Lenient adapt non-Publisher subscription result

Closes gh-1213

spring-graphql/src/main/java/org/springframework/graphql/execution/ReactiveAdapterRegistryHelper.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2024 the original author or authors.
+ * Copyright 2002-2025 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.
@@ -25,7 +25,6 @@ import reactor.core.publisher.Mono;
 
 import org.springframework.core.ReactiveAdapter;
 import org.springframework.core.ReactiveAdapterRegistry;
-import org.springframework.util.Assert;
 
 /**
  * Helper to adapt a result Object to {@link Mono} or {@link Flux} through
@@ -90,7 +89,8 @@ public abstract class ReactiveAdapterRegistryHelper {
 
 	/**
 	 * Return a {@link Flux} for the given result Object, adapting to a
-	 * {@link Publisher} first if necessary via {@link ReactiveAdapterRegistry}.
+	 * {@link Publisher} via {@link ReactiveAdapterRegistry} or wrapping it as
+	 * {@code Flux} if necessary.
 	 * @param result the result Object to adapt
 	 * @return a {@link Flux}, possibly empty if the result is {@code null}
 	 */
@@ -102,8 +102,7 @@ public abstract class ReactiveAdapterRegistryHelper {
 			return Flux.from(publisher);
 		}
 		ReactiveAdapter adapter = registry.getAdapter(result.getClass());
-		Assert.state(adapter != null, "Expected Publisher for a subscription");
-		return Flux.from(adapter.toPublisher(result));
+		return ((adapter != null) ? Flux.from(adapter.toPublisher(result)) : Flux.just(result));
 	}
 
 	/**

spring-graphql/src/test/java/org/springframework/graphql/server/webflux/GraphQlSseHandlerTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2020-2024 the original author or authors.
+ * Copyright 2020-2025 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.
@@ -104,6 +104,26 @@ class GraphQlSseHandlerTests {
 				""");
 	}
 
+	@Test // gh-1213
+	void shouldHandleNonPublisherValue() {
+
+		SerializableGraphQlRequest request = initRequest(
+				"subscription TestSubscription { bookSearch(author:\"Orwell\") { id name } }");
+
+		GraphQlSseHandler handler = createHandler(env -> BookSource.getBook(1L));
+		MockServerHttpResponse response = handleRequest(this.httpRequest, handler, request);
+
+		assertThat(response.getHeaders().getContentType().isCompatibleWith(MediaType.TEXT_EVENT_STREAM)).isTrue();
+		assertThat(response.getBodyAsString().block()).isEqualTo("""
+				event:next
+				data:{"data":{"bookSearch":{"id":"1","name":"Nineteen Eighty-Four"}}}
+
+				event:complete
+				data:{}
+
+				""");
+	}
+
 	@Test
 	void shouldWriteEventsAndTerminalError() {
 

spring-graphql/src/test/java/org/springframework/graphql/server/webmvc/GraphQlSseHandlerTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2020-2024 the original author or authors.
+ * Copyright 2020-2025 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.
@@ -111,6 +111,25 @@ class GraphQlSseHandlerTests {
 				""");
 	}
 
+	@Test // gh-1213
+	void shouldHandleNonPublisherValue() throws Exception {
+		GraphQlSseHandler handler = createSseHandler(env -> BookSource.getBook(1L));
+		MockHttpServletRequest request = createServletRequest("""
+				{ "query": "subscription TestSubscription { bookSearch { id name } }" }
+				""");
+		MockHttpServletResponse response = handleAndAwait(request, handler);
+
+		assertThat(response.getContentType()).isEqualTo(MediaType.TEXT_EVENT_STREAM_VALUE);
+		assertThat(response.getContentAsString()).isEqualTo("""
+				event:next
+				data:{"data":{"bookSearch":{"id":"1","name":"Nineteen Eighty-Four"}}}
+
+				event:complete
+				data:
+
+				""");
+	}
+
 	@Test
 	void shouldWriteEventsAndTerminalError() throws Exception {