INT-4186: Add @reactive to Messaging Annotations

JIRA: https://jira.spring.io/browse/INT-4186

Just a PoC how it would look from the end-user perspective

* Add `@Reactive` annotations' option alongside with `@Poller` to allow to use `ReactiveConsumer` underneath with provided backpressure strategy

Author: artembilan

spring-integration-core/src/main/java/org/springframework/integration/annotation/Reactive.java

@@ -0,0 +1,79 @@
+/*
+ * Copyright 2017 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
+ *
+ *      http://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.annotation;
+
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+import org.springframework.core.annotation.AliasFor;
+import org.springframework.integration.reactive.BackpressureType;
+
+import reactor.core.publisher.BufferOverflowStrategy;
+
+/**
+ * Provides backpressure options for the Messaging annotations for
+ * reactive endpoints.
+ * <p>
+ * It is an analogue of the XML {@code <reactive/>} element.
+ * <p>
+ * Non-reference attributes support Property Placeholder resolutions.
+ *
+ * @author Artem Bilan
+ *
+ * @since 5.0
+ *
+ * @see reactor.core.publisher.Flux#onBackpressureBuffer
+ * @see reactor.core.publisher.Flux#onBackpressureDrop
+ * @see reactor.core.publisher.Flux#onBackpressureError
+ * @see reactor.core.publisher.Flux#onBackpressureLatest
+ */
+@Target({})
+@Retention(RetentionPolicy.RUNTIME)
+public @interface Reactive {
+
+	/**
+	 * @return The {@link BackpressureType} to use.
+	 */
+	@AliasFor("backpressure")
+	BackpressureType value() default BackpressureType.NONE;
+
+	/**
+	 * @return The {@link BackpressureType} to use.
+	 */
+	@AliasFor("value")
+	BackpressureType backpressure() default BackpressureType.NONE;
+
+	/**
+	 * @return The {@link java.util.function.Consumer} bean name, called as a callback on backpressure.
+	 */
+	String consumer() default "";
+
+	/**
+	 * @return The {@link BufferOverflowStrategy} which is used
+	 * in case of {@link BackpressureType#BUFFER} for the {@link #backpressure()}.
+	 */
+	BufferOverflowStrategy bufferOverflowStrategy() default BufferOverflowStrategy.ERROR;
+
+	/**
+	 * @return the maximum buffer backlog size before immediate error
+	 * in case of {@link BackpressureType#BUFFER} for the {@link #backpressure()}.
+	 * Defaults to {@link Integer#MIN_VALUE} meaning {@code unbounded}.
+	 */
+	String bufferMaxSize() default "-2147483648";
+
+}

spring-integration-core/src/main/java/org/springframework/integration/annotation/ServiceActivator.java

@@ -115,7 +115,17 @@
 	 * ({@link org.springframework.integration.scheduling.PollerMetadata}).
 	 * This attribute is an {@code array} just to allow an empty default (no poller).
 	 * Only one {@link Poller} element is allowed.
+	 * Mutually exclusive with {@link #reactive()}.
 	 */
 	Poller[] poller() default {};
 
+	/**
+	 * @return the {@link Reactive} options for a reactive endpoint.
+	 * This attribute is an {@code array} just to allow an empty default (not reactive).
+	 * Only one {@link Reactive} element is allowed.
+	 * Mutually exclusive with {@link #poller()}.
+	 * @since 5.0
+	 */
+	Reactive[] reactive() default { };
+
 }

spring-integration-core/src/main/java/org/springframework/integration/dsl/IntegrationFlowDefinition.java

@@ -38,7 +38,6 @@
 import org.springframework.integration.channel.ChannelInterceptorAware;
 import org.springframework.integration.channel.DirectChannel;
 import org.springframework.integration.channel.FixedSubscriberChannel;
-import org.springframework.integration.channel.MessageChannelReactiveUtils;
 import org.springframework.integration.channel.ReactiveChannel;
 import org.springframework.integration.channel.interceptor.WireTap;
 import org.springframework.integration.config.ConsumerEndpointFactoryBean;
@@ -68,6 +67,7 @@
 import org.springframework.integration.handler.MessageTriggerAction;
 import org.springframework.integration.handler.MethodInvokingMessageProcessor;
 import org.springframework.integration.handler.ServiceActivatingHandler;
+import org.springframework.integration.reactive.MessageChannelReactiveUtils;
 import org.springframework.integration.router.AbstractMappingMessageRouter;
 import org.springframework.integration.router.AbstractMessageRouter;
 import org.springframework.integration.router.ExpressionEvaluatingRouter;

spring-integration-core/src/main/java/org/springframework/integration/endpoint/ReactiveConsumer.java

@@ -23,8 +23,8 @@
 import org.reactivestreams.Subscription;
 
 import org.springframework.context.Lifecycle;
-import org.springframework.integration.channel.MessageChannelReactiveUtils;
 import org.springframework.integration.channel.MessagePublishingErrorHandler;
+import org.springframework.integration.reactive.MessageChannelReactiveUtils;
 import org.springframework.integration.support.channel.BeanFactoryChannelResolver;
 import org.springframework.messaging.Message;
 import org.springframework.messaging.MessageChannel;
@@ -36,6 +36,7 @@
 import reactor.core.Exceptions;
 import reactor.core.Receiver;
 import reactor.core.Trackable;
+import reactor.core.publisher.Flux;
 import reactor.core.publisher.Operators;
 
 
@@ -49,7 +50,7 @@ public class ReactiveConsumer extends AbstractEndpoint {
 
 	private final Lifecycle lifecycleDelegate;
 
-	private volatile Publisher<Message<?>> publisher;
+	private volatile Flux<Message<?>> publisher;
 
 	private ErrorHandler errorHandler;
 
@@ -67,7 +68,7 @@ public ReactiveConsumer(MessageChannel inputChannel, Subscriber<Message<?>> subs
 		Assert.notNull(subscriber, "'subscriber' must not be null");
 
 		Publisher<?> messagePublisher = MessageChannelReactiveUtils.toPublisher(inputChannel);
-		this.publisher = (Publisher<Message<?>>) messagePublisher;
+		this.publisher = Flux.from((Publisher<Message<?>>) messagePublisher);
 
 		this.subscriber = new Operators.SubscriberAdapter<Message<?>, Message<?>>(subscriber) {
 
@@ -104,7 +105,8 @@ protected void doStart() {
 		if (this.lifecycleDelegate != null) {
 			this.lifecycleDelegate.start();
 		}
-		this.publisher.subscribe(this.subscriber);
+		this.publisher
+				.subscribe(this.subscriber);
 	}
 
 	@Override

spring-integration-core/src/main/java/org/springframework/integration/reactive/BackpressureType.java

@@ -0,0 +1,36 @@
+/*
+ * Copyright 2017 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
+ *
+ *      http://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.reactive;
+
+/**
+ * The {@link reactor.core.publisher.Flux} backpressure strategy type.
+ *
+ * @author Artem Bilan
+ *
+ * @since 5.0
+ *
+ * @see org.springframework.integration.annotation.Reactive
+ * @see reactor.core.publisher.Flux#onBackpressureBuffer
+ * @see reactor.core.publisher.Flux#onBackpressureDrop
+ * @see reactor.core.publisher.Flux#onBackpressureError
+ * @see reactor.core.publisher.Flux#onBackpressureLatest
+ */
+public enum BackpressureType {
+
+	BUFFER, DROP, ERROR, LATEST, NONE
+
+}

spring-integration-core/src/main/java/org/springframework/integration/channel/MessageChannelReactiveUtils.java renamed to spring-integration-core/src/main/java/org/springframework/integration/reactive/MessageChannelReactiveUtils.java

@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-package org.springframework.integration.channel;
+package org.springframework.integration.reactive;
 
 import java.util.Iterator;
 

spring-integration-core/src/main/java/org/springframework/integration/reactive/package-info.java

@@ -0,0 +1,4 @@
+/**
+ * Provides utility classes for Reactive Streams integration.
+ */
+package org.springframework.integration.reactive;

spring-integration-core/src/test/java/org/springframework/integration/channel/reactive/ReactiveChannelTests.java

@@ -36,7 +36,7 @@
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.integration.annotation.ServiceActivator;
-import org.springframework.integration.channel.MessageChannelReactiveUtils;
+import org.springframework.integration.reactive.MessageChannelReactiveUtils;
 import org.springframework.integration.channel.QueueChannel;
 import org.springframework.integration.channel.ReactiveChannel;
 import org.springframework.integration.config.EnableIntegration;

spring-integration-core/src/test/java/org/springframework/integration/configuration/EnableIntegrationTests.java

@@ -83,6 +83,7 @@
 import org.springframework.integration.annotation.MessagingGateway;
 import org.springframework.integration.annotation.Poller;
 import org.springframework.integration.annotation.Publisher;
+import org.springframework.integration.annotation.Reactive;
 import org.springframework.integration.annotation.Role;
 import org.springframework.integration.annotation.ServiceActivator;
 import org.springframework.integration.annotation.Transformer;
@@ -109,6 +110,7 @@
 import org.springframework.integration.history.MessageHistory;
 import org.springframework.integration.history.MessageHistoryConfigurer;
 import org.springframework.integration.json.JsonPropertyAccessor;
+import org.springframework.integration.reactive.BackpressureType;
 import org.springframework.integration.scheduling.PollerMetadata;
 import org.springframework.integration.support.MessageBuilder;
 import org.springframework.integration.support.MutableMessageBuilder;
@@ -138,6 +140,7 @@
 import org.springframework.util.ClassUtils;
 import org.springframework.util.MultiValueMap;
 
+import reactor.core.publisher.BufferOverflowStrategy;
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
 
@@ -987,7 +990,11 @@ public AtomicReference<Thread> asyncAnnotationProcessThread() {
 		}
 
 		@Bean
-		@ServiceActivator(inputChannel = "sendAsyncChannel")
+		@ServiceActivator(inputChannel = "sendAsyncChannel",
+				reactive = @Reactive(
+						backpressure = BackpressureType.BUFFER,
+						bufferOverflowStrategy = BufferOverflowStrategy.ERROR,
+						bufferMaxSize = "100"))
 		@Role("foo")
 		public MessageHandler sendAsyncHandler() {
 			return new MessageHandler() {