GH-3067: Draft of mapping multiple headers with same key with SimpleK…

…afkaHeaderMapper

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2018-2022 the original author or authors.
+ * Copyright 2018-2024 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.
@@ -17,12 +17,16 @@
 package org.springframework.kafka.support;
 
 import java.nio.ByteBuffer;
+import java.util.Collection;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Set;
+import java.util.stream.Collectors;
 
+import org.apache.kafka.common.header.Header;
 import org.apache.kafka.common.header.Headers;
 import org.apache.kafka.common.header.internals.RecordHeader;
+import org.assertj.core.util.Streams;
 
 import org.springframework.messaging.MessageHeaders;
 
@@ -36,7 +40,6 @@
  *
  * @author Gary Russell
  * @since 2.1.3
- *
  */
 public class SimpleKafkaHeaderMapper extends AbstractKafkaHeaderMapper {
 
@@ -69,6 +72,7 @@ public SimpleKafkaHeaderMapper() {
 	 * generally should not map the {@code "id" and "timestamp"} headers. Note:
 	 * most of the headers in {@link KafkaHeaders} are never mapped as headers since they
 	 * represent data in consumer/producer records.
+	 *
 	 * @param patterns the patterns.
 	 * @see org.springframework.util.PatternMatchUtils#simpleMatch(String, String)
 	 */
@@ -82,6 +86,7 @@ private SimpleKafkaHeaderMapper(boolean outbound, String... patterns) {
 
 	/**
 	 * Create an instance for inbound mapping only with pattern matching.
+	 *
 	 * @param patterns the patterns to match.
 	 * @return the header mapper.
 	 * @since 2.8.8
@@ -94,27 +99,40 @@ public static SimpleKafkaHeaderMapper forInboundOnlyWithMatchers(String... patte
 	public void fromHeaders(MessageHeaders headers, Headers target) {
 		headers.forEach((key, value) -> {
 			if (!NEVER.contains(key)) {
-				Object valueToAdd = headerValueToAddOut(key, value);
-				if (valueToAdd instanceof byte[] && matches(key, valueToAdd)) {
-					target.add(new RecordHeader(key, (byte[]) valueToAdd));
+				if (value instanceof Collection<?> values) {
+					values.forEach(v -> mapIfMatched(target, key, v));
+				} else {
+					mapIfMatched(target, key, value);
 				}
 			}
 		});
 	}
 
+	private void mapIfMatched(Headers target, String key, Object value) {
+		Object valueToAdd = headerValueToAddOut(key, value);
+		if (valueToAdd instanceof byte[] && matches(key, valueToAdd)) {
+			target.add(new RecordHeader(key, (byte[]) valueToAdd));
+		}
+	}
+
 	@Override
 	public void toHeaders(Headers source, Map<String, Object> target) {
-		source.forEach(header -> {
-			String headerName = header.key();
-			if (matchesForInbound(headerName)) {
-				if (headerName.equals(KafkaHeaders.DELIVERY_ATTEMPT)) {
-					target.put(headerName, ByteBuffer.wrap(header.value()).getInt());
-				}
-				else {
-					target.put(headerName, headerValueToAddIn(header));
-				}
-			}
-		});
+		Streams.stream(source)
+				.collect(Collectors.groupingBy(Header::key))
+				.forEach((headerName, headers) -> {
+					if (matchesForInbound(headerName)) {
+						if (headerName.equals(KafkaHeaders.DELIVERY_ATTEMPT)) {
+							target.put(headerName, ByteBuffer.wrap(headers.get(headers.size() - 1).value()).getInt());
+						} else {
+							var values = headers.stream().map(super::headerValueToAddIn).toList();
+							if (values.size() == 1) {
+								target.put(headerName, values.get(0));
+							} else {
+								target.put(headerName, values);
+							}
+						}
+					}
+				});
 	}
 
 }

spring-kafka/src/test/java/org/springframework/kafka/support/SimpleKafkaHeaderMapperTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2019-2022 the original author or authors.
+ * Copyright 2019-2024 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.
@@ -22,6 +22,7 @@
 
 import java.nio.ByteBuffer;
 import java.util.HashMap;
+import java.util.List;
 import java.util.Map;
 
 import org.apache.kafka.common.header.Header;
@@ -65,6 +66,26 @@ public void testSpecificStringConvert() {
 				entry("neverConverted", "baz".getBytes()));
 	}
 
+	@Test
+	void testIterableHeaderConvert() {
+		SimpleKafkaHeaderMapper mapper = new SimpleKafkaHeaderMapper();
+		Map<String, Boolean> rawMappedHeaders = new HashMap<>();
+		rawMappedHeaders.put("stringHeader", true);
+		mapper.setRawMappedHeaders(rawMappedHeaders);
+		Map<String, Object> headersMap = new HashMap<>();
+		headersMap.put("stringHeader", List.of("firstValue", "secondValue"));
+		MessageHeaders headers = new MessageHeaders(headersMap);
+		Headers target = new RecordHeaders();
+		mapper.fromHeaders(headers, target);
+		assertThat(target).containsExactlyInAnyOrder(
+				new RecordHeader("stringHeader", "firstValue".getBytes()),
+				new RecordHeader("stringHeader", "secondValue".getBytes())
+		);
+		headersMap.clear();
+		mapper.toHeaders(target, headersMap);
+		assertThat(headersMap).contains(entry("stringHeader", List.of("firstValue", "secondValue")));
+	}
+
 	@Test
 	public void testNotStringConvert() {
 		SimpleKafkaHeaderMapper mapper = new SimpleKafkaHeaderMapper();