GH-3763: Add handleReactive() for Java DSL

Fixes #3763

Add a convenient terminal operator to `BaseIntegrationFlowDefinition`
based on a `ReactiveMessageHandler`.
Also add an overload like `handleReactive(ReactiveMessageHandlerSpec)`
to let end-user to choose a protocol-specific channel adapter

Author: artembilan

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

@@ -65,6 +65,7 @@
 import org.springframework.integration.handler.LoggingHandler;
 import org.springframework.integration.handler.MessageProcessor;
 import org.springframework.integration.handler.MessageTriggerAction;
+import org.springframework.integration.handler.ReactiveMessageHandlerAdapter;
 import org.springframework.integration.handler.ServiceActivatingHandler;
 import org.springframework.integration.router.AbstractMessageRouter;
 import org.springframework.integration.router.ErrorMessageExceptionTypeRouter;
@@ -91,6 +92,7 @@
 import org.springframework.messaging.Message;
 import org.springframework.messaging.MessageChannel;
 import org.springframework.messaging.MessageHandler;
+import org.springframework.messaging.ReactiveMessageHandler;
 import org.springframework.messaging.support.ChannelInterceptor;
 import org.springframework.messaging.support.InterceptableChannel;
 import org.springframework.util.Assert;
@@ -2916,6 +2918,66 @@ public IntegrationFlow nullChannel() {
 				.get();
 	}
 
+	/**
+	 * Populate a terminal consumer endpoint for the selected protocol specific
+	 * {@link MessageHandler} implementation from {@code Namespace Factory}:
+	 * In addition, accept options for the integration endpoint using {@link GenericEndpointSpec}.
+	 * @param messageHandlerSpec the {@link MessageHandlerSpec} to configure protocol specific
+	 * {@link MessageHandler}.
+	 * @param <H> the {@link MessageHandler} type.
+	 * @return the current {@link BaseIntegrationFlowDefinition}.
+	 * @since 6.1
+	 */
+	public <H extends ReactiveMessageHandler> IntegrationFlow handleReactive(
+			ReactiveMessageHandlerSpec<?, H> messageHandlerSpec) {
+
+		return handleReactive(messageHandlerSpec, null);
+	}
+
+	/**
+	 * Populate a terminal consumer endpoint for the selected protocol specific
+	 * {@link MessageHandler} implementation from {@code Namespace Factory}:
+	 * In addition, accept options for the integration endpoint using {@link GenericEndpointSpec}.
+	 * @param messageHandlerSpec the {@link MessageHandlerSpec} to configure protocol specific
+	 * {@link MessageHandler}.
+	 * @param endpointConfigurer the {@link Consumer} to provide integration endpoint options.
+	 * @param <H> the {@link MessageHandler} type.
+	 * @return the current {@link BaseIntegrationFlowDefinition}.
+	 * @since 6.1
+	 */
+	public <H extends ReactiveMessageHandler> IntegrationFlow handleReactive(
+			ReactiveMessageHandlerSpec<?, H> messageHandlerSpec,
+			@Nullable Consumer<GenericEndpointSpec<ReactiveMessageHandlerAdapter>> endpointConfigurer) {
+
+		return
+				addComponents(messageHandlerSpec.getComponentsToRegister()).
+						handleReactive(messageHandlerSpec.getObject().getDelegate(), endpointConfigurer);
+	}
+
+	/**
+	 * Add a {@link ReactiveMessageHandler} as a terminal {@link IntegrationFlow} operator.
+	 * @param reactiveMessageHandler the {@link ReactiveMessageHandler} to finish the flow.
+	 * @return The {@link IntegrationFlow} instance based on this definition.
+	 * @since 6.1
+	 */
+	public IntegrationFlow handleReactive(ReactiveMessageHandler reactiveMessageHandler) {
+		return handleReactive(reactiveMessageHandler, null);
+	}
+
+	/**
+	 * Add a {@link ReactiveMessageHandler} as a terminal {@link IntegrationFlow} operator.
+	 * @param reactiveMessageHandler the {@link ReactiveMessageHandler} to finish the flow.
+	 * @param endpointConfigurer the {@link Consumer} to configure a target endpoint for the handler.
+	 * @return The {@link IntegrationFlow} instance based on this definition.
+	 * @since 6.1
+	 */
+	public IntegrationFlow handleReactive(ReactiveMessageHandler reactiveMessageHandler,
+			@Nullable Consumer<GenericEndpointSpec<ReactiveMessageHandlerAdapter>> endpointConfigurer) {
+
+		return handle(new ReactiveMessageHandlerAdapter(reactiveMessageHandler), endpointConfigurer)
+				.get();
+	}
+
 	/**
 	 * Finish this flow with delegation to other {@link IntegrationFlow} instance.
 	 * @param other the {@link IntegrationFlow} to compose with.

spring-integration-core/src/test/java/org/springframework/integration/dsl/flows/IntegrationFlowTests.java

@@ -70,7 +70,6 @@
 import org.springframework.integration.gateway.GatewayProxyFactoryBean;
 import org.springframework.integration.handler.AbstractReplyProducingMessageHandler;
 import org.springframework.integration.handler.LoggingHandler;
-import org.springframework.integration.handler.ReactiveMessageHandlerAdapter;
 import org.springframework.integration.handler.advice.ErrorMessageSendingRecoverer;
 import org.springframework.integration.handler.advice.ExpressionEvaluatingRequestHandlerAdvice;
 import org.springframework.integration.handler.advice.RequestHandlerRetryAdvice;
@@ -718,8 +717,7 @@ public MessageHandler loggingMessageHandler() {
 		public IntegrationFlow wireTapFlow1() {
 			return IntegrationFlow.from("tappedChannel1")
 					.wireTap("tapChannel", wt -> wt.selector(m -> m.getPayload().equals("foo")))
-					.handle(new ReactiveMessageHandlerAdapter((message) -> Mono.just(message).log().then()))
-					.get();
+					.handleReactive((message) -> Mono.just(message).log().then());
 		}
 
 		@Bean

spring-integration-mongodb/src/test/java/org/springframework/integration/mongodb/dsl/MongoDbTests.java

@@ -425,7 +425,7 @@ private MongoDbOutboundGatewaySpec collectionCallbackOutboundGateway(
 		public IntegrationFlow reactiveStore() {
 			return f -> f
 					.channel(MessageChannels.flux())
-					.handle(MongoDb.reactiveOutboundChannelAdapter(REACTIVE_MONGO_DATABASE_FACTORY));
+					.handleReactive(MongoDb.reactiveOutboundChannelAdapter(REACTIVE_MONGO_DATABASE_FACTORY));
 		}
 
 	}

spring-integration-r2dbc/src/test/java/org/springframework/integration/r2dbc/dsl/R2dbcDslTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2020-2022 the original author or authors.
+ * Copyright 2020-2023 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.
@@ -114,12 +114,12 @@ IntegrationFlow r2dbcDslFlow(R2dbcEntityTemplate r2dbcEntityTemplate) {
 							e -> e.poller(p -> p.fixedDelay(100)).autoStartup(false).id("r2dbcInboundChannelAdapter"))
 					.<Mono<?>>handle((p, h) -> p, e -> e.async(true))
 					.channel(MessageChannels.flux())
-					.handle(R2dbc.outboundChannelAdapter(r2dbcEntityTemplate)
-							.queryType(R2dbcMessageHandler.Type.UPDATE)
-							.tableNameExpression("payload.class.simpleName")
-							.criteria((message) -> Criteria.where("id").is(2))
-							.values("{age:36}"))
-					.get();
+					.handleReactive(
+							R2dbc.outboundChannelAdapter(r2dbcEntityTemplate)
+									.queryType(R2dbcMessageHandler.Type.UPDATE)
+									.tableNameExpression("payload.class.simpleName")
+									.criteria((message) -> Criteria.where("id").is(2))
+									.values("{age:36}"));
 		}
 
 	}

src/reference/asciidoc/r2dbc.adoc

@@ -110,10 +110,10 @@ With Java DSL a configuration for this channel adapter is like this:
 ====
 [source, java]
 ----
-.handle(R2dbc.outboundChannelAdapter(r2dbcEntityTemplate)
+.handleReactive(R2dbc.outboundChannelAdapter(r2dbcEntityTemplate)
         .queryType(R2dbcMessageHandler.Type.UPDATE)
         .tableNameExpression("payload.class.simpleName")
         .criteria((message) -> Criteria.where("id").is(message.getHeaders().get("personId")))
         .values("{age:36}"))
 ----
-====
+====

src/reference/asciidoc/reactive-streams.adoc

@@ -154,6 +154,29 @@ However, when a `ReactiveStreamsConsumer` is involved in the flow (e.g. when cha
 One of the out-of-the-box `ReactiveMessageHandler` implementation is a `ReactiveMongoDbStoringMessageHandler` for Outbound Channel Adapter.
 See <<./mongodb.adoc#mongodb-reactive-channel-adapters,MongoDB Reactive Channel Adapters>> for more information.
 
+Starting with version 6.1, the `IntegrationFlowDefinition` expose a convenient `handleReactive(ReactiveMessageHandler)` terminal operator.
+Any `ReactiveMessageHandler` implementation (even just plain lambda with `Mono` API) can be used for this operator.
+The framework subscribes to the returned `Mono<Void>` automatically.
+Here is a simple sample of possible configuration for this operator:
+
+====
+[source, java]
+----
+@Bean
+public IntegrationFlow wireTapFlow1() {
+    return IntegrationFlow.from("tappedChannel1")
+            .wireTap("tapChannel", wt -> wt.selector(m -> m.getPayload().equals("foo")))
+            .handleReactive((message) -> Mono.just(message).log().then());
+}
+----
+====
+
+An overloaded version of this operator accepts a `Consumer<GenericEndpointSpec<ReactiveMessageHandlerAdapter>>` to customize a consumer endpoint around the provided `ReactiveMessageHandler`.
+
+In addition, a `ReactiveMessageHandlerSpec`-based variants also provided.
+In most cases they are used for protocol-specific channel adapter implementations.
+See the next section following links to the target technologies with respective reactive channel adapters.
+
 [[reactive-channel-adapters]]
 === Reactive Channel Adapters
 

src/reference/asciidoc/whats-new.adoc

@@ -29,6 +29,11 @@ See <<./zip.adoc#zip,Zip Support>>  for more information.
 The `ContextHolderRequestHandlerAdvice` allows to store a value from a request message into some context around `MessageHandler` execution.
 See <<./handler-advice.adoc#context-holder-advice, Context Holder Advice>> for more information.
 
+[[x6.1-handle-reactive]]
+==== The `handleReactive()` operator for Java DSL
+The `IntegrationFlow` can now end with a convenient `handleReactive(ReactiveMessageHandler)` operator.
+See <<./reactive-streams.adoc#reactive-message-handler, `ReactiveMessageHandler`>> for more information.
+
 [[x6.1-general]]
 === General Changes