Keep mainThreadPrefix exposed until background init threads finished

Closes gh-35409

Author: jhoeller

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

@@ -1113,11 +1113,10 @@ public void preInstantiateSingletons() throws BeansException {
 		List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);
 
 		// Trigger initialization of all non-lazy singleton beans...
-		List<CompletableFuture<?>> futures = new ArrayList<>();
-
 		this.preInstantiationThread.set(PreInstantiation.MAIN);
 		this.mainThreadPrefix = getThreadNamePrefix();
 		try {
+			List<CompletableFuture<?>> futures = new ArrayList<>();
 			for (String beanName : beanNames) {
 				RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
 				if (!mbd.isAbstract() && mbd.isSingleton()) {
@@ -1127,21 +1126,20 @@ public void preInstantiateSingletons() throws BeansException {
 					}
 				}
 			}
+			if (!futures.isEmpty()) {
+				try {
+					CompletableFuture.allOf(futures.toArray(new CompletableFuture<?>[0])).join();
+				}
+				catch (CompletionException ex) {
+					ReflectionUtils.rethrowRuntimeException(ex.getCause());
+				}
+			}
 		}
 		finally {
 			this.mainThreadPrefix = null;
 			this.preInstantiationThread.remove();
 		}
 
-		if (!futures.isEmpty()) {
-			try {
-				CompletableFuture.allOf(futures.toArray(new CompletableFuture<?>[0])).join();
-			}
-			catch (CompletionException ex) {
-				ReflectionUtils.rethrowRuntimeException(ex.getCause());
-			}
-		}
-
 		// Trigger post-initialization callback for all applicable beans...
 		for (String beanName : beanNames) {
 			Object singletonInstance = getSingleton(beanName, false);