spring-projectsGH-1189: support Mono and Future

* Support `Mono` and `Future`
* Support auto ack at async return scenario when manual commit
* Support `KafkaListenerErrorHandler`
* Add warn log if the container is not configured for out-of-order manual commit
* Add async return test in `BatchMessagingMessageListenerAdapterTests`
  and `MessagingMessageListenerAdapterTests`
* Add unit test async listener with `@SendTo` in `AsyncListenerTests`
* Add `async-returns.adoc` and `whats-new.adoc`

spring-kafka-docs/src/main/antora/modules/ROOT/pages/kafka/receiving-messages/async-returns.adoc

@@ -0,0 +1,31 @@
+[[async-returns]]
+= Asynchronous `@KafkaListener` Return Types
+
+`@KafkaListener` (and `@KafkaHandler`) methods can be specified with asynchronous return types `CompletableFuture<?>` and `Mono<?>`, letting the reply be sent asynchronously.
+
+[source, java]
+----
+@KafkaListener(id = "myListener", topics = "myTopic")
+public CompletableFuture<String> listen(String data) {
+    ...
+    CompletableFuture<String> future = new CompletableFuture<>();
+    future.complete("done");
+    return future;
+}
+----
+
+[source, java]
+----
+@KafkaListener(id = "myListener", topics = "myTopic")
+public Mono<Void> listen(String data) {
+    ...
+    return Mono.empty();
+}
+----
+
+IMPORTANT: The listener container factory must be configured with manual ack mode and async ack to enable out-of-order commits; instead, the asynchronous completion will ack or nack the message when the async operation completes.
+When the async result is completed with an error, whether the message is recover or not depends on the container error handler.
+If some exception occurs within the listener method that prevents creation of the async result object, you MUST catch that exception and return an appropriate return object that will cause the message to be ack or recover.
+
+If a `KafkaListenerErrorHandler` is configured on a listener with an async return type, the error handler is invoked after a failure.
+See xref:kafka/annotation-error-handling.adoc[Handling Exceptions] for more information about this error handler and its purpose.

spring-kafka-docs/src/main/antora/modules/ROOT/pages/whats-new.adoc

@@ -12,4 +12,10 @@ For changes in earlier version, see xref:appendix/change-history.adoc[Change His
 
 A new `TransactionIdSuffixStrategy` interface was introduced to manage `transactional.id` suffix.
 The default implementation is `DefaultTransactionIdSuffixStrategy` when setting `maxCache` greater than zero can reuse `transactional.id` within a specific range, otherwise suffixes will be generated on the fly by incrementing a counter.
-See xref:kafka/transactions.adoc#transaction-id-suffix-fixed[Fixed TransactionIdSuffix] for more information.
+See xref:kafka/transactions.adoc#transaction-id-suffix-fixed[Fixed TransactionIdSuffix] for more information.
+
+[[x32-async-return]]
+=== Async @KafkaListener Return
+
+`@KafkaListener` (and `@KafkaHandler`) methods can now return asynchronous return types `CompletableFuture<?>` and `Mono<?>`.
+See xref:kafka/receiving-messages/async-returns.adoc[Async Returns] for more information.

spring-kafka/src/main/java/org/springframework/kafka/config/MethodKafkaListenerEndpoint.java

@@ -147,8 +147,8 @@ private String getReplyTopic() {
 				}
 				String topic = destinations.length == 1 ? destinations[0] : "";
 				BeanFactory beanFactory = getBeanFactory();
-				if (beanFactory instanceof ConfigurableListableBeanFactory) {
-					topic = ((ConfigurableListableBeanFactory) beanFactory).resolveEmbeddedValue(topic);
+				if (beanFactory instanceof ConfigurableListableBeanFactory configurableListableBeanFactory) {
+					topic = configurableListableBeanFactory.resolveEmbeddedValue(topic);
 					if (topic != null) {
 						topic = resolve(topic);
 					}

spring-kafka/src/main/java/org/springframework/kafka/listener/KafkaMessageListenerContainer.java

@@ -3395,6 +3395,11 @@ public void nack(Duration sleep) {
 				ListenerConsumer.this.nackSleepDurationMillis = sleep.toMillis();
 			}
 
+			@Override
+			public boolean isAsyncAcks() {
+				return !ListenerConsumer.this.containerProperties.isAsyncAcks();
+			}
+
 			@Override
 			public String toString() {
 				return "Acknowledgment for " + KafkaUtils.format(this.cRecord);
@@ -3493,6 +3498,11 @@ public void nack(int index, Duration sleep) {
 				processAcks(new ConsumerRecords<K, V>(newRecords));
 			}
 
+			@Override
+			public boolean isAsyncAcks() {
+				return !ListenerConsumer.this.containerProperties.isAsyncAcks();
+			}
+
 			@Override
 			public String toString() {
 				return "Acknowledgment for " + this.records;

spring-kafka/src/main/java/org/springframework/kafka/listener/adapter/MessagingMessageListenerAdapter.java

@@ -26,6 +26,7 @@
 import java.util.Iterator;
 import java.util.List;
 import java.util.Map;
+import java.util.concurrent.CompletableFuture;
 import java.util.stream.Collectors;
 
 import org.apache.commons.logging.LogFactory;
@@ -47,6 +48,7 @@
 import org.springframework.kafka.listener.ConsumerSeekAware;
 import org.springframework.kafka.listener.KafkaListenerErrorHandler;
 import org.springframework.kafka.listener.ListenerExecutionFailedException;
+import org.springframework.kafka.listener.MessageListener;
 import org.springframework.kafka.support.Acknowledgment;
 import org.springframework.kafka.support.KafkaHeaders;
 import org.springframework.kafka.support.KafkaNull;
@@ -66,9 +68,12 @@
 import org.springframework.messaging.support.GenericMessage;
 import org.springframework.messaging.support.MessageBuilder;
 import org.springframework.util.Assert;
+import org.springframework.util.ClassUtils;
 import org.springframework.util.ObjectUtils;
 import org.springframework.util.StringUtils;
 
+import reactor.core.publisher.Mono;
+
 /**
  * An abstract {@link org.springframework.kafka.listener.MessageListener} adapter
  * providing the necessary infrastructure to extract the payload of a
@@ -95,6 +100,9 @@ public abstract class MessagingMessageListenerAdapter<K, V> implements ConsumerS
 	 */
 	protected static final Message<KafkaNull> NULL_MESSAGE = new GenericMessage<>(KafkaNull.INSTANCE); // NOSONAR
 
+	private static final boolean monoPresent =
+			ClassUtils.isPresent("reactor.core.publisher.Mono", MessageListener.class.getClassLoader());
+
 	private final Object bean;
 
 	protected final LogAccessor logger = new LogAccessor(LogFactory.getLog(getClass())); //NOSONAR
@@ -368,7 +376,7 @@ protected void invoke(Object records, @Nullable Acknowledgment acknowledgment, C
 		try {
 			Object result = invokeHandler(records, acknowledgment, message, consumer);
 			if (result != null) {
-				handleResult(result, records, message);
+				handleResult(result, records, acknowledgment, consumer, message);
 			}
 		}
 		catch (ListenerExecutionFailedException e) { // NOSONAR ex flow control
@@ -436,19 +444,58 @@ private RuntimeException checkAckArg(@Nullable Acknowledgment acknowledgment, Me
 	 * response message to the SendTo topic.
 	 * @param resultArg the result object to handle (never <code>null</code>)
 	 * @param request the original request message
+	 * @param acknowledgment the acknowledgment to manual ack
+	 * @param consumer the consumer to handler error
 	 * @param source the source data for the method invocation - e.g.
 	 * {@code o.s.messaging.Message<?>}; may be null
 	 */
-	protected void handleResult(Object resultArg, Object request, Object source) {
+	protected void handleResult(Object resultArg, Object request, @Nullable Acknowledgment acknowledgment,
+			Consumer<?, ?> consumer, @Nullable Message<?> source) {
+
 		this.logger.debug(() -> "Listener method returned result [" + resultArg
 				+ "] - generating response message for it");
-		boolean isInvocationResult = resultArg instanceof InvocationResult;
-		Object result = isInvocationResult ? ((InvocationResult) resultArg).getResult() : resultArg;
 		String replyTopic = evaluateReplyTopic(request, source, resultArg);
 		Assert.state(replyTopic == null || this.replyTemplate != null,
 				"a KafkaTemplate is required to support replies");
-		sendResponse(result, replyTopic, source, isInvocationResult
-				? ((InvocationResult) resultArg).isMessageReturnType() : this.messageReturnType);
+		Object result;
+		boolean messageReturnType;
+		if (resultArg instanceof InvocationResult invocationResult) {
+			result = invocationResult.getResult();
+			messageReturnType = invocationResult.isMessageReturnType();
+		}
+		else {
+			result = resultArg;
+			messageReturnType = this.messageReturnType;
+		}
+		if (result instanceof CompletableFuture<?> completable) {
+			if (acknowledgment == null || acknowledgment.isAsyncAcks()) {
+				this.logger.warn("Container 'Acknowledgment' must be async ack for Future<?> return type; "
+						+ "otherwise the container will ack the message immediately");
+			}
+			completable.whenComplete((r, t) -> {
+				if (t == null) {
+					asyncSuccess(r, replyTopic, source, messageReturnType);
+					acknowledge(acknowledgment);
+				}
+				else {
+					asyncFailure(request, acknowledgment, consumer, t, source);
+				}
+			});
+		}
+		else if (monoPresent && result instanceof Mono<?> mono) {
+			if (acknowledgment == null || acknowledgment.isAsyncAcks()) {
+				this.logger.warn("Container 'Acknowledgment' must be async ack for Mono<?> return type; " +
+						"otherwise the container will ack the message immediately");
+			}
+			mono.subscribe(
+					r -> asyncSuccess(r, replyTopic, source, messageReturnType),
+					t -> asyncFailure(request, acknowledgment, consumer, t, source),
+					() -> acknowledge(acknowledgment)
+			);
+		}
+		else {
+			sendResponse(result, replyTopic, source, messageReturnType);
+		}
 	}
 
 	@Nullable
@@ -586,6 +633,37 @@ private void sendReplyForMessageSource(Object result, String topic, Message<?> s
 		this.replyTemplate.send(builder.build());
 	}
 
+	protected void asyncSuccess(@Nullable Object result, String replyTopic, Message<?> source, boolean returnTypeMessage) {
+		if (result == null) {
+			if (this.logger.isDebugEnabled()) {
+				this.logger.debug("Async result is null, ignoring");
+			}
+		}
+		else {
+			sendResponse(result, replyTopic, source, returnTypeMessage);
+		}
+	}
+
+	protected void acknowledge(@Nullable Acknowledgment acknowledgment) {
+		if (acknowledgment != null) {
+			acknowledgment.acknowledge();
+		}
+	}
+
+	protected void asyncFailure(Object request, @Nullable Acknowledgment acknowledgment, Consumer<?, ?> consumer,
+			Throwable t, Message<?> source) {
+		try {
+			handleException(request, acknowledgment, consumer, source,
+					new ListenerExecutionFailedException(createMessagingErrorMessage(
+							"Async Fail", source.getPayload()), t));
+			return;
+		}
+		catch (Exception ex) {
+		}
+		this.logger.error(t, "Future, Mono, or suspend function was completed with an exception for " + source);
+		acknowledge(acknowledgment);
+	}
+
 	protected void handleException(Object records, @Nullable Acknowledgment acknowledgment, Consumer<?, ?> consumer,
 			Message<?> message, ListenerExecutionFailedException e) {
 
@@ -596,7 +674,7 @@ protected void handleException(Object records, @Nullable Acknowledgment acknowle
 				}
 				Object result = this.errorHandler.handleError(message, e, consumer, acknowledgment);
 				if (result != null) {
-					handleResult(result, records, message);
+					handleResult(result, records, acknowledgment, consumer, message);
 				}
 			}
 			catch (Exception ex) {

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

@@ -81,4 +81,8 @@ default void nack(int index, Duration sleep) {
 		throw new UnsupportedOperationException("nack(index, sleep) is not supported by this Acknowledgment");
 	}
 
+	default boolean isAsyncAcks() {
+		return false;
+	}
+
 }