Avoid blocking on session id generation

see #2393

spring-session-core/src/main/java/org/springframework/session/ReactiveMapSessionRepository.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2014-2022 the original author or authors.
+ * Copyright 2014-2023 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.
@@ -20,6 +20,7 @@
 import java.util.Map;
 
 import reactor.core.publisher.Mono;
+import reactor.core.scheduler.Schedulers;
 
 import org.springframework.session.events.SessionDeletedEvent;
 import org.springframework.session.events.SessionExpiredEvent;
@@ -37,6 +38,7 @@
  * </p>
  *
  * @author Rob Winch
+ * @author Yanming Zhou
  * @since 2.0
  */
 public class ReactiveMapSessionRepository implements ReactiveSessionRepository<MapSession> {
@@ -98,11 +100,17 @@ public Mono<Void> deleteById(String id) {
 
 	@Override
 	public Mono<MapSession> createSession() {
-		return Mono.defer(() -> {
-			MapSession result = new MapSession(this.sessionIdGenerator);
-			result.setMaxInactiveInterval(this.defaultMaxInactiveInterval);
-			return Mono.just(result);
-		});
+		// @formatter:off
+		return Mono.fromSupplier(() -> this.sessionIdGenerator.generate())
+				.subscribeOn(Schedulers.boundedElastic())
+				.publishOn(Schedulers.parallel())
+				.map((sessionId) -> {
+					MapSession result = new MapSession(sessionId);
+					result.setMaxInactiveInterval(this.defaultMaxInactiveInterval);
+					result.setSessionIdGenerator(this.sessionIdGenerator);
+					return result;
+				});
+		// @formatter:on
 	}
 
 	/**