Manage asynchronous EventListener with replies

This commit makes sure to reject an `@EventListener` annotated method
that also uses `@Async`. In such scenario, the method is invoked in a
separate thread and the infrastructure has no handle on the actual reply,
if any.

The documentation has been improved to refer to that scenario.

Issue: SPR-14113

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

@@ -35,8 +35,10 @@ import org.springframework.context.expression.AnnotatedElementKey;
 import org.springframework.core.BridgeMethodResolver;
 import org.springframework.core.ResolvableType;
 import org.springframework.core.annotation.AnnotatedElementUtils;
+import org.springframework.core.annotation.AnnotationUtils;
 import org.springframework.core.annotation.Order;
 import org.springframework.expression.EvaluationContext;
+import org.springframework.scheduling.annotation.Async;
 import org.springframework.util.Assert;
 import org.springframework.util.ObjectUtils;
 import org.springframework.util.ReflectionUtils;
@@ -82,6 +84,7 @@ public class ApplicationListenerMethodAdapter implements GenericApplicationListe
 
 
 	public ApplicationListenerMethodAdapter(String beanName, Class<?> targetClass, Method method) {
+		validateMethod(method);
 		this.beanName = beanName;
 		this.method = method;
 		this.targetClass = targetClass;
@@ -90,6 +93,14 @@ public class ApplicationListenerMethodAdapter implements GenericApplicationListe
 		this.methodKey = new AnnotatedElementKey(this.method, this.targetClass);
 	}
 
+	private static void validateMethod(Method method) {
+		if (method.getReturnType() != void.class &&
+				AnnotationUtils.findAnnotation(method, Async.class) != null) {
+			throw new IllegalStateException(
+					"Asynchronous @EventListener method is not allowed to return reply events: " + method);
+		}
+	}
+
 
 	/**
 	 * Initialize this instance.