* Fix Checkstyle violations

* Add JavaDocs to new classes
* Fix some code style inconsistency in the `AbstractMessageProducingHandler`

Author: artembilan

spring-integration-core/src/main/java/org/springframework/integration/handler/AbstractMessageProducingHandler.java

@@ -27,6 +27,7 @@
 
 import org.reactivestreams.Publisher;
 
+import org.springframework.beans.factory.BeanFactory;
 import org.springframework.core.ReactiveAdapter;
 import org.springframework.core.ReactiveAdapterRegistry;
 import org.springframework.integration.IntegrationMessageHeaderAccessor;
@@ -202,8 +203,9 @@ protected void onInit() {
 		super.onInit();
 		Assert.state(!(this.outputChannelName != null && this.outputChannel != null), //NOSONAR (inconsistent sync)
 				"'outputChannelName' and 'outputChannel' are mutually exclusive.");
-		if (getBeanFactory() != null) {
-			this.messagingTemplate.setBeanFactory(getBeanFactory());
+		BeanFactory beanFactory = getBeanFactory();
+		if (beanFactory != null) {
+			this.messagingTemplate.setBeanFactory(beanFactory);
 		}
 		this.messagingTemplate.setDestinationResolver(getChannelResolver());
 	}
@@ -222,11 +224,11 @@ public MessageChannel getOutputChannel() {
 	protected void sendOutputs(Object result, Message<?> requestMessage) {
 		if (result instanceof Iterable<?> && shouldSplitOutput((Iterable<?>) result)) {
 			for (Object o : (Iterable<?>) result) {
-				this.produceOutput(o, requestMessage);
+				produceOutput(o, requestMessage);
 			}
 		}
 		else if (result != null) {
-			this.produceOutput(result, requestMessage);
+			produceOutput(result, requestMessage);
 		}
 	}
 
@@ -246,8 +248,7 @@ protected void produceOutput(Object replyArg, final Message<?> requestMessage) {
 		if (getOutputChannel() == null) {
 			Map<?, ?> routingSlipHeader = obtainRoutingSlipHeader(requestHeaders, reply);
 			if (routingSlipHeader != null) {
-				Assert.isTrue(routingSlipHeader.size() == 1,
-						"The RoutingSlip header value must be a SingletonMap");
+				Assert.isTrue(routingSlipHeader.size() == 1, "The RoutingSlip header value must be a SingletonMap");
 				Object key = routingSlipHeader.keySet().iterator().next();
 				Object value = routingSlipHeader.values().iterator().next();
 				Assert.isInstanceOf(List.class, key, "The RoutingSlip key must be List");
@@ -298,7 +299,7 @@ else if (reply instanceof AbstractIntegrationMessageBuilder<?>) {
 	}
 
 	private void doProduceOutput(Message<?> requestMessage, MessageHeaders requestHeaders, Object reply,
-			Object replyChannel) {
+			@Nullable Object replyChannel) {
 
 		if (this.async && (reply instanceof ListenableFuture<?> || reply instanceof Publisher<?>)) {
 			MessageChannel messageChannel = getOutputChannel();
@@ -341,7 +342,7 @@ else if (reply instanceof AbstractIntegrationMessageBuilder) {
 		return builder;
 	}
 
-	private void asyncNonReactiveReply(Message<?> requestMessage, Object reply, Object replyChannel) {
+	private void asyncNonReactiveReply(Message<?> requestMessage, Object reply, @Nullable Object replyChannel) {
 		ListenableFuture<?> future;
 		if (reply instanceof ListenableFuture<?>) {
 			future = (ListenableFuture<?>) reply;
@@ -508,9 +509,10 @@ private final class ReplyFutureCallback implements ListenableFutureCallback<Obje
 
 		private final Message<?> requestMessage;
 
+		@Nullable
 		private final Object replyChannel;
 
-		ReplyFutureCallback(Message<?> requestMessage, Object replyChannel) {
+		ReplyFutureCallback(Message<?> requestMessage, @Nullable Object replyChannel) {
 			this.requestMessage = requestMessage;
 			this.replyChannel = replyChannel;
 		}

spring-integration-core/src/main/java/org/springframework/integration/handler/AbstractReactiveReplyProducingMessageHandler.java

@@ -1,9 +1,33 @@
+/*
+ * Copyright 2020 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.integration.handler;
 
 import org.springframework.messaging.Message;
 
 import reactor.core.publisher.Mono;
 
+/**
+ * A {@link BaseReplyProducingMessageHandler} extension for reply-producing message handlers with
+ * reactive type reply.
+ *
+ * @author Artem Bilan
+ *
+ * @since 5.3
+ */
 public abstract class AbstractReactiveReplyProducingMessageHandler
 		extends BaseReplyProducingMessageHandler<AbstractReactiveReplyProducingMessageHandler.RequestHandler> {
 

spring-integration-core/src/main/java/org/springframework/integration/handler/AbstractReplyProducingMessageHandler.java

@@ -16,19 +16,8 @@
 
 package org.springframework.integration.handler;
 
-import java.util.LinkedList;
-import java.util.List;
-
-import org.aopalliance.aop.Advice;
-
-import org.springframework.aop.framework.ProxyFactory;
-import org.springframework.beans.factory.BeanClassLoaderAware;
-import org.springframework.integration.IntegrationPatternType;
-import org.springframework.integration.handler.advice.HandleMessageAdvice;
 import org.springframework.lang.Nullable;
 import org.springframework.messaging.Message;
-import org.springframework.util.Assert;
-import org.springframework.util.ClassUtils;
 
 /**
  * Base class for MessageHandlers that are capable of producing replies.

spring-integration-core/src/main/java/org/springframework/integration/handler/BaseReplyProducingMessageHandler.java

@@ -1,3 +1,19 @@
+/*
+ * Copyright 2020 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.integration.handler;
 
 import java.util.LinkedList;
@@ -14,6 +30,16 @@
 import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
 
+/**
+ * An {@link AbstractMessageProducingHandler} extension for an {@link Advice}s logic
+ * to applied for the target implementation.
+ *
+ * @param <H> an internal advice request handler.
+ *
+ * @author Artem Bilan
+ *
+ * @since 5.3
+ */
 public abstract class BaseReplyProducingMessageHandler<H> extends AbstractMessageProducingHandler
 		implements BeanClassLoaderAware {