Rethrow failure on main thread

Previously, if a failure occurred when evaluating conditions on a
separate thread, an NPE would occur on the main thread as the
expected array of outcomes was null.

This commit avoids the NPE and the lack of error reporting by
rethrowing on the main thread any failure that occurs on the
separate thread that's spawned to parallelize the evaluation.

Closes gh-41492

Author: wilkinsona

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2023 the original author or authors.
+ * Copyright 2012-2024 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.
@@ -29,6 +29,7 @@
 import org.springframework.core.annotation.Order;
 import org.springframework.core.type.AnnotatedTypeMetadata;
 import org.springframework.util.MultiValueMap;
+import org.springframework.util.ReflectionUtils;
 import org.springframework.util.StringUtils;
 
 /**
@@ -142,8 +143,17 @@ private static final class ThreadedOutcomesResolver implements OutcomesResolver
 
 		private volatile ConditionOutcome[] outcomes;
 
+		private volatile Throwable failure;
+
 		private ThreadedOutcomesResolver(OutcomesResolver outcomesResolver) {
-			this.thread = new Thread(() -> this.outcomes = outcomesResolver.resolveOutcomes());
+			this.thread = new Thread(() -> {
+				try {
+					this.outcomes = outcomesResolver.resolveOutcomes();
+				}
+				catch (Throwable ex) {
+					this.failure = ex;
+				}
+			});
 			this.thread.start();
 		}
 
@@ -155,6 +165,9 @@ public ConditionOutcome[] resolveOutcomes() {
 			catch (InterruptedException ex) {
 				Thread.currentThread().interrupt();
 			}
+			if (this.failure != null) {
+				ReflectionUtils.rethrowRuntimeException(this.failure);
+			}
 			return this.outcomes;
 		}