Fix issue returning CompletableFuture

Closes gh-628

spring-graphql/src/main/java/org/springframework/graphql/data/method/annotation/support/DataFetcherHandlerMethod.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2023 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.
@@ -16,6 +16,7 @@
 package org.springframework.graphql.data.method.annotation.support;
 
 import java.util.Arrays;
+import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.Executor;
 
 import graphql.schema.DataFetchingEnvironment;
@@ -133,11 +134,14 @@ public class DataFetcherHandlerMethod extends InvocableHandlerMethodSupport {
 				}) :
 				toArgsMono(args).flatMap(argValues -> {
 					Object result = validateAndInvoke(argValues, environment);
-					if (result instanceof Mono) {
+					if (result instanceof Mono<?>) {
 						return (Mono<?>) result;
 					}
-					else if (result instanceof Flux) {
-						return Flux.from((Flux<?>) result).collectList();
+					else if (result instanceof Flux<?>) {
+						return ((Flux<?>) result).collectList();
+					}
+					else if (result instanceof CompletableFuture<?>) {
+						return Mono.fromFuture((CompletableFuture<?>) result);
 					}
 					else {
 						return Mono.justOrEmpty(result);

spring-graphql/src/test/java/org/springframework/graphql/data/method/annotation/support/DataFetcherHandlerMethodTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2023 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.
@@ -26,6 +26,7 @@ import graphql.schema.DataFetchingEnvironment;
 import graphql.schema.DataFetchingEnvironmentImpl;
 import org.junit.jupiter.api.Test;
 import org.mockito.Mockito;
+import reactor.core.publisher.Mono;
 
 import org.springframework.core.task.SimpleAsyncTaskExecutor;
 import org.springframework.graphql.data.GraphQlArgumentBinder;
@@ -35,6 +36,8 @@ import org.springframework.graphql.data.method.HandlerMethodArgumentResolverComp
 import org.springframework.graphql.data.method.annotation.Argument;
 import org.springframework.graphql.data.method.annotation.QueryMapping;
 import org.springframework.lang.Nullable;
+import org.springframework.security.core.annotation.AuthenticationPrincipal;
+import org.springframework.security.core.userdetails.User;
 import org.springframework.util.ClassUtils;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -86,6 +89,23 @@ public class DataFetcherHandlerMethodTests {
 		assertThat(future.get()).isEqualTo("A");
 	}
 
+	@Test
+	void completableFutureReturnValue() {
+
+		HandlerMethodArgumentResolverComposite resolvers = new HandlerMethodArgumentResolverComposite();
+		resolvers.addResolver(new AuthenticationPrincipalArgumentResolver((beanName, context) -> null));
+		resolvers.addResolver(new ArgumentMethodArgumentResolver(new GraphQlArgumentBinder()));
+
+		DataFetcherHandlerMethod handlerMethod = new DataFetcherHandlerMethod(
+				handlerMethodFor(new TestController(), "handleAndReturnFuture"), resolvers,
+				null, null, false);
+
+		Object result = handlerMethod.invoke(DataFetchingEnvironmentImpl.newDataFetchingEnvironment().build());
+
+		assertThat(result).isInstanceOf(Mono.class);
+		assertThat(((Mono<String>) result).block()).isEqualTo("B");
+	}
+
 	private static HandlerMethod handlerMethodFor(Object controller, String methodName) {
 		Method method = ClassUtils.getMethod(controller.getClass(), methodName, (Class<?>[]) null);
 		return new HandlerMethod(controller, method);
@@ -112,6 +132,10 @@ public class DataFetcherHandlerMethodTests {
 			return () -> "A";
 		}
 
+		public CompletableFuture<String> handleAndReturnFuture(@AuthenticationPrincipal User user) {
+			return CompletableFuture.completedFuture("B");
+		}
+
 	}
 
 }