Fix GraphQL Java 21 compatibility issue

Closes gh-963

Author: rstoyanchev

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

@@ -16,6 +16,7 @@
 
 package org.springframework.graphql.execution;
 
+import java.lang.reflect.Field;
 import java.util.ArrayList;
 import java.util.List;
 import java.util.function.BiFunction;
@@ -33,6 +34,8 @@
 import org.springframework.graphql.ExecutionGraphQlResponse;
 import org.springframework.graphql.ExecutionGraphQlService;
 import org.springframework.graphql.support.DefaultExecutionGraphQlResponse;
+import org.springframework.util.ClassUtils;
+import org.springframework.util.ReflectionUtils;
 
 /**
  * {@link ExecutionGraphQlService} that uses a {@link GraphQlSource} to obtain a
@@ -43,6 +46,9 @@
  */
 public class DefaultExecutionGraphQlService implements ExecutionGraphQlService {
 
+	private static final boolean belowGraphQlJava22 = ClassUtils.isPresent(
+			DataLoaderRegistry21Initializer.CLASS_NAME, ExecutionGraphQlService.class.getClassLoader());
+
 	private static final BiFunction<ExecutionInput, ExecutionInput.Builder, ExecutionInput> RESET_EXECUTION_ID_CONFIGURER =
 			(executionInput, builder) -> builder.executionId(null).build();
 
@@ -55,11 +61,15 @@ public class DefaultExecutionGraphQlService implements ExecutionGraphQlService {
 
 	private final boolean isDefaultExecutionIdProvider;
 
+	private final Object emptyDataLoaderRegistryInstance;
+
 
 	public DefaultExecutionGraphQlService(GraphQlSource graphQlSource) {
 		this.graphQlSource = graphQlSource;
 		this.isDefaultExecutionIdProvider =
 				(graphQlSource.graphQl().getIdProvider() == ExecutionIdProvider.DEFAULT_EXECUTION_ID_PROVIDER);
+		this.emptyDataLoaderRegistryInstance = (belowGraphQlJava22) ?
+				DataLoaderRegistry21Initializer.getInstance() : DataLoaderRegistry22Initializer.getInstance();
 	}
 
 
@@ -105,7 +115,7 @@ public final Mono<ExecutionGraphQlResponse> execute(ExecutionGraphQlRequest requ
 	private ExecutionInput registerDataLoaders(ExecutionInput executionInput) {
 		GraphQLContext graphQLContext = executionInput.getGraphQLContext();
 		DataLoaderRegistry existingRegistry = executionInput.getDataLoaderRegistry();
-		if (existingRegistry == EmptyDataLoaderRegistryInstance.EMPTY_DATALOADER_REGISTRY) {
+		if (existingRegistry == this.emptyDataLoaderRegistryInstance) {
 			DataLoaderRegistry newRegistry = DataLoaderRegistry.newRegistry().build();
 			applyDataLoaderRegistrars(newRegistry, graphQLContext);
 			executionInput = executionInput.transform((builder) -> builder.dataLoaderRegistry(newRegistry));
@@ -120,4 +130,30 @@ private void applyDataLoaderRegistrars(DataLoaderRegistry registry, GraphQLConte
 		this.dataLoaderRegistrars.forEach((registrar) -> registrar.registerDataLoaders(registry, graphQLContext));
 	}
 
+
+	private static final class DataLoaderRegistry22Initializer {
+
+		public static Object getInstance() {
+			return EmptyDataLoaderRegistryInstance.EMPTY_DATALOADER_REGISTRY;
+		}
+	}
+
+
+	private static final class DataLoaderRegistry21Initializer {
+
+		public static final String CLASS_NAME =
+				"graphql.execution.instrumentation.dataloader.DataLoaderDispatcherInstrumentationState";
+
+		@SuppressWarnings("DataFlowIssue")
+		public static Object getInstance() {
+			try {
+				Field field = ReflectionUtils.findField(Class.forName(CLASS_NAME), "EMPTY_DATALOADER_REGISTRY");
+				return ReflectionUtils.getField(field, null);
+			}
+			catch (ClassNotFoundException ex) {
+				throw new RuntimeException(ex);
+			}
+		}
+	}
+
 }