Addressing PR review

Signed-off-by: Sanghyeok An <ojt90902@naver.com>

Author: chickenchickenlove

spring-kafka/src/main/java/org/springframework/kafka/support/AbstractKafkaHeaderMapper.java

@@ -196,6 +196,7 @@ public void addRawMappedHeader(String name, boolean toString) {
 	/**
 	 * Add patterns for matching multi-value headers under the same key.
 	 * @param patterns the patterns for header.
+	 * @since 4.0
 	 */
 	public void setMultiValueHeaderPatterns(String ... patterns) {
 		this.multiValueHeaderMatchers.addAll(Arrays
@@ -268,6 +269,7 @@ protected Object headerValueToAddOut(String key, Object value) {
 	 * Check whether the header value should be mapped to multiple values.
 	 * @param headerName the header name.
 	 * @return True for multiple values at the same key.
+	 * @since 4.0
 	 */
 	protected boolean doesMatchMultiValueHeader(String headerName) {
 		for (HeaderMatcher headerMatcher : this.multiValueHeaderMatchers) {
@@ -283,7 +285,7 @@ protected boolean doesMatchMultiValueHeader(String headerName) {
 	 * @param headerName the header name.
 	 * @param header the header instance.
 	 * @param headers the target headers.
-	 * @since 4.0.0
+	 * @since 4.0
 	 */
 	protected void fromUserHeader(String headerName, Header header, final Map<String, Object> headers) {
 		if (!doesMatchMultiValueHeader(headerName)) {
@@ -294,7 +296,7 @@ protected void fromUserHeader(String headerName, Header header, final Map<String
 			List<Object> headerValues = (List<Object>)
 					headers.computeIfAbsent(headerName, key -> new ArrayList<>());
 			headerValues.add(headerValueToAddIn(header));
-			headers.put(headerName, headerValues);
+//			headers.put(headerName, headerValues);
 		}
 	}
 

spring-kafka/src/main/java/org/springframework/kafka/support/DefaultKafkaHeaderMapper.java

@@ -268,14 +268,12 @@ public void fromHeaders(MessageHeaders headers, Headers target) {
 		headers.forEach((key, rawValue) -> {
 			if (matches(key, rawValue)) {
 				if (doesMatchMultiValueHeader(key)) {
-					Iterable<?> valuesToMap;
-					if (rawValue instanceof Iterable<?> iterable) {
-						valuesToMap = iterable;
+					if (rawValue instanceof Iterable<?> valuesToMap) {
+						valuesToMap.forEach(o -> fromHeader(key, o, jsonHeaders, headerObjectMapper, target));
 					}
 					else {
-						valuesToMap = List.of(rawValue);
+						fromHeader(key, rawValue, jsonHeaders, headerObjectMapper, target);
 					}
-					valuesToMap.forEach(o -> fromHeader(key, o, jsonHeaders, headerObjectMapper, target));
 				}
 				else {
 					fromHeader(key, rawValue, jsonHeaders, headerObjectMapper, target);
@@ -341,7 +339,7 @@ private void fromHeader(String key, Object rawValue, Map<String, String> jsonHea
 				else {
 					target.add(new RecordHeader(key, headerObjectMapper.writeValueAsBytes(valueToAdd)));
 				}
-				jsonHeaders.put(key, className);
+				jsonHeaders.putIfAbsent(key, className);
 			}
 			catch (Exception e) {
 				logger.error(e, () -> "Could not map " + key + " with type " + rawValue.getClass().getName());

spring-kafka/src/main/java/org/springframework/kafka/support/SimpleKafkaHeaderMapper.java

@@ -97,14 +97,12 @@ public void fromHeaders(MessageHeaders headers, Headers target) {
 		headers.forEach((key, value) -> {
 			if (!NEVER.contains(key)) {
 				if (doesMatchMultiValueHeader(key)) {
-					Iterable<?> valuesToMap;
-					if (value instanceof Iterable<?> iterable) {
-						valuesToMap = iterable;
+					if (value instanceof Iterable<?> valuesToMap) {
+						valuesToMap.forEach(o -> fromHeader(key, o, target));
 					}
 					else {
-						valuesToMap = List.of(value);
+						fromHeader(key, value, target);
 					}
-					valuesToMap.forEach(o -> fromHeader(key, o, target));
 				}
 				else {
 					fromHeader(key, value, target);