Merge pull request #16115 from JWThewes

* pr/16115:
  Polish "Check for multiple processors in OnClassCondition"
  Check for multiple processors in OnClassCondition

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

@@ -46,9 +46,23 @@ class OnClassCondition extends FilteringSpringBootCondition {
 	@Override
 	protected final ConditionOutcome[] getOutcomes(String[] autoConfigurationClasses,
 			AutoConfigurationMetadata autoConfigurationMetadata) {
-		// Split the work and perform half in a background thread. Using a single
+		// Split the work and perform half in a background thread if more than one
-		// additional thread seems to offer the best performance. More threads make
+		// processor is available. Using a single additional thread seems to offer the
-		// things worse
+		// best performance. More threads make things worse.
+		if (Runtime.getRuntime().availableProcessors() > 1) {
+			return resolveOutcomesThreaded(autoConfigurationClasses,
+					autoConfigurationMetadata);
+		}
+		else {
+			OutcomesResolver outcomesResolver = new StandardOutcomesResolver(
+					autoConfigurationClasses, 0, autoConfigurationClasses.length,
+					autoConfigurationMetadata, getBeanClassLoader());
+			return outcomesResolver.resolveOutcomes();
+		}
+	}
+
+	private ConditionOutcome[] resolveOutcomesThreaded(String[] autoConfigurationClasses,
+			AutoConfigurationMetadata autoConfigurationMetadata) {
 		int split = autoConfigurationClasses.length / 2;
 		OutcomesResolver firstHalfResolver = createOutcomesResolver(
 				autoConfigurationClasses, 0, split, autoConfigurationMetadata);