Update docs on @sendto and @SendToUser

1. Explain that both can be used on the same method
2. Better describe semantics for class vs method level
3. General improvements

Issue: SPR-16336

Author: rstoyanchev

spring-messaging/src/main/java/org/springframework/messaging/handler/annotation/SendTo.java

@@ -32,9 +32,8 @@
  * convey the destination to use for the reply. In that case, that destination
  * should take precedence.
  *
- * <p>The annotation may also be placed at class-level if the provider supports
- * it to indicate that all related methods should use this destination if none
- * is specified otherwise.
+* <p>This annotation may be placed class-level in which case it is inherited by
+ * methods of the class.
  *
  * @author Rossen Stoyanchev
  * @author Stephane Nicoll

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 the original author or authors.
+ * Copyright 2002-2018 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.
@@ -25,13 +25,17 @@
 import org.springframework.core.annotation.AliasFor;
 
 /**
- * Annotation that indicates that the return value of a message-handling method
- * should be sent as a {@link org.springframework.messaging.Message} to the specified
- * destination(s) prepended with <code>"/user/{username}"</code> where the user name
+ * Indicates the return value of a message-handling method should be sent as a
+ * {@link org.springframework.messaging.Message} to the specified destination(s)
+ * further prepended with <code>"/user/{username}"</code> where the user name
  * is extracted from the headers of the input message being handled.
  *
- * <p>The annotation may also be placed at class-level in which case all methods
- * in the class where the annotation applies will inherit it.
+ * <p>Both {@code @SendTo} and {@code @SendToUser} may be used on the same method
+ * in which case a message is sent to the destinations of both annotations.
+ *
+ * <p>This annotation may be placed class-level in which case it is inherited
+ * by methods of the class. At the same time, method-level {@code @SendTo} or
+ * {@code @SendToUser} annotations override any such at the class level.
 
  * @author Rossen Stoyanchev
  * @author Sam Brannen

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

@@ -330,15 +330,14 @@ protected List<? extends HandlerMethodReturnValueHandler> initReturnValueHandler
 		List<HandlerMethodReturnValueHandler> handlers = new ArrayList<>();
 
 		// Single-purpose return value types
+
 		handlers.add(new ListenableFutureReturnValueHandler());
 		handlers.add(new CompletableFutureReturnValueHandler());
 
 		// Annotation-based return value types
-		SendToMethodReturnValueHandler sendToHandler =
-				new SendToMethodReturnValueHandler(this.brokerTemplate, true);
-		if (this.headerInitializer != null) {
-			sendToHandler.setHeaderInitializer(this.headerInitializer);
-		}
+
+		SendToMethodReturnValueHandler sendToHandler = new SendToMethodReturnValueHandler(this.brokerTemplate, true);
+		sendToHandler.setHeaderInitializer(this.headerInitializer);
 		handlers.add(sendToHandler);
 
 		SubscriptionMethodReturnValueHandler subscriptionHandler =
@@ -350,6 +349,7 @@ protected List<? extends HandlerMethodReturnValueHandler> initReturnValueHandler
 		handlers.addAll(getCustomReturnValueHandlers());
 
 		// catch-all
+
 		sendToHandler = new SendToMethodReturnValueHandler(this.brokerTemplate, false);
 		sendToHandler.setHeaderInitializer(this.headerInitializer);
 		handlers.add(sendToHandler);

src/docs/asciidoc/web/websocket.adoc

@@ -1214,22 +1214,17 @@ methods as described next.
 [[websocket-stomp-message-mapping]]
 ==== `@MessageMapping`
 
-The `@MessageMapping` annotation can be used on methods to route messages based on their
+`@MessageMapping` can be used to annotate methods to route messages based on their
 destination. It is supported at the method level as well as at the type level. At type
 level `@MessageMapping` is used to express shared mappings across all methods in a
 controller.
 
-By default destination mappings are expected to be Ant-style, path patterns, e.g. "/foo*",
-"/foo/**". The patterns include support for template variables, e.g. "/foo/{id}", that can
-be referenced with `@DestinationVariable` method arguments.
+The mapping values are Ant-style path patterns by default, e.g. "/foo*", "/foo/**"
+including support for template variables, e.g. "/foo/{id}", that can be referenced via
+`@DestinationVariable` method arguments. Applications can also switch to a dot-separated
+destination convention for mappings, as explained in <<websocket-stomp-destination-separator>>.
 
-[TIP]
-====
-Applications can choose to switch to a dot-separated destination convention.
-See <<websocket-stomp-destination-separator>>.
-====
-
-`@MessageMapping` methods can have flexible signatures with the following arguments:
+*Supported Method Arguments*
 
 [cols="1,2", options="header"]
 |===
@@ -1271,47 +1266,52 @@ Values will be converted to the declared method argument type as necessary.
 
 |===
 
-When an `@MessageMapping` method returns a value, by default the value is serialized to
-a payload through a configured `MessageConverter`, and then sent as a `Message` to the
-`"brokerChannel"` from where it is broadcast to subscribers. The destination of the
-outbound message is the same as that of the inbound message but prefixed with `"/topic"`.
+*Return Values*
+
+By default, the return value from an `@MessageMapping` method is serialized to a payload
+through a matching `MessageConverter`, and sent as a `Message` to the `"brokerChannel"`
+from where it is broadcast to subscribers. The destination of the outbound message is the
+same as that of the inbound message but prefixed with `"/topic"`.
+
+The `@SendTo` and `@SendToUser` annotations can be used to customize the destination of
+the output message. `@SendTo` is used to simply customize target destination, or to
+specify multiple destinations. `@SendToUser` is used to direct the output message only
+to the user associated with the input message, see <<websocket-stomp-user-destination>>.
 
-You can use the `@SendTo` method annotation to customize the destination to send
-the payload to. `@SendTo` can also be used at the class level to share a default target
-destination to send messages to. `@SendToUser` is an variant for sending messages only to
-the user associated with a message. See <<websocket-stomp-user-destination>> for details.
+`@SendTo` and `@SendToUser` may both be used at the same time on the same method, and both
+are supported at the class level in which case they act as a default for methods in the
+class. However keep in mind that _any_ method-level `@SendTo` or `@SendToUser` annotations
+override _any_ such annotations at the class level.
 
-The return value from an `@MessageMapping` method may be wrapped with `ListenableFuture`,
-`CompletableFuture`, or `CompletionStage` in order to produce the payload asynchronously.
+Messages may be handled asynchronously and a `@MessageMapping` method may return
+`ListenableFuture`, `CompletableFuture`, or `CompletionStage`.
 
-As an alternative to returning a payload from an `@MessageMapping` method you can also
-send messages using the `SimpMessagingTemplate`, which is also how return values are
-handled under the covers. See <<websocket-stomp-handle-send>>.
+Note that `@SendTo` and `@SendToUser` are merely a convenience that amounts to using the
+`SimpMessagingTemplate` to send messages. If necessary, for more advanced scenarios,
+`@MessageMapping` methods can fall back on using the `SimpMessagingTemplate` directly.
+This can be done instead of, or possibly in addition to returning a value.
+See <<websocket-stomp-handle-send>>.
 
 
 [[websocket-stomp-subscribe-mapping]]
 ==== `@SubscribeMapping`
 
-The `@SubscribeMapping` annotation is used in combination with `@MessageMapping` in order
-to narrow the mapping to subscription messages. In such scenarios, the `@MessageMapping`
-annotation specifies the destination while `@SubscribeMapping` indicates interest in
-subscription messages only.
-
-An `@SubscribeMapping` method is generally no different from any `@MessageMapping`
-method with respect to mapping and input arguments. For example you can combine it with a
-type-level `@MessageMapping` to express a shared destination prefix, and you can use the
-same <<websocket-stomp-message-mapping,method arguments>> as any @MessageMapping` method.
-
-The key difference with `@SubscribeMapping` is that the return value of the method is
-serialized as a payload and sent, not to the "brokerChannel" but to the
-"clientOutboundChannel", effectively replying directly to the client rather than
-broadcasting through the broker. This is useful for implementing one-off, request-reply
-message exchanges, and never holding on to the subscription. A common scenario for this
-pattern is application initialization when data must be loaded and presented.
-
-A `@SubscribeMapping` method can also be annotated with `@SendTo` in which case the
-return value is sent to the `"brokerChannel"` with the explicitly specified target
-destination.
+`@SubscribeMapping` is used together with `@MessageMapping` to narrow the mapping to
+subscription messages. In this scenario `@MessageMapping` expresses message destination
+mappings for routing purposes, which can be done at the class or at the method level,
+while `@SubscribeMapping` narrows the mapping to subscription messages only.
+
+Methods with `@MessageMapping` and `@SubscribeMapping` support the same
+<<websocket-stomp-message-mapping,method arguments>> as methods annotated only with
+`@MessageMapping` do. However for the return value, in the absence of `@SendTo` and
+`@SendToUser`, a message is sent directly as a reply to the subscription, via the
+"clientOutboundChannel" channel. Effectively the subscription is used as a one-time,
+request-reply message exchange with the subscription never stored. This is useful for
+loading data on startup and for initializing a front-end UI.
+
+If an `@SubscribeMapping` method is annotated with `@SendTo` and `@SendToUser` the return
+value is sent to the `"brokerChannel"` as usual, sending a message subscribers of the
+specified destination(s).
 
 
 [[websocket-stomp-exception-handler]]