Ensure correct ordering of delegates regardless of user input

spring-kafka/src/main/java/org/springframework/kafka/support/serializer/DelegatingByTypeSerializer.java

@@ -16,8 +16,7 @@
 
 package org.springframework.kafka.support.serializer;
 
-import java.util.LinkedHashMap;
-import java.util.Map;
+import java.util.*;
 import java.util.Map.Entry;
 
 import org.apache.kafka.common.errors.SerializationException;
@@ -32,13 +31,27 @@
  * @author Gary Russell
  * @author Artem Bilan
  * @author Wang Zhiyang
+ * @author Mahesh Aravind V
  *
  * @since 2.7.9
  *
  */
 public class DelegatingByTypeSerializer implements Serializer<Object> {
 
-	private final Map<Class<?>, Serializer<?>> delegates = new LinkedHashMap<>();
+	private static final Comparator<Class<?>> DELEGATES_ASSIGNABILITY_COMPARATOR =
+			(type1, type2) -> {
+
+				if (type1.isAssignableFrom(type2)) {
+					return 1;
+				}
+				if (type2.isAssignableFrom(type1)) {
+					return -1;
+				}
+
+				return 0;
+			};
+
+	private final Map<Class<?>, Serializer<?>> delegates = new TreeMap<>(DELEGATES_ASSIGNABILITY_COMPARATOR);
 
 	private final boolean assignable;
 
@@ -51,17 +64,23 @@ public DelegatingByTypeSerializer(Map<Class<?>, Serializer<?>> delegates) {
 	}
 
 	/**
-	 * Construct an instance with the map of delegates; keys matched exactly or if the
-	 * target object is assignable to the key, depending on the assignable argument.
-	 * If assignable, entries are checked in the natural entry order so an ordered map
-	 * such as a {@link LinkedHashMap} is recommended.
-	 * @param delegates the delegates.
-	 * @param assignable whether the target is assignable to the key.
+	 * Construct an instance with the map of delegates.
+	 * If {@code assignable} is {@code false}, only exact key matches are considered.
+	 * If {@code assignable} is {@code true}, a delegate is selected if its key class
+	 * is assignable from the target object's class. When multiple matches are possible,
+	 * the most specific matching class is selected — that is, the closest match in the
+	 * class hierarchy.
+	 *
+	 * @param delegates the delegates
+	 * @param assignable true if {@link #findDelegate(Object, Map)} should consider assignability to
+	 * the key rather than an exact match.
+	 *
 	 * @since 2.8.3
 	 */
 	public DelegatingByTypeSerializer(Map<Class<?>, Serializer<?>> delegates, boolean assignable) {
 		Assert.notNull(delegates, "'delegates' cannot be null");
 		Assert.noNullElements(delegates.values(), "Serializers in delegates map cannot be null");
+
 		this.delegates.putAll(delegates);
 		this.assignable = assignable;
 	}
@@ -101,7 +120,7 @@ public byte[] serialize(String topic, Headers headers, Object data) {
 		return delegate.serialize(topic, headers, data);
 	}
 
-	private  <T> Serializer<T> findDelegate(T data) {
+	protected  <T> Serializer<T> findDelegate(T data) {
 		return findDelegate(data, this.delegates);
 	}
 

spring-kafka/src/test/java/org/springframework/kafka/support/serializer/DelegatingByTypeSerializerTests.java

@@ -0,0 +1,61 @@
+/*
+ * Copyright 2021-2025 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.kafka.support.serializer;
+
+import java.util.LinkedHashMap;
+import java.util.Map;
+
+import org.apache.kafka.common.serialization.Serializer;
+import org.junit.jupiter.api.Nested;
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.Mockito.mock;
+
+/**
+ * @author Mahesh Aravind V
+ *
+ */
+class DelegatingByTypeSerializerTests {
+	@Nested
+	class AssignableTest {
+		@Test
+		void shouldOrderDelegatesSoChildComesBeforeParent() {
+			class Parent { }
+
+			class Child extends Parent { }
+
+			Serializer mockParentSerializer = mock(Serializer.class);
+			Serializer mockChildSerializer = mock(Serializer.class);
+
+			// Using LinkedHashMap to ensure the order is always wrong
+			Map<Class<?>, Serializer<?>> delegates = new LinkedHashMap<>();
+			delegates.put(Parent.class, mockParentSerializer);
+			delegates.put(Child.class, mockChildSerializer);
+
+			DelegatingByTypeSerializer serializer = new DelegatingByTypeSerializer(delegates, true);
+
+
+			Serializer childSerializer = serializer.findDelegate(mock(Child.class));
+			Serializer parentSerializer = serializer.findDelegate(mock(Parent.class));
+
+
+			assertThat(childSerializer).isEqualTo(mockChildSerializer);
+			assertThat(parentSerializer).isEqualTo(mockParentSerializer);
+		}
+	}
+}