spring-projectsGH-3635: Add Future<Void> & Mono<Void> to gateway

Fixes spring-projects#3635

When `Future<Void>` & `Mono<Void>` is used as a messaging gateway return type,
the application hangs out on this barrier which may lead to the out of memory eventually

* Add support for the `Future<Void>` & `Mono<Void>` messaging gateway return type
and ensure an asynchronous call for the `gateway.send(Message)` operation and
its exception handling.
In case of successful call, the `Future` is fulfilled with `null` and `Mono` is completed as empty

Author: artembilan

spring-integration-core/src/main/java/org/springframework/integration/gateway/GatewayProxyFactoryBean.java

@@ -564,7 +564,7 @@ private Object invokeGatewayMethod(MethodInvocation invocation, boolean runningO
 		}
 		boolean shouldReturnMessage =
 				Message.class.isAssignableFrom(gateway.returnType) || (!runningOnCallerThread && gateway.expectMessage);
-		boolean shouldReply = gateway.returnType != void.class;
+		boolean shouldReply = !gateway.isVoidReturn;
 		int paramCount = method.getParameterTypes().length;
 		Object response;
 		boolean hasPayloadExpression = findPayloadExpression(method);
@@ -640,7 +640,12 @@ private Object sendOrSendAndReceive(MethodInvocation invocation, MethodInvocatio
 			}
 		}
 		else {
-			gateway.send(args);
+			if (gateway.isMonoReturn) {
+				return Mono.fromRunnable(() -> gateway.send(args));
+			}
+			else {
+				gateway.send(args);
+			}
 		}
 		return null;
 	}

spring-integration-core/src/test/java/org/springframework/integration/gateway/AsyncGatewayTests.java

@@ -30,10 +30,13 @@
 
 import org.junit.jupiter.api.Test;
 
+import org.springframework.beans.DirectFieldAccessor;
 import org.springframework.beans.factory.BeanFactory;
+import org.springframework.integration.MessageDispatchingException;
 import org.springframework.integration.annotation.Gateway;
 import org.springframework.integration.annotation.GatewayHeader;
 import org.springframework.integration.channel.DirectChannel;
+import org.springframework.integration.channel.NullChannel;
 import org.springframework.integration.channel.QueueChannel;
 import org.springframework.messaging.Message;
 import org.springframework.messaging.MessageChannel;
@@ -42,6 +45,7 @@
 import org.springframework.messaging.support.MessageBuilder;
 
 import reactor.core.publisher.Mono;
+import reactor.test.StepVerifier;
 
 /**
  * @author Mark Fisher
@@ -70,7 +74,7 @@ public void futureWithMessageReturned() throws Exception {
 	}
 
 	@Test
-	public void futureWithError() throws Exception {
+	public void futureWithError() {
 		final Error error = new Error("error");
 		DirectChannel channel = new DirectChannel() {
 
@@ -140,7 +144,7 @@ public void customFutureReturned() {
 	}
 
 	@Test
-	public void nonAsyncFutureReturned() throws Exception {
+	public void nonAsyncFutureReturned() {
 		QueueChannel requestChannel = new QueueChannel();
 		addThreadEnricher(requestChannel);
 		startResponder(requestChannel);
@@ -204,6 +208,30 @@ public void futureWithWildcardReturned() throws Exception {
 		assertThat(result).isEqualTo("foobar");
 	}
 
+	@Test
+	public void futureVoid() throws Exception {
+		GatewayProxyFactoryBean proxyFactory = new GatewayProxyFactoryBean(TestEchoService.class);
+		proxyFactory.setDefaultRequestChannel(new NullChannel());
+		proxyFactory.setBeanName("testGateway");
+		proxyFactory.setBeanFactory(mock(BeanFactory.class));
+		proxyFactory.afterPropertiesSet();
+		TestEchoService service = (TestEchoService) proxyFactory.getObject();
+		Future<Void> f = service.asyncSendAndForget("test1");
+		Object result = f.get(10, TimeUnit.SECONDS);
+		assertThat(result).isNull();
+
+		new DirectFieldAccessor(proxyFactory).setPropertyValue("initialized", false);
+		proxyFactory.setDefaultRequestChannel((message, timeout) -> {
+			throw new MessageDispatchingException(message, "intentional dispatcher error");
+		});
+		proxyFactory.afterPropertiesSet();
+
+		Future<Void> futureError = service.asyncSendAndForget("test2");
+		assertThatExceptionOfType(ExecutionException.class)
+				.isThrownBy(() -> futureError.get(10, TimeUnit.SECONDS))
+				.withCauseInstanceOf(MessageDispatchingException.class)
+				.withMessageContaining("intentional dispatcher error");
+	}
 
 	@Test
 	public void monoWithMessageReturned() {
@@ -252,7 +280,7 @@ public void monoWithWildcardReturned() {
 	}
 
 	@Test
-	public void monoWithConsumer() throws Exception {
+	public void monoWithConsumer() {
 		QueueChannel requestChannel = new QueueChannel();
 		startResponder(requestChannel);
 		GatewayProxyFactoryBean proxyFactory = new GatewayProxyFactoryBean(TestEchoService.class);
@@ -263,16 +291,38 @@ public void monoWithConsumer() throws Exception {
 		TestEchoService service = (TestEchoService) proxyFactory.getObject();
 		Mono<String> mono = service.returnStringPromise("foo");
 
-		final AtomicReference<String> result = new AtomicReference<>();
-		final CountDownLatch latch = new CountDownLatch(1);
+		StepVerifier.create(mono)
+				.expectNext("foobar")
+				.verifyComplete();
+	}
 
-		mono.subscribe(s -> {
-			result.set(s);
-			latch.countDown();
+	@Test
+	public void monoVoid() throws InterruptedException {
+		GatewayProxyFactoryBean proxyFactory = new GatewayProxyFactoryBean(TestEchoService.class);
+		proxyFactory.setDefaultRequestChannel(new NullChannel());
+		proxyFactory.setBeanName("testGateway");
+		proxyFactory.setBeanFactory(mock(BeanFactory.class));
+		proxyFactory.afterPropertiesSet();
+		TestEchoService service = (TestEchoService) proxyFactory.getObject();
+		Mono<Void> mono = service.monoVoid("test1");
+
+		CountDownLatch emptyMonoLatch = new CountDownLatch(1);
+		mono.switchIfEmpty(Mono.empty().doOnSuccess(v -> emptyMonoLatch.countDown()).then()).subscribe();
+
+		assertThat(emptyMonoLatch.await(10, TimeUnit.SECONDS)).isTrue();
+
+		new DirectFieldAccessor(proxyFactory).setPropertyValue("initialized", false);
+		proxyFactory.setDefaultRequestChannel((message, timeout) -> {
+			throw new MessageDispatchingException(message, "intentional dispatcher error");
 		});
+		proxyFactory.afterPropertiesSet();
 
-		latch.await(10, TimeUnit.SECONDS);
-		assertThat(result.get()).isEqualTo("foobar");
+		Mono<Void> monoError = service.monoVoid("test2");
+
+		StepVerifier.create(monoError)
+				.expectSubscription()
+				.expectError(MessageDispatchingException.class)
+				.verify(Duration.ofSeconds(10));
 	}
 
 	private static void startResponder(final PollableChannel requestChannel) {
@@ -323,18 +373,13 @@ private interface TestEchoService {
 
 		Mono<?> returnSomethingPromise(String s);
 
-	}
+		Future<Void> asyncSendAndForget(String s);
 
-	private static class CustomFuture implements Future<String> {
+		Mono<Void> monoVoid(String s);
 
-		private final String result;
-
-		private final Thread thread;
+	}
 
-		private CustomFuture(String result, Thread thread) {
-			this.result = result;
-			this.thread = thread;
-		}
+	private record CustomFuture(String result, Thread thread) implements Future<String> {
 
 		@Override
 		public boolean cancel(boolean mayInterruptIfRunning) {

src/reference/asciidoc/gateway.adoc

@@ -460,7 +460,7 @@ If you provide a one-way flow, nothing would be sent back to the caller.
 If you want to completely suppress exceptions, you can provide a reference to the global `nullChannel` (essentially a `/dev/null` approach).
 Finally, as mentioned above, if no `error-channel` is defined, then the exceptions propagate as usual.
 
-When you use the `@MessagingGateway` annotation (see `<<messaging-gateway-annotation>>`), you can use use the `errorChannel` attribute.
+When you use the `@MessagingGateway` annotation (see `<<messaging-gateway-annotation>>`), you can use an `errorChannel` attribute.
 
 Starting with version 5.0, when you use a gateway method with a `void` return type (one-way flow), the `error-channel` reference (if provided) is populated in the standard `errorChannel` header of each sent message.
 This feature allows a downstream asynchronous flow, based on the standard `ExecutorChannel` configuration (or a `QueueChannel`), to override a default global `errorChannel` exceptions sending behavior.
@@ -469,7 +469,7 @@ The `error-channel` property was ignored for `void` methods with an asynchronous
 Instead, error messages were sent to the default `errorChannel`.
 
 
-IMPORTANT: Exposing the messaging system through simple POJI Gateways  provides benefits, but "`hiding`" the reality of the underlying messaging system does come at a price, so there are certain things you should consider.
+IMPORTANT: Exposing the messaging system through simple POJI Gateways provides benefits, but "`hiding`" the reality of the underlying messaging system does come at a price, so there are certain things you should consider.
 We want our Java method to return as quickly as possible and not hang for an indefinite amount of time while the caller is waiting on it to return (whether void, a return value, or a thrown Exception).
 When regular methods are used as a proxies in front of the messaging system, we have to take into account the potentially asynchronous nature of the underlying messaging.
 This means that there might be a chance that a message that was initiated by a gateway could be dropped by a filter and never reach a component that is responsible for producing a reply.
@@ -782,10 +782,9 @@ The calling thread continues, with `handleInvoice()` being called when the flow
 As mentioned in the <<gateway-asynctaskexecutor>> section above, if you wish some downstream component to return a message with an async payload (`Future`, `Mono`, and others), you must explicitly set the async executor to `null` (or `""` when using XML configuration).
 The flow is then invoked on the caller thread and the result can be retrieved later.
 
-===== `void` Return Type
+===== Asynchronous `void` Return Type
 
-Unlike the return types mentioned earlier, when the method return type is `void`, the framework cannot implicitly determine that you wish the downstream flow to run asynchronously, with the caller thread returning immediately.
-In this case, you must annotate the interface method with `@Async`, as the following example shows:
+The messaging gateway method can be declared like this:
 
 ====
 [source, java]
@@ -801,7 +800,9 @@ public interface MyGateway {
 ----
 ====
 
-Unlike the `Future<?>` return types, there is no way to inform the caller if some exception is thrown by the flow, unless some custom `TaskExecutor` (such as an `ErrorHandlingTaskExecutor`) is associated with the `@Async` annotation.
+But downstream exceptions are not going to be propagated back to the caller.
+To ensure asynchronous behaviour for downstream flow invocation and exception propagation to the caller, starting with version 6.0, the framework provides support for the `Future<Void>` and `Mono<Void>` return types.
+The use-case is similar to send-and-forget behavior described before for plain `void` return type, but with a difference that flow execution happens asynchronously and returned `Future` (or `Mono`) is complete with a `null` or exceptionally according to the `send` operation result.
 
 [[gateway-no-response]]
 ==== Gateway Behavior When No response Arrives

src/reference/asciidoc/whats-new.adoc

@@ -80,7 +80,9 @@ See <<./dsl.adoc#java-dsl,Java DSL>> for more information.
 The `org.springframework.util.concurrent.ListenableFuture` has been deprecated starting with Spring Framework `6.0`.
 All Spring Integration async API has been migrated to the `CompletableFuture`.
 
-See <<./gateway.adoc#gw-completable-future, CompletableFuture support>> for more information.
+Also Messaging Gateway interface method can now return `Future<Void>` and `Mono<Void>` with a proper asynchronous execution of the downstream flow.
+
+See <<./gateway.adoc#async-gateway, Asynchronous Gateway>> for more information.
 
 The `integrationGlobalProperties` bean is now declared by the framework as an instance of `org.springframework.integration.context.IntegrationProperties` instead of the previously deprecated `java.util.Properties`.