Spring MVC recognizes gRPC streams

For this to work, a compatible message converter is necessary,
but only available in spring-grpc.

See gh-35401

Author: rstoyanchev

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/ReactiveTypeHandler.java

@@ -81,6 +81,8 @@ class ReactiveTypeHandler {
 
 	private static final MediaType WILDCARD_SUBTYPE_SUFFIXED_BY_NDJSON = MediaType.valueOf("application/*+x-ndjson");
 
+	private static final MediaType APPLICATION_GRPC = MediaType.valueOf("application/grpc");
+
 	private static final boolean isContextPropagationPresent = ClassUtils.isPresent(
 			"io.micrometer.context.ContextSnapshot", ReactiveTypeHandler.class.getClassLoader());
 
@@ -165,9 +167,14 @@ public boolean isReactiveType(Class<?> type) {
 				new SseEmitterSubscriber(emitter, this.taskExecutor, taskDecorator).connect(adapter, returnValue);
 				return emitter;
 			}
+			if (mediaTypes.stream().anyMatch(APPLICATION_GRPC::includes)) {
+				ResponseBodyEmitter emitter = getEmitter(mediaType.orElse(APPLICATION_GRPC));
+				new BasicEmitterSubscriber(emitter, APPLICATION_GRPC, this.taskExecutor).connect(adapter, returnValue);
+				return emitter;
+			}
 			if (CharSequence.class.isAssignableFrom(elementClass)) {
 				ResponseBodyEmitter emitter = getEmitter(mediaType.orElse(MediaType.TEXT_PLAIN));
-				new TextEmitterSubscriber(emitter, this.taskExecutor).connect(adapter, returnValue);
+				new BasicEmitterSubscriber(emitter, MediaType.TEXT_PLAIN, this.taskExecutor).connect(adapter, returnValue);
 				return emitter;
 			}
 			MediaType streamingResponseType = findConcreteJsonStreamMediaType(mediaTypes);
@@ -475,15 +482,18 @@ protected void send(Object element) throws IOException {
 	}
 
 
-	private static class TextEmitterSubscriber extends AbstractEmitterSubscriber {
+	private static class BasicEmitterSubscriber extends AbstractEmitterSubscriber {
+
+		private final MediaType mediaType;
 
-		TextEmitterSubscriber(ResponseBodyEmitter emitter, TaskExecutor executor) {
+		BasicEmitterSubscriber(ResponseBodyEmitter emitter, MediaType mediaType, TaskExecutor executor) {
 			super(emitter, executor, null);
+			this.mediaType = mediaType;
 		}
 
 		@Override
 		protected void send(Object element) throws IOException {
-			getEmitter().send(element, MediaType.TEXT_PLAIN);
+			getEmitter().send(element, this.mediaType);
 		}
 	}