Lazy use of Reactor Scheduler in DefaultPartHttpMessageReader

Closes gh-33218

spring-web/src/main/java/org/springframework/http/codec/multipart/DefaultPartHttpMessageReader.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2023 the original author or authors.
+ * Copyright 2002-2024 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.
@@ -66,7 +66,8 @@ public class DefaultPartHttpMessageReader extends LoggingCodecSupport implements
 
 	private int maxParts = -1;
 
-	private Scheduler blockingOperationScheduler = Schedulers.boundedElastic();
+	@Nullable
+	private Scheduler blockingOperationScheduler;
 
 	private FileStorage fileStorage = FileStorage.tempDirectory(this::getBlockingOperationScheduler);
 
@@ -152,7 +153,8 @@ public class DefaultPartHttpMessageReader extends LoggingCodecSupport implements
 	}
 
 	private Scheduler getBlockingOperationScheduler() {
-		return this.blockingOperationScheduler;
+		return (this.blockingOperationScheduler != null ?
+				this.blockingOperationScheduler : Schedulers.boundedElastic());
 	}
 
 	/**
@@ -206,7 +208,7 @@ public class DefaultPartHttpMessageReader extends LoggingCodecSupport implements
 						else {
 							return PartGenerator.createPart(partsTokens,
 									this.maxInMemorySize, this.maxDiskUsagePerPart,
-									this.fileStorage.directory(), this.blockingOperationScheduler);
+									this.fileStorage.directory(), getBlockingOperationScheduler());
 						}
 					});
 		});