Support for reactive result values from event listener methods

Closes gh-21831

spring-context/src/main/java/org/springframework/context/event/ApplicationListenerMethodAdapter.java

@@ -24,9 +24,12 @@ import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Collections;
 import java.util.List;
+import java.util.concurrent.CompletionStage;
 
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
+import org.reactivestreams.Subscriber;
+import org.reactivestreams.Subscription;
 
 import org.springframework.aop.support.AopUtils;
 import org.springframework.context.ApplicationContext;
@@ -34,20 +37,24 @@ import org.springframework.context.ApplicationEvent;
 import org.springframework.context.PayloadApplicationEvent;
 import org.springframework.context.expression.AnnotatedElementKey;
 import org.springframework.core.BridgeMethodResolver;
+import org.springframework.core.ReactiveAdapter;
+import org.springframework.core.ReactiveAdapterRegistry;
 import org.springframework.core.ResolvableType;
 import org.springframework.core.annotation.AnnotatedElementUtils;
 import org.springframework.core.annotation.Order;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
+import org.springframework.util.ClassUtils;
 import org.springframework.util.ObjectUtils;
 import org.springframework.util.ReflectionUtils;
 import org.springframework.util.StringUtils;
+import org.springframework.util.concurrent.ListenableFuture;
 
 /**
  * {@link GenericApplicationListener} adapter that delegates the processing of
  * an event to an {@link EventListener} annotated method.
  *
- * <p>Delegates to {@link #processEvent(ApplicationEvent)} to give sub-classes
+ * <p>Delegates to {@link #processEvent(ApplicationEvent)} to give subclasses
  * a chance to deviate from the default. Unwraps the content of a
  * {@link PayloadApplicationEvent} if necessary to allow a method declaration
  * to define any arbitrary event type. If a condition is defined, it is
@@ -60,6 +67,10 @@ import org.springframework.util.StringUtils;
  */
 public class ApplicationListenerMethodAdapter implements GenericApplicationListener {
 
+	private static final boolean reactiveStreamsPresent = ClassUtils.isPresent(
+			"org.reactivestreams.Publisher", ApplicationListenerMethodAdapter.class.getClassLoader());
+
+
 	protected final Log logger = LogFactory.getLog(getClass());
 
 	private final String beanName;
@@ -213,6 +224,30 @@ public class ApplicationListenerMethodAdapter implements GenericApplicationListe
 	}
 
 	protected void handleResult(Object result) {
+		if (reactiveStreamsPresent && new ReactiveResultHandler().subscribeToPublisher(result)) {
+			if (logger.isTraceEnabled()) {
+				logger.trace("Adapted to reactive result: " + result);
+			}
+		}
+		else if (result instanceof CompletionStage) {
+			((CompletionStage<?>) result).whenComplete((event, ex) -> {
+				if (ex != null) {
+					handleAsyncError(ex);
+				}
+				else if (event != null) {
+					publishEvent(event);
+				}
+			});
+		}
+		else if (result instanceof ListenableFuture) {
+			((ListenableFuture<?>) result).addCallback(this::publishEvents, this::handleAsyncError);
+		}
+		else {
+			publishEvents(result);
+		}
+	}
+
+	private void publishEvents(Object result) {
 		if (result.getClass().isArray()) {
 			Object[] events = ObjectUtils.toObjectArray(result);
 			for (Object event : events) {
@@ -237,6 +272,10 @@ public class ApplicationListenerMethodAdapter implements GenericApplicationListe
 		}
 	}
 
+	protected void handleAsyncError(Throwable t) {
+		logger.error("Unexpected error occurred in asynchronous listener", t);
+	}
+
 	private boolean shouldHandle(ApplicationEvent event, @Nullable Object[] args) {
 		if (args == null) {
 			return false;
@@ -376,4 +415,40 @@ public class ApplicationListenerMethodAdapter implements GenericApplicationListe
 		return this.method.toGenericString();
 	}
 
+
+	private class ReactiveResultHandler {
+
+		public boolean subscribeToPublisher(Object result) {
+			ReactiveAdapter adapter = ReactiveAdapterRegistry.getSharedInstance().getAdapter(result.getClass());
+			if (adapter != null) {
+				adapter.toPublisher(result).subscribe(new EventPublicationSubscriber());
+				return true;
+			}
+			return false;
+		}
+	}
+
+
+	private class EventPublicationSubscriber implements Subscriber<Object> {
+
+		@Override
+		public void onSubscribe(Subscription s) {
+			s.request(Integer.MAX_VALUE);
+		}
+
+		@Override
+		public void onNext(Object o) {
+			publishEvents(o);
+		}
+
+		@Override
+		public void onError(Throwable t) {
+			handleAsyncError(t);
+		}
+
+		@Override
+		public void onComplete() {
+		}
+	}
+
 }

spring-context/src/main/java/org/springframework/scheduling/support/TaskUtils.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2013 the original author or authors.
+ * Copyright 2002-2019 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.
@@ -92,9 +92,7 @@ public abstract class TaskUtils {
 
 		@Override
 		public void handleError(Throwable t) {
-			if (logger.isErrorEnabled()) {
-				logger.error("Unexpected error occurred in scheduled task.", t);
-			}
+			logger.error("Unexpected error occurred in scheduled task", t);
 		}
 	}