Only perform background pre-init with multiple processors

Closes gh-15722

Author: wilkinsona

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/BackgroundPreinitializer.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2018 the original author or authors.
+ * Copyright 2012-2019 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.
@@ -70,7 +70,7 @@ public class BackgroundPreinitializer
 	@Override
 	public void onApplicationEvent(SpringApplicationEvent event) {
 		if (!Boolean.getBoolean(IGNORE_BACKGROUNDPREINITIALIZER_PROPERTY_NAME)
-				&& event instanceof ApplicationStartingEvent
+				&& event instanceof ApplicationStartingEvent && multipleProcessors()
 				&& preinitializationStarted.compareAndSet(false, true)) {
 			performPreinitialization();
 		}
@@ -86,6 +86,10 @@ public void onApplicationEvent(SpringApplicationEvent event) {
 		}
 	}
 
+	private boolean multipleProcessors() {
+		return Runtime.getRuntime().availableProcessors() > 1;
+	}
+
 	private void performPreinitialization() {
 		try {
 			Thread thread = new Thread(new Runnable() {