Add ability to customize message channels

@EnableWebSocketMessageBroker message channel configuration can now be
customized via WebSocketMessageBrokerConfigurer. It is necessary to
make this easy and even required as part of the basic configuration
since by default the message channels are backed by a thread pool of
size 1, not suitable for production use.

Issue: SPR-11023

spring-websocket/src/test/java/org/springframework/web/socket/messaging/SimpAnnotationMethodIntegrationTests.java

@@ -36,6 +36,7 @@ import org.springframework.context.annotation.ComponentScan;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.messaging.handler.annotation.MessageExceptionHandler;
 import org.springframework.messaging.handler.annotation.MessageMapping;
+import org.springframework.messaging.simp.config.ChannelRegistration;
 import org.springframework.messaging.simp.config.MessageBrokerRegistry;
 import org.springframework.messaging.simp.stomp.StompCommand;
 import org.springframework.messaging.support.channel.AbstractSubscribableChannel;
@@ -215,6 +216,14 @@ public class SimpAnnotationMethodIntegrationTests extends AbstractWebSocketInteg
 			registry.addEndpoint("/ws").setHandshakeHandler(this.handshakeHandler);
 		}
 
+		@Override
+		public void configureClientInboundChannel(ChannelRegistration registration) {
+		}
+
+		@Override
+		public void configureClientOutboundChannel(ChannelRegistration registration) {
+		}
+
 		@Override
 		public void configureMessageBroker(MessageBrokerRegistry configurer) {
 			configurer.setApplicationDestinationPrefixes("/app");

spring-websocket/src/test/java/org/springframework/web/socket/messaging/config/WebSocketMessageBrokerConfigurationSupportTests.java

@@ -27,6 +27,7 @@ import org.springframework.messaging.handler.annotation.MessageMapping;
 import org.springframework.messaging.handler.annotation.SendTo;
 import org.springframework.messaging.simp.SimpMessageType;
 import org.springframework.messaging.simp.annotation.SubscribeMapping;
+import org.springframework.messaging.simp.config.ChannelRegistration;
 import org.springframework.messaging.simp.config.MessageBrokerRegistry;
 import org.springframework.messaging.simp.stomp.StompCommand;
 import org.springframework.messaging.simp.stomp.StompHeaderAccessor;
@@ -119,20 +120,29 @@ public class WebSocketMessageBrokerConfigurationSupportTests {
 	@Configuration
 	static class TestSimpleMessageBrokerConfig implements WebSocketMessageBrokerConfigurer {
 
+		@Bean
+		public TestController subscriptionController() {
+			return new TestController();
+		}
+
 		@Override
 		public void registerStompEndpoints(StompEndpointRegistry registry) {
 			registry.addEndpoint("/simpleBroker");
 		}
 
 		@Override
-		public void configureMessageBroker(MessageBrokerRegistry configurer) {
+		public void configureClientInboundChannel(ChannelRegistration registration) {
+		}
+
+		@Override
+		public void configureClientOutboundChannel(ChannelRegistration registration) {
+		}
+
+		@Override
+		public void configureMessageBroker(MessageBrokerRegistry registry) {
 			// SimpleBroker used by default
 		}
 
-		@Bean
-		public TestController subscriptionController() {
-			return new TestController();
-		}
 	}
 
 	@Configuration