrelationIdFormatter.java b/spring-boot-project/spring-boot/src/main/java/org/springframework/boot/logging/CorrelationIdFormatter.java
index 1388aecca0b..e701fba05cf 100644
--- a/spring-boot-project/spring-boot/src/main/java/org/springframework/boot/logging/CorrelationIdFormatter.java
+++ b/spring-boot-project/spring-boot/src/main/java/org/springframework/boot/logging/CorrelationIdFormatter.java
@@ -22,8 +22,8 @@ import java.util.Collection;
 import java.util.Collections;
 import java.util.Iterator;
 import java.util.List;
-import java.util.function.Function;
 import java.util.function.Predicate;
+import java.util.function.UnaryOperator;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
 import java.util.stream.Collectors;
@@ -84,7 +84,7 @@ public final class CorrelationIdFormatter {
 	 * @param resolver the resolver used to resolve named values
 	 * @return a formatted correlation id
 	 */
-	public String format(Function<String, String> resolver) {
+	public String format(UnaryOperator<String> resolver) {
 		StringBuilder result = new StringBuilder();
 		formatTo(resolver, result);
 		return result.toString();
@@ -96,7 +96,7 @@ public final class CorrelationIdFormatter {
 	 * @param resolver the resolver used to resolve named values
 	 * @param appendable the appendable for the formatted correlation id
 	 */
-	public void formatTo(Function<String, String> resolver, Appendable appendable) {
+	public void formatTo(UnaryOperator<String> resolver, Appendable appendable) {
 		Predicate<Part> canResolve = (part) -> StringUtils.hasLength(resolver.apply(part.name()));
 		try {
 			if (this.parts.stream().anyMatch(canResolve)) {
@@ -169,7 +169,7 @@ public final class CorrelationIdFormatter {
 
 		private static final Pattern pattern = Pattern.compile("^(.+?)\\((\\d+)\\)$");
 
-		String resolve(Function<String, String> resolver) {
+		String resolve(UnaryOperator<String> resolver) {
 			String resolved = resolver.apply(name());
 			if (resolved == null) {
 				return blank();