GH-3280: NullChannel as reply for void gateways (#3292)

* GH-3280: NullChannel as reply for void gateways

Fixes #3280

To properly support a one-way gateway (`void` return type), it is
better to ignore any possible replies from downstream instead of
unexpected `no output-channel or replyChannel header available`
error

* Populate a `nullChannel` into a `replyChannel` header
for `void` gateway methods when `replyChannel` is not set explicitly
* Remove redundant `GatewayProxyFactoryBean.PARSER` in favor of
similar `EXPRESSION_PARSER` in the super class
* Test and document the feature

* * Fix language in the `whats-new.adoc`

Co-authored-by: Gary Russell <grussell@vmware.com>

* * Fix language in gateway.adoc

Co-authored-by: Gary Russell <grussell@vmware.com>

Co-authored-by: Gary Russell <grussell@vmware.com>

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

@@ -54,11 +54,11 @@
 import org.springframework.expression.EvaluationContext;
 import org.springframework.expression.Expression;
 import org.springframework.expression.common.LiteralExpression;
-import org.springframework.expression.spel.standard.SpelExpressionParser;
 import org.springframework.expression.spel.support.StandardEvaluationContext;
 import org.springframework.integration.IntegrationPatternType;
 import org.springframework.integration.annotation.Gateway;
 import org.springframework.integration.annotation.GatewayHeader;
+import org.springframework.integration.context.IntegrationContextUtils;
 import org.springframework.integration.endpoint.AbstractEndpoint;
 import org.springframework.integration.expression.ExpressionUtils;
 import org.springframework.integration.expression.ValueExpression;
@@ -103,8 +103,6 @@
 public class GatewayProxyFactoryBean extends AbstractEndpoint
 		implements TrackableComponent, FactoryBean<Object>, MethodInterceptor, BeanClassLoaderAware {
 
-	private static final SpelExpressionParser PARSER = new SpelExpressionParser();
-
 	private final Object initializationMonitor = new Object();
 
 	private final Map<Method, MethodInvocationGateway> gatewayMap = new HashMap<>();
@@ -718,7 +716,7 @@ private Expression extractPayloadExpressionFromAnnotationOrMetadata(@Nullable Ga
 			 * no longer work as expected; they will need to use, say, -1 instead.
 			 */
 			if (payloadExpression == null && StringUtils.hasText(gatewayAnnotation.payloadExpression())) {
-				payloadExpression = PARSER.parseExpression(gatewayAnnotation.payloadExpression());
+				payloadExpression = EXPRESSION_PARSER.parseExpression(gatewayAnnotation.payloadExpression());
 			}
 		}
 		else if (methodMetadata != null && methodMetadata.getPayloadExpression() != null) {
@@ -880,11 +878,24 @@ private Map<String, Object> headers(Method method, Map<String, Expression> heade
 		Map<String, Object> headers = null;
 		// We don't want to eagerly resolve the error channel here
 		Object errorChannelForVoidReturn = this.errorChannel == null ? this.errorChannelName : this.errorChannel;
-		if (errorChannelForVoidReturn != null && method.getReturnType().equals(void.class)) {
+		boolean isVoidReturnType = method.getReturnType().equals(void.class);
+		if (errorChannelForVoidReturn != null && isVoidReturnType) {
 			headers = new HashMap<>();
 			headers.put(MessageHeaders.ERROR_CHANNEL, errorChannelForVoidReturn);
 		}
 
+		if (isVoidReturnType &&
+				(!headerExpressions.containsKey(MessageHeaders.REPLY_CHANNEL) ||
+						(this.globalMethodMetadata != null &&
+								!this.globalMethodMetadata.getHeaderExpressions()
+										.containsKey(MessageHeaders.REPLY_CHANNEL)))) {
+
+			if (headers == null) {
+				headers = new HashMap<>();
+			}
+			headers.put(MessageHeaders.REPLY_CHANNEL, IntegrationContextUtils.NULL_CHANNEL_BEAN_NAME);
+		}
+
 		if (getMessageBuilderFactory() instanceof DefaultMessageBuilderFactory) {
 			Set<String> headerNames = new HashSet<>(headerExpressions.keySet());
 
@@ -987,16 +998,12 @@ else if (StringUtils.hasText(channelName2)) {
 
 	@Override // guarded by super#lifecycleLock
 	protected void doStart() {
-		for (MethodInvocationGateway gateway : this.gatewayMap.values()) {
-			gateway.start();
-		}
+		this.gatewayMap.values().forEach(MethodInvocationGateway::start);
 	}
 
 	@Override // guarded by super#lifecycleLock
 	protected void doStop() {
-		for (MethodInvocationGateway gateway : this.gatewayMap.values()) {
-			gateway.stop();
-		}
+		this.gatewayMap.values().forEach(MethodInvocationGateway::stop);
 	}
 
 	@SuppressWarnings("unchecked")

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 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.
@@ -19,6 +19,7 @@
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
 import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
+import static org.mockito.BDDMockito.willReturn;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.spy;
 
@@ -48,7 +49,9 @@
 import org.springframework.integration.annotation.GatewayHeader;
 import org.springframework.integration.channel.DirectChannel;
 import org.springframework.integration.channel.QueueChannel;
+import org.springframework.integration.context.IntegrationContextUtils;
 import org.springframework.integration.endpoint.EventDrivenConsumer;
+import org.springframework.integration.handler.BridgeHandler;
 import org.springframework.integration.support.utils.IntegrationUtils;
 import org.springframework.messaging.Message;
 import org.springframework.messaging.MessageChannel;
@@ -129,6 +132,32 @@ public void testOneWay() {
 		assertThat(message.getPayload()).isEqualTo("test");
 	}
 
+	@Test
+	public void testOneWayIgnoreReply() {
+		DirectChannel requestChannel = new DirectChannel();
+		BeanFactory beanFactory = mock(BeanFactory.class);
+		QueueChannel nullChannel = new QueueChannel();
+		willReturn(nullChannel)
+				.given(beanFactory)
+				.getBean(IntegrationContextUtils.NULL_CHANNEL_BEAN_NAME, MessageChannel.class);
+		BridgeHandler handler = new BridgeHandler();
+		handler.setBeanFactory(beanFactory);
+		handler.afterPropertiesSet();
+		requestChannel.subscribe(handler);
+		GatewayProxyFactoryBean proxyFactory = new GatewayProxyFactoryBean(TestService.class);
+		proxyFactory.setDefaultRequestChannel(requestChannel);
+		proxyFactory.setBeanName("testGateway");
+		proxyFactory.setBeanFactory(beanFactory);
+		proxyFactory.afterPropertiesSet();
+		TestService service = (TestService) proxyFactory.getObject();
+		service.oneWay("test");
+		Message<?> message = nullChannel.receive(1000);
+		assertThat(message)
+				.isNotNull()
+				.extracting(Message::getPayload)
+				.isEqualTo("test");
+	}
+
 	@Test
 	public void testSolicitResponse() {
 		QueueChannel replyChannel = new QueueChannel();

src/reference/asciidoc/gateway.adoc

@@ -82,6 +82,9 @@ The gateway creates a bridge from it to the temporary, anonymous reply channel t
 You might also want to explicitly provide a reply channel for monitoring or auditing through an interceptor (for example, <<./channel.adoc#channel-wiretap,wiretap>>).
 To configure a channel interceptor, you need a named channel.
 
+NOTE: Starting with version 5.4, when gateway method return type is `void`, the framework populates a `replyChannel` header as a `nullChannel` bean reference if such a header is not provided explicitly.
+This allows any possible reply from the downstream flow to be discarded, meeting the one-way gateway contract.
+
 [[gateway-configuration-annotations]]
 ==== Gateway Configuration with Annotations and XML
 

src/reference/asciidoc/whats-new.adoc

@@ -12,3 +12,11 @@ If you are interested in the changes and features that were introduced in earlie
 
 If you are interested in more details, see the Issue Tracker tickets that were resolved as part of the 5.4 development process.
 
+[[x5.4-new-components]]
+=== New Components
+
+[[x5.4-general]]
+=== General Changes
+
+The one-way messaging gateway (the `void` method return type) now sets a `nullChannel` explicitly into the `replyChannel` header to ignore any possible downstream replies.
+See <<./gateway.adoc#gateway-default-reply-channel,Setting the Default Reply Channel>> for more information.