Check for multiple processors in OnClassCondition

See gh-16115

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

@@ -49,6 +49,25 @@ class OnClassCondition extends FilteringSpringBootCondition {
 		// Split the work and perform half in a background thread. Using a single
 		// additional thread seems to offer the best performance. More threads make
 		// things worse
+		if (Runtime.getRuntime().availableProcessors() > 1) {
+			return resolveOutcomesThreaded(autoConfigurationClasses,
+					autoConfigurationMetadata);
+		}
+		else {
+			return resolveOutcomes(autoConfigurationClasses, autoConfigurationMetadata);
+		}
+	}
+
+	private ConditionOutcome[] resolveOutcomes(String[] autoConfigurationClasses,
+			AutoConfigurationMetadata autoConfigurationMetadata) {
+		OutcomesResolver outcomesResolver = createOutcomesResolver(
+				autoConfigurationClasses, 0, autoConfigurationClasses.length,
+				autoConfigurationMetadata);
+		return outcomesResolver.resolveOutcomes();
+	}
+
+	private ConditionOutcome[] resolveOutcomesThreaded(String[] autoConfigurationClasses,
+			AutoConfigurationMetadata autoConfigurationMetadata) {
 		int split = autoConfigurationClasses.length / 2;
 		OutcomesResolver firstHalfResolver = createOutcomesResolver(
 				autoConfigurationClasses, 0, split, autoConfigurationMetadata);