Gh-2924: Fix KafkaNull for JSONConverters

 Fixes: gh-2924

 Add the possibility of sending KafkaNull message payload with JsonMessageConverter.

 **Cherry-pick to 3.0.x**

spring-kafka/src/main/java/org/springframework/kafka/support/converter/ByteArrayJsonMessageConverter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2019-2021 the original author or authors.
+ * Copyright 2019-2023 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.
@@ -16,6 +16,7 @@
 
 package org.springframework.kafka.support.converter;
 
+import org.springframework.kafka.support.KafkaNull;
 import org.springframework.messaging.Message;
 
 import com.fasterxml.jackson.core.JsonProcessingException;
@@ -29,6 +30,7 @@
  * {@code String<->byte[]} conversion is avoided.
  *
  * @author Gary Russell
+ * @author Vladimir Loginov
  * @since 2.3
  *
  */
@@ -44,7 +46,9 @@ public ByteArrayJsonMessageConverter(ObjectMapper objectMapper) {
 	@Override
 	protected Object convertPayload(Message<?> message) {
 		try {
-			return getObjectMapper().writeValueAsBytes(message.getPayload());
+			return message.getPayload() instanceof KafkaNull
+					? null
+					:  getObjectMapper().writeValueAsBytes(message.getPayload());
 		}
 		catch (JsonProcessingException e) {
 			throw new ConversionException("Failed to convert to JSON", message, e);

spring-kafka/src/main/java/org/springframework/kafka/support/converter/BytesJsonMessageConverter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2018-2021 the original author or authors.
+ * Copyright 2018-2023 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.
@@ -18,6 +18,7 @@
 
 import org.apache.kafka.common.utils.Bytes;
 
+import org.springframework.kafka.support.KafkaNull;
 import org.springframework.messaging.Message;
 
 import com.fasterxml.jackson.core.JsonProcessingException;
@@ -31,6 +32,7 @@
  * {@code String<->byte[]} conversion is avoided.
  *
  * @author Gary Russell
+ * @author Vladimir Loginov
  * @since 2.1.7
  *
  */
@@ -46,7 +48,9 @@ public BytesJsonMessageConverter(ObjectMapper objectMapper) {
 	@Override
 	protected Object convertPayload(Message<?> message) {
 		try {
-			return Bytes.wrap(getObjectMapper().writeValueAsBytes(message.getPayload()));
+			return message.getPayload() instanceof KafkaNull
+					? null
+					: Bytes.wrap(getObjectMapper().writeValueAsBytes(message.getPayload()));
 		}
 		catch (JsonProcessingException e) {
 			throw new ConversionException("Failed to convert to JSON", message, e);

spring-kafka/src/main/java/org/springframework/kafka/support/converter/JsonMessageConverter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2019-2021 the original author or authors.
+ * Copyright 2019-2023 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.

spring-kafka/src/main/java/org/springframework/kafka/support/converter/StringJsonMessageConverter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2016-2021 the original author or authors.
+ * Copyright 2016-2023 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.
@@ -16,6 +16,7 @@
 
 package org.springframework.kafka.support.converter;
 
+import org.springframework.kafka.support.KafkaNull;
 import org.springframework.messaging.Message;
 
 import com.fasterxml.jackson.core.JsonProcessingException;
@@ -31,6 +32,7 @@
  * @author Gary Russell
  * @author Artem Bilan
  * @author Dariusz Szablinski
+ * @author Vladimir Loginov
  */
 public class StringJsonMessageConverter extends JsonMessageConverter {
 
@@ -44,12 +46,12 @@ public StringJsonMessageConverter(ObjectMapper objectMapper) {
 	@Override
 	protected Object convertPayload(Message<?> message) {
 		try {
-			return getObjectMapper()
-					.writeValueAsString(message.getPayload());
+			return message.getPayload() instanceof KafkaNull
+					? null
+					: getObjectMapper().writeValueAsString(message.getPayload());
 		}
 		catch (JsonProcessingException e) {
 			throw new ConversionException("Failed to convert to JSON", message, e);
 		}
 	}
-
 }

spring-kafka/src/test/java/org/springframework/kafka/annotation/BatchListenerConversionTests.java

@@ -17,6 +17,7 @@
 package org.springframework.kafka.annotation;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatNoException;
 import static org.mockito.Mockito.never;
 import static org.mockito.Mockito.spy;
 import static org.mockito.Mockito.verify;
@@ -56,6 +57,7 @@
 import org.springframework.kafka.listener.DeadLetterPublishingRecoverer;
 import org.springframework.kafka.listener.DefaultErrorHandler;
 import org.springframework.kafka.support.KafkaHeaders;
+import org.springframework.kafka.support.KafkaNull;
 import org.springframework.kafka.support.converter.BatchMessagingMessageConverter;
 import org.springframework.kafka.support.converter.BytesJsonMessageConverter;
 import org.springframework.kafka.support.converter.ConversionException;
@@ -75,6 +77,7 @@
 /**
  * @author Gary Russell
  * @author Artem Bilan
+ * @author Vladimir Loginov
  *
  * @since 1.3.2
  *
@@ -131,6 +134,8 @@ public void testBatchOfPojoMessages(@Autowired KafkaAdmin admin) throws Exceptio
 		assertThat(listener.received.size()).isGreaterThan(0);
 		assertThat(listener.received.get(0).getPayload()).isInstanceOf(Foo.class);
 		assertThat(listener.received.get(0).getPayload().getBar()).isEqualTo("bar");
+		assertThatNoException().isThrownBy(() -> this.template.send(
+				new GenericMessage<>(KafkaNull.INSTANCE, Collections.singletonMap(KafkaHeaders.TOPIC, topic))));
 		verify(admin, never()).clusterId();
 	}