Defensively register ReactiveReturnValueHandler for messaging methods

Closes gh-23207

Author: jhoeller

spring-messaging/src/main/java/org/springframework/messaging/handler/invocation/HandlerMethodReturnValueHandlerComposite.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 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.
@@ -26,7 +26,6 @@
 import org.springframework.core.MethodParameter;
 import org.springframework.lang.Nullable;
 import org.springframework.messaging.Message;
-import org.springframework.util.Assert;
 import org.springframework.util.concurrent.ListenableFuture;
 
 /**
@@ -139,9 +138,10 @@ public boolean isAsyncReturnValue(Object returnValue, MethodParameter returnType
 	@Nullable
 	public ListenableFuture<?> toListenableFuture(Object returnValue, MethodParameter returnType) {
 		HandlerMethodReturnValueHandler handler = getReturnValueHandler(returnType);
-		Assert.state(handler instanceof AsyncHandlerMethodReturnValueHandler,
-				"AsyncHandlerMethodReturnValueHandler required");
-		return ((AsyncHandlerMethodReturnValueHandler) handler).toListenableFuture(returnValue, returnType);
+		if (handler instanceof AsyncHandlerMethodReturnValueHandler) {
+			return ((AsyncHandlerMethodReturnValueHandler) handler).toListenableFuture(returnValue, returnType);
+		}
+		return null;
 	}
 
 }

spring-messaging/src/main/java/org/springframework/messaging/handler/invocation/ReactiveReturnValueHandler.java

@@ -21,7 +21,6 @@
 import org.springframework.core.MethodParameter;
 import org.springframework.core.ReactiveAdapter;
 import org.springframework.core.ReactiveAdapterRegistry;
-import org.springframework.util.Assert;
 import org.springframework.util.concurrent.ListenableFuture;
 import org.springframework.util.concurrent.MonoToListenableFutureAdapter;
 
@@ -60,8 +59,10 @@ public boolean isAsyncReturnValue(Object returnValue, MethodParameter returnType
 	@Override
 	public ListenableFuture<?> toListenableFuture(Object returnValue, MethodParameter returnType) {
 		ReactiveAdapter adapter = this.adapterRegistry.getAdapter(returnType.getParameterType(), returnValue);
-		Assert.state(adapter != null, () -> "No ReactiveAdapter found for " + returnType.getParameterType());
-		return new MonoToListenableFutureAdapter<>(Mono.from(adapter.toPublisher(returnValue)));
+		if (adapter != null) {
+			return new MonoToListenableFutureAdapter<>(Mono.from(adapter.toPublisher(returnValue)));
+		}
+		return null;
 	}
 
 }

spring-messaging/src/main/java/org/springframework/messaging/simp/annotation/support/SimpAnnotationMethodMessageHandler.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 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.
@@ -74,6 +74,7 @@
 import org.springframework.stereotype.Controller;
 import org.springframework.util.AntPathMatcher;
 import org.springframework.util.Assert;
+import org.springframework.util.ClassUtils;
 import org.springframework.util.CollectionUtils;
 import org.springframework.util.PathMatcher;
 import org.springframework.util.StringValueResolver;
@@ -93,6 +94,10 @@
 public class SimpAnnotationMethodMessageHandler extends AbstractMethodMessageHandler<SimpMessageMappingInfo>
 		implements EmbeddedValueResolverAware, SmartLifecycle {
 
+	private static final boolean reactorPresent = ClassUtils.isPresent(
+			"reactor.core.publisher.Flux", SimpAnnotationMethodMessageHandler.class.getClassLoader());
+
+
 	private final SubscribableChannel clientInboundChannel;
 
 	private final SimpMessageSendingOperations clientMessagingTemplate;
@@ -328,7 +333,9 @@ protected List<? extends HandlerMethodReturnValueHandler> initReturnValueHandler
 
 		handlers.add(new ListenableFutureReturnValueHandler());
 		handlers.add(new CompletableFutureReturnValueHandler());
-		handlers.add(new ReactiveReturnValueHandler());
+		if (reactorPresent) {
+			handlers.add(new ReactiveReturnValueHandler());
+		}
 
 		// Annotation-based return value types