allow returning CompletableFuture's from datafetchers

When returning `CompletableFuture`s from datafetchers we sometimes encounter an error in the lines of `Unknown value 'java.util.concurrent.CompletableFuture@e77cd7a[Completed normally]'`.
By converting the future to a mono it should be handled properly.

Author: koenpunt

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

@@ -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;
@@ -136,12 +137,16 @@ public Object invoke(DataFetchingEnvironment environment) {
 					if (result instanceof Mono) {
 						return (Mono<?>) result;
 					}
-					else if (result instanceof Flux) {
+
+					if (result instanceof Flux) {
 						return Flux.from((Flux<?>) result).collectList();
 					}
-					else {
-						return Mono.justOrEmpty(result);
+
+					if (result instanceof CompletableFuture<?>) {
+						return Mono.fromFuture((CompletableFuture<?>) result);
 					}
+
+					return Mono.justOrEmpty(result);
 				});
 	}
 

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

@@ -24,7 +24,6 @@
 import graphql.GraphQLContext;
 import graphql.schema.DataFetchingEnvironment;
 import graphql.schema.DataFetchingEnvironmentImpl;
-import io.micrometer.context.ContextSnapshot;
 import org.junit.jupiter.api.Test;
 import org.mockito.Mockito;
 
@@ -37,6 +36,7 @@
 import org.springframework.graphql.data.method.annotation.QueryMapping;
 import org.springframework.lang.Nullable;
 import org.springframework.util.ClassUtils;
+import reactor.core.publisher.Mono;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
@@ -86,6 +86,27 @@ void callableReturnValue() throws Exception {
 		assertThat(future.get()).isEqualTo("A");
 	}
 
+	@Test
+	void completableFutureReturnValue() throws Exception {
+
+		HandlerMethodArgumentResolverComposite resolvers = new HandlerMethodArgumentResolverComposite();
+		resolvers.addResolver(new ArgumentMethodArgumentResolver(new GraphQlArgumentBinder()));
+
+		DataFetcherHandlerMethod handlerMethod = new DataFetcherHandlerMethod(
+				handlerMethodFor(new TestController(), "handleAndReturnsCompletableFuture"), resolvers, null,
+				new SimpleAsyncTaskExecutor(), false);
+
+		DataFetchingEnvironment environment = DataFetchingEnvironmentImpl
+				.newDataFetchingEnvironment()
+				.build();
+
+		Object result = handlerMethod.invoke(environment);
+
+		assertThat(result).isInstanceOf(Mono.class);
+		Mono<String> mono = (Mono<String>) result;
+		assertThat(mono.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 +133,9 @@ public Callable<String> handleAndReturnCallable() {
 			return () -> "A";
 		}
 
+		public CompletableFuture<String> handleAndReturnsCompletableFuture() {
+			return CompletableFuture.completedFuture("B");
+		}
 	}
 
 }