Skip nullness check for Map-like return types of @BatchMapping methods

Prior to this commit, the schema inspector would perform nullness checks
for fields and data fetchers. `@BatchMapping` methods can return
`Map<Book, Author>` types and such cases can be raised as inconsistent
with the schema declaration. This check is invalid because we cannot
infer the nullness of the actual schema field given a `Map` fetched
using a `DataLoader`.

This commit ensures that such cases are skipped.

Closes gh-1334

Author: bclozel

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

@@ -235,10 +235,17 @@ private void checkDataFetcherNullness(FieldCoordinates fieldCoordinates, SelfDes
 		Method dataFetcherMethod = dataFetcher.asMethod();
 		if (dataFetcherMethod != null) {
 			Nullness applicationNullness = Nullness.forMethodReturnType(dataFetcherMethod);
-			// we cannot infer nullness if wrapped by reactive types
 			ReactiveAdapter reactiveAdapter = ReactiveAdapterRegistry.getSharedInstance()
 					.getAdapter(dataFetcherMethod.getReturnType());
-			if (reactiveAdapter == null && isMismatch(schemaNullness, applicationNullness)) {
+			if (reactiveAdapter != null) {
+				// we cannot infer nullness if wrapped by reactive types
+				logger.debug("Skip nullness check for data fetcher '" + dataFetcherMethod.getName() + "' because of Reactive return type.");
+			}
+			else if (dataFetcher.usesDataLoader() && Map.class.isAssignableFrom(dataFetcherMethod.getReturnType())) {
+				// we cannot infer nullness if batch loader method returns a Map
+				logger.debug("Skip nullness check for data fetcher '" + dataFetcherMethod.getName() + "' because of batch loading.");
+			}
+			else if (isMismatch(schemaNullness, applicationNullness)) {
 				DescribedAnnotatedElement annotatedElement = new DescribedAnnotatedElement(dataFetcherMethod, dataFetcher.getDescription());
 				this.reportBuilder.fieldNullnessMismatch(fieldCoordinates,
 						new DefaultNullnessMismatch(schemaNullness, applicationNullness, annotatedElement));

spring-graphql/src/test/java/org/springframework/graphql/execution/SchemaMappingInspectorNullnessTests.java

@@ -16,6 +16,8 @@
 
 package org.springframework.graphql.execution;
 
+import java.util.List;
+import java.util.Map;
 import java.util.concurrent.CompletableFuture;
 
 import graphql.Scalars;
@@ -36,6 +38,7 @@
 import org.junit.jupiter.api.Nested;
 import org.junit.jupiter.api.Test;
 
+import org.springframework.graphql.data.method.annotation.BatchMapping;
 import org.springframework.graphql.data.method.annotation.QueryMapping;
 import org.springframework.stereotype.Controller;
 
@@ -360,6 +363,27 @@ void reportIsEmptyWhenSchemaFieldNonNullAndAsyncTypeNullable() {
 			assertThatReport(report).isEmpty();
 		}
 
+		@Test
+		void reportIsEmptyWhenBatchMapping() {
+			String schema = """
+						type Query {
+							bookById(id: ID!): Book
+						}
+						type Book {
+							id: ID!
+							title: String!
+							author: Author
+						}
+						type Author {
+							name: String!
+						}
+					""";
+			SchemaReport report = inspectSchema(schema, BatchMappingBookController.class);
+			assertThatReport(report).isEmpty();
+		}
+
+
+
 		@Test
 		void doesNotFailWhenNullFieldDefinitionType() {
 			String schemaContent = """
@@ -422,10 +446,29 @@ static class NullableAsyncBookController {
 
 		}
 
+		@Controller
+		@NullMarked
+		static class BatchMappingBookController {
+
+			@QueryMapping
+			public @Nullable Book bookById(String id) {
+				return new Book("42", "Spring for GraphQL Book");
+			}
+
+			@BatchMapping
+			public Map<Book, Author> author(List<Book> books) {
+				return Map.of();
+			}
+		}
+
 		record Book(String id, String title) {
 
 		}
 
+		record Author(String name) {
+
+		}
+
 		static class NullFieldTypeVisitor extends GraphQLTypeVisitorStub {
 			@Override
 			public TraversalControl visitGraphQLFieldDefinition(GraphQLFieldDefinition node, TraverserContext<GraphQLSchemaElement> context) {