Fix IntegrationMBeanExporter logic for endpoints

Related to https://stackoverflow.com/questions/72851234/error-in-startup-application-when-using-serviceactivator-in-spring-cloud-stream

The `MessagingAnnotationPostProcessor` register an `endpoint` bean for Messaging Annotation on POJO methods.
The `IntegrationMBeanExporter` post-process such a bean and registers respective MBean.
It does this not in optimal way scanning all the `IntegrationConsumer` beans for requested `MessageHandler`
which may cause a `BeanCurrentlyInCreationException`.

* Rework the logic of the `IntegrationMBeanExporter.postProcessAbstractEndpoint()` to propagate provided endpoint
for the monitor registration to bypass application context scanning for matched name.
* Swap `equals()` for `monitor` since an `extractTarget()` may return `null`
* Some other code clean in the `IntegrationMBeanExporter`
* Change one of the `@ServiceActivator` in the configuration for the `ScatterGatherHandlerIntegrationTests`
to POJO method.
However, this didn't fail for me with original code unlike in the sample application provided in the mentioned SO thread.

**Cherry-pick to `main`**

spring-integration-jmx/src/main/java/org/springframework/integration/monitor/IntegrationMBeanExporter.java

@@ -334,7 +334,7 @@ private void postProcessAbstractEndpoint(Object bean) {
 			MessageHandler handler = integrationConsumer.getHandler();
 			MessageHandler monitor = (MessageHandler) extractTarget(handler);
 			if (monitor instanceof IntegrationManagement) {
-				registerHandler((IntegrationManagement) monitor);
+				registerHandler((IntegrationManagement) monitor, integrationConsumer);
 				this.handlers.put(((IntegrationManagement) monitor).getComponentName(),
 						(IntegrationManagement) monitor);
 				this.runtimeBeans.add(monitor);
@@ -656,8 +656,12 @@ private void registerHandlers() {
 
 	}
 
-	private void registerHandler(IntegrationManagement monitor2) {
-		IntegrationManagement monitor = enhanceHandlerMonitor(monitor2);
+	private void registerHandler(IntegrationManagement monitor) {
+		registerHandler(monitor, null);
+	}
+
+	private void registerHandler(IntegrationManagement monitor2, @Nullable IntegrationConsumer consumer) {
+		IntegrationManagement monitor = enhanceHandlerMonitor(monitor2, consumer);
 		String name = monitor.getComponentName();
 		if (!this.objectNames.containsKey(monitor2) && matches(this.componentNamePatterns, name)) {
 			String beanKey = getHandlerBeanKey(monitor);
@@ -809,44 +813,47 @@ private String getStaticNames() {
 				.collect(Collectors.joining(","));
 	}
 
-	@SuppressWarnings("unlikely-arg-type")
-	private IntegrationManagement enhanceHandlerMonitor(IntegrationManagement monitor2) {
+	private IntegrationManagement enhanceHandlerMonitor(IntegrationManagement monitor,
+			@Nullable IntegrationConsumer consumer) {
 
-		if (monitor2.getManagedName() != null && monitor2.getManagedType() != null) {
-			return monitor2;
+		if (monitor.getManagedName() != null && monitor.getManagedType() != null) {
+			return monitor;
 		}
 
-		// Assignment algorithm and bean id, with bean id pulled reflectively out of enclosing endpoint if possible
-		String[] names = this.applicationContext.getBeanNamesForType(IntegrationConsumer.class);
-
-		String name = null;
 		String endpointName = null;
 		String source = "endpoint";
-		IntegrationConsumer endpoint = null;
+		IntegrationConsumer endpoint = consumer;
 
-		for (String beanName : names) {
-			endpoint = this.applicationContext.getBean(beanName, IntegrationConsumer.class);
-			try {
-				MessageHandler handler = endpoint.getHandler();
-				if (handler.equals(monitor2) ||
-						extractTarget(handlerInAnonymousWrapper(handler)).equals(monitor2)) {
-					name = beanName;
-					endpointName = beanName;
-					break;
+		if (endpoint == null) {
+			// Assignment algorithm and bean id, with bean id pulled reflectively out of enclosing endpoint if possible
+			String[] names = this.applicationContext.getBeanNamesForType(IntegrationConsumer.class);
+
+			for (String beanName : names) {
+				endpoint = this.applicationContext.getBean(beanName, IntegrationConsumer.class);
+				try {
+					MessageHandler handler = endpoint.getHandler();
+					if (handler.equals(monitor) || monitor.equals(extractTarget(handlerInAnonymousWrapper(handler)))) {
+						endpointName = beanName;
+						break;
+					}
+				}
+				catch (Exception ex) {
+					logger.trace("Could not get handler from bean = " + beanName, ex);
+					endpoint = null;
 				}
-			}
-			catch (Exception e) {
-				logger.trace("Could not get handler from bean = " + beanName, e);
-				endpoint = null;
 			}
 		}
+		else {
+			endpointName = endpoint.getBeanName();
+		}
 
 		IntegrationManagement messageHandlerMetrics =
-				buildMessageHandlerMetrics(monitor2, name, source, endpoint);
+				buildMessageHandlerMetrics(monitor, endpointName, source, endpoint);
 		if (endpointName != null) {
 			this.endpointsByMonitor.put(messageHandlerMetrics, endpointName);
 		}
 		return messageHandlerMetrics;
+
 	}
 
 	private IntegrationManagement buildMessageHandlerMetrics(
@@ -869,7 +876,7 @@ private IntegrationManagement buildMessageHandlerMetrics(
 		}
 
 		if (managedName == null) {
-			managedName = ((NamedComponent) monitor2).getComponentName();
+			managedName = monitor2.getComponentName();
 			if (managedName == null) {
 				managedName = monitor2.toString();
 			}
@@ -902,7 +909,6 @@ private String getInternalComponentName(String name) {
 	}
 
 	private IntegrationInboundManagement enhanceSourceMonitor(IntegrationInboundManagement source2) {
-
 		if (source2.getManagedName() != null) {
 			return source2;
 		}
@@ -958,8 +964,8 @@ private IntegrationInboundManagement buildMessageSourceMetricsIfAny(
 				try {
 					target = targetSource.getTarget();
 				}
-				catch (Exception e) {
-					logger.error("Could not get handler from bean = " + managedName);
+				catch (Exception ex) {
+					logger.error("Could not get handler from bean = " + managedName, ex);
 				}
 			}
 

spring-integration-jmx/src/test/java/org/springframework/integration/monitor/ScatterGatherHandlerIntegrationTests.java

@@ -333,21 +333,9 @@ public MessageChannel serviceChannel2() {
 			return new DirectChannel();
 		}
 
-		@Bean
-		@ServiceActivator(inputChannel = "serviceChannel2")
-		public MessageHandler service2() {
-			return new AbstractReplyProducingMessageHandler() {
-
-				{
-					setOutputChannel(gatherChannel());
-				}
-
-				@Override
-				protected Object handleRequestMessage(Message<?> requestMessage) {
-					return Math.random();
-				}
-
-			};
+		@ServiceActivator(inputChannel = "serviceChannel2", outputChannel = "gatherChannel")
+		public double service2() {
+			return Math.random();
 		}
 
 	}