Support virtual threads in webmvc.GraphQlWebSocketHandler

This commit ensures that the WebMVC `GraphQlWebSocketHandler` variant
uses a virtual threads factory on Java 21+. On lower Java versions, the
usual `Schedulers.newSingle` scheduler will be used.

Closes gh-1326

Author: bclozel

spring-graphql/build.gradle

@@ -1,7 +1,20 @@
+plugins {
+    id 'org.springframework.graphql.multiReleaseJar'
+}
+
 description = "GraphQL Support for Spring Applications"
 
 apply plugin: "kotlin"
 
+multiRelease {
+    releaseVersions 21
+}
+
+configurations {
+    java21Api.extendsFrom(api)
+    java21Implementation.extendsFrom(implementation)
+}
+
 dependencies {
 	api 'com.graphql-java:graphql-java'
 	api 'io.projectreactor:reactor-core'
@@ -99,3 +112,6 @@ dependencies {
 test {
 	useJUnitPlatform()
 }
+java21Test {
+    useJUnitPlatform()
+}

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

@@ -0,0 +1,38 @@
+/*
+ * Copyright 2025-present 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.graphql.execution;
+
+import reactor.core.scheduler.Scheduler;
+import reactor.core.scheduler.Schedulers;
+
+/**
+ * Factory for Reactor {@link Scheduler schedulers}.
+ * @author Brian Clozel
+ * @since 2.0.0
+ */
+public abstract class ReactorSchedulers {
+
+	/**
+	 * Create a scheduler backed by a single new thread, using {@link Schedulers#newSingle(String)} on Java < 21
+	 * and a custom Virtual Thread factory on Java >= 21.
+	 * @param name component and thread name prefix
+	 */
+	public static Scheduler singleThread(String name) {
+		return Schedulers.newSingle(name);
+	}
+
+}

spring-graphql/src/main/java/org/springframework/graphql/server/webmvc/GraphQlWebSocketHandler.java

@@ -45,10 +45,10 @@
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
 import reactor.core.scheduler.Scheduler;
-import reactor.core.scheduler.Schedulers;
 
 import org.springframework.aot.hint.annotation.RegisterReflectionForBinding;
 import org.springframework.graphql.execution.ErrorType;
+import org.springframework.graphql.execution.ReactorSchedulers;
 import org.springframework.graphql.execution.SubscriptionPublisherException;
 import org.springframework.graphql.server.WebGraphQlHandler;
 import org.springframework.graphql.server.WebGraphQlResponse;
@@ -487,7 +487,7 @@ private static class SessionState {
 
 		SessionState(String graphQlSessionId, WebMvcSessionInfo sessionInfo) {
 			this.sessionInfo = sessionInfo;
-			this.scheduler = Schedulers.newSingle("GraphQL-WsSession-" + graphQlSessionId);
+			this.scheduler = ReactorSchedulers.singleThread("GraphQL-WsSession-" + graphQlSessionId);
 			this.keepAliveSubscriber = new KeepAliveSubscriber(sessionInfo.getSession());
 		}
 

spring-graphql/src/main/java21/org/springframework/graphql/execution/ReactorSchedulers.java

@@ -0,0 +1,38 @@
+/*
+ * Copyright 2025-present 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.graphql.execution;
+
+import reactor.core.scheduler.Scheduler;
+import reactor.core.scheduler.Schedulers;
+
+/**
+ * Factory for Reactor {@link Scheduler schedulers}.
+ * @author Brian Clozel
+ * @since 2.0.0
+ */
+public abstract class ReactorSchedulers {
+
+	/**
+	 * Create a scheduler backed by a single new thread, using {@link Schedulers#newSingle(String)} on Java < 21
+	 * and a custom Virtual Thread factory on Java >= 21.
+	 * @param name component and thread name prefix
+	 */
+	public static Scheduler singleThread(String name) {
+		return Schedulers.newSingle(Thread.ofVirtual().name(name).factory());
+	}
+
+}

spring-graphql/src/main/java21/org/springframework/graphql/execution/package-info.java

@@ -0,0 +1,24 @@
+/*
+ * Copyright 2020-present 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/**
+ * Support for GraphQL request execution, including abstractions to configure and invoke
+ * {@code graphql.GraphQL}.
+ */
+@NullMarked
+package org.springframework.graphql.execution;
+
+import org.jspecify.annotations.NullMarked;

spring-graphql/src/test/java21/org/springframework/graphql/execution/ReactorSchedulersTests.java

@@ -0,0 +1,37 @@
+/*
+ * Copyright 2025-present 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.graphql.execution;
+
+import org.junit.jupiter.api.Test;
+import reactor.core.Disposable;
+import reactor.core.scheduler.Scheduler;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.awaitility.Awaitility.await;
+
+class ReactorSchedulersTests {
+
+	@Test
+	void singleThreadReturnsVirtualThread() {
+		Scheduler scheduler = ReactorSchedulers.singleThread("testThread");
+		scheduler.init();
+		Disposable disposable = scheduler.schedule(() -> assertThat(Thread.currentThread().isVirtual()).isTrue());
+		await().until(disposable::isDisposed);
+		scheduler.dispose();
+	}
+
+}