Use putIfAbsent in Netty5HeadersAdapter

See gh-30343

spring-web/src/main/java/org/springframework/http/client/reactive/Netty5HeadersAdapter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2023 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.
@@ -91,11 +91,9 @@ class Netty5HeadersAdapter implements MultiValueMap<String, String> {
 	@Override
 	public Map<String, String> toSingleValueMap() {
 		Map<String, String> singleValueMap = CollectionUtils.newLinkedHashMap(this.headers.size());
-		this.headers.forEach(entry -> {
+		this.headers.forEach(entry ->
-			if (!singleValueMap.containsKey(entry.getKey())) {
+				singleValueMap.putIfAbsent(entry.getKey().toString(), entry.getValue().toString())
-				singleValueMap.put(entry.getKey().toString(), entry.getValue().toString());
+		);
-			}
-		});
 		return singleValueMap;
 	}
 

spring-web/src/main/java/org/springframework/http/server/reactive/Netty5HeadersAdapter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2023 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.
@@ -89,11 +89,9 @@ final class Netty5HeadersAdapter implements MultiValueMap<String, String> {
 	@Override
 	public Map<String, String> toSingleValueMap() {
 		Map<String, String> singleValueMap = CollectionUtils.newLinkedHashMap(this.headers.size());
-		this.headers.forEach(entry -> {
+		this.headers.forEach(entry ->
-					if (!singleValueMap.containsKey(entry.getKey())) {
+				singleValueMap.putIfAbsent(entry.getKey().toString(), entry.getValue().toString())
-						singleValueMap.put(entry.getKey().toString(), entry.getValue().toString());
+		);
-					}
-				});
 		return singleValueMap;
 	}