Document (de)serialization config properties

When reading the reference manual, it is likely that one would encounter the
"Introduction" and start with the "consumer properties map" approach to
configuring deserialization. However, in the course of troubleshooting consumer
failures (eg. deserialization errors), one might read further into the manual
and encounter the "Serialization, Deserialization, and Message Conversion"
section, and pass instances of deserializers to the consumer factory in an
attempt to fix the failures, and might miss out that in fact consumer
properties map now has no effect.

Mention this in the documentation, and add a test case illustrating this is the
expected behavior.

Author: rctay

spring-kafka/src/test/java/org/springframework/kafka/listener/KafkaMessageListenerContainerTests.java

@@ -113,6 +113,7 @@
  * @author Artem Bilan
  * @author Loic Talhouarne
  * @author Lukasz Kaminski
+ * @author Ray Chuan Tay
  */
 @EmbeddedKafka(topics = { KafkaMessageListenerContainerTests.topic1, KafkaMessageListenerContainerTests.topic2,
 		KafkaMessageListenerContainerTests.topic3, KafkaMessageListenerContainerTests.topic4,
@@ -176,6 +177,8 @@ public class KafkaMessageListenerContainerTests {
 
 	public static final String topic23 = "testTopic23";
 
+	public static final String topic24 = "testTopic24";
+
 	private static EmbeddedKafkaBroker embeddedKafka;
 
 	@BeforeAll
@@ -1893,6 +1896,47 @@ public void testJsonSerDeConfiguredType() throws Exception {
 		this.logger.info("Stop JSON1");
 	}
 
+	@Test
+	public void testJsonSerDeWithInstanceDoesNotUseConfiguration() throws Exception {
+		this.logger.info("Start JSON1a");
+		Class<Foo1> consumerConfigValueDefaultType = Foo1.class;
+		Map<String, Object> props = KafkaTestUtils.consumerProps("testJson", "false", embeddedKafka);
+		props.put(ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG, JsonDeserializer.class);
+		props.put(JsonDeserializer.VALUE_DEFAULT_TYPE, consumerConfigValueDefaultType);
+		DefaultKafkaConsumerFactory<Integer, Foo> cf = new DefaultKafkaConsumerFactory<>(props, null, new JsonDeserializer<>(Foo.class));
+		ContainerProperties containerProps = new ContainerProperties(topic24);
+
+		final CountDownLatch latch = new CountDownLatch(1);
+		final AtomicReference<ConsumerRecord<?, ?>> received = new AtomicReference<>();
+		containerProps.setMessageListener((MessageListener<Integer, Foo>) record -> {
+			KafkaMessageListenerContainerTests.this.logger.info("json: " + record);
+			received.set(record);
+			latch.countDown();
+		});
+
+		KafkaMessageListenerContainer<Integer, Foo> container =
+				new KafkaMessageListenerContainer<>(cf, containerProps);
+		container.setBeanName("testJson1a");
+		container.start();
+
+		ContainerTestUtils.waitForAssignment(container, embeddedKafka.getPartitionsPerTopic());
+
+		Map<String, Object> senderProps = KafkaTestUtils.producerProps(embeddedKafka);
+		senderProps.put(ProducerConfig.VALUE_SERIALIZER_CLASS_CONFIG, JsonSerializer.class);
+		DefaultKafkaProducerFactory<Integer, Foo> pf = new DefaultKafkaProducerFactory<>(senderProps);
+		KafkaTemplate<Integer, Foo> template = new KafkaTemplate<>(pf);
+		template.setDefaultTopic(topic24);
+		template.sendDefault(0, new Foo("bar"));
+		template.flush();
+		assertThat(latch.await(60, TimeUnit.SECONDS)).isTrue();
+		assertThat(received.get().value())
+				.isInstanceOf(Foo.class)
+				.isNotInstanceOf(consumerConfigValueDefaultType);
+		container.stop();
+		pf.destroy();
+		this.logger.info("Stop JSON1a");
+	}
+
 	@Test
 	public void testJsonSerDeHeaderSimpleType() throws Exception {
 		this.logger.info("Start JSON2");

src/reference/asciidoc/kafka.adoc

@@ -2740,7 +2740,8 @@ A `JacksonMimeTypeModule` can be registered as a bean in the application context
 Also starting with version 2.3, the `JsonDeserializer` provides `TypeReference`-based constructors for better handling of target generic container types.
 
 Starting with version 2.1, you can convey type information in record `Headers`, allowing the handling of multiple types.
-In addition, you can configure the serializer and deserializer by using the following Kafka properties:
+In addition, you can configure the serializer and deserializer by using the following Kafka properties.
+They have no effect if you have provided `Serializer` and `Deserializer` instances for `KafkaConsumer` and `KafkaProducer`, respectively.
 
 * `JsonSerializer.ADD_TYPE_INFO_HEADERS` (default `true`): You can set it to `false` to disable this feature on the `JsonSerializer` (sets the `addTypeInfo` property).
 * `JsonSerializer.TYPE_MAPPINGS` (default `empty`): See <<serdes-mapping-types>>.