Consistently use local copy of volatile mainThreadPrefix field

Closes gh-34746

Author: jhoeller

spring-beans/src/main/java/org/springframework/beans/factory/support/DefaultListableBeanFactory.java

@@ -1066,8 +1066,9 @@ protected void checkMergedBeanDefinition(RootBeanDefinition mbd, String beanName
 	@Nullable
 	protected Boolean isCurrentThreadAllowedToHoldSingletonLock() {
 		String mainThreadPrefix = this.mainThreadPrefix;
-		if (this.mainThreadPrefix != null) {
-			// We only differentiate in the preInstantiateSingletons phase.
+		if (mainThreadPrefix != null) {
+			// We only differentiate in the preInstantiateSingletons phase, using
+			// the volatile mainThreadPrefix field as an indicator for that phase.
 
 			PreInstantiation preInstantiation = this.preInstantiationThread.get();
 			if (preInstantiation != null) {
@@ -1087,7 +1088,7 @@ protected Boolean isCurrentThreadAllowedToHoldSingletonLock() {
 			}
 			else if (this.strictLocking == null) {
 				// No explicit locking configuration -> infer appropriate locking.
-				if (mainThreadPrefix != null && !getThreadNamePrefix().equals(mainThreadPrefix)) {
+				if (!getThreadNamePrefix().equals(mainThreadPrefix)) {
 					// An unmanaged thread (assumed to be application-internal) with lenient locking,
 					// and not part of the same thread pool that provided the main bootstrap thread
 					// (excluding scenarios where we are hit by multiple external bootstrap threads).