Avoid lenient locking for additional external bootstrap threads

Includes spring.locking.strict revision to differentiate between true, false, not set.
Includes checkFlag accessor on SpringProperties, also used in StatementCreatorUtils.

Closes gh-34729
See gh-34303

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

@@ -133,6 +133,11 @@ public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFacto
 	 * System property that instructs Spring to enforce strict locking during bean creation,
 	 * rather than the mix of strict and lenient locking that 6.2 applies by default. Setting
 	 * this flag to "true" restores 6.1.x style locking in the entire pre-instantiation phase.
+	 * <p>By default, the factory infers strict locking from the encountered thread names:
+	 * If additional threads have names that match the thread prefix of the main bootstrap thread,
+	 * they are considered external (multiple external bootstrap threads calling into the factory)
+	 * and therefore have strict locking applied to them. This inference can be turned off through
+	 * explicitly setting this flag to "false" rather than leaving it unspecified.
 	 * @since 6.2.6
 	 * @see #preInstantiateSingletons()
 	 */
@@ -157,8 +162,9 @@ public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFacto
 	private static final Map<String, Reference<DefaultListableBeanFactory>> serializableFactories =
 			new ConcurrentHashMap<>(8);
 
-	/** Whether lenient locking is allowed in this factory. */
-	private final boolean lenientLockingAllowed = !SpringProperties.getFlag(STRICT_LOCKING_PROPERTY_NAME);
+	/** Whether strict locking is enforced or relaxed in this factory. */
+	@Nullable
+	private final Boolean strictLocking = SpringProperties.checkFlag(STRICT_LOCKING_PROPERTY_NAME);
 
 	/** Optional id for this factory, for serialization purposes. */
 	@Nullable
@@ -214,6 +220,9 @@ public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFacto
 
 	private volatile boolean preInstantiationPhase;
 
+	@Nullable
+	private volatile String mainThreadPrefix;
+
 	private final NamedThreadLocal<PreInstantiation> preInstantiationThread =
 			new NamedThreadLocal<>("Pre-instantiation thread marker");
 
@@ -1045,7 +1054,7 @@ public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFacto
 			}
 		}
 		else {
-			// Bean intended to be initialized in main bootstrap thread
+			// Bean intended to be initialized in main bootstrap thread.
 			if (this.preInstantiationThread.get() == PreInstantiation.BACKGROUND) {
 				throw new BeanCurrentlyInCreationException(beanName, "Bean marked for mainline initialization " +
 						"but requested in background thread - enforce early instantiation in mainline thread " +
@@ -1057,8 +1066,28 @@ public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFacto
 	@Override
 	@Nullable
 	protected Boolean isCurrentThreadAllowedToHoldSingletonLock() {
-		return (this.lenientLockingAllowed && this.preInstantiationPhase ?
-				this.preInstantiationThread.get() != PreInstantiation.BACKGROUND : null);
+		if (this.preInstantiationPhase) {
+			// We only differentiate in the preInstantiateSingletons phase.
+			PreInstantiation preInstantiation = this.preInstantiationThread.get();
+			if (preInstantiation != null) {
+				// A Spring-managed thread:
+				// MAIN is allowed to lock (true) or even forced to lock (null),
+				// BACKGROUND is never allowed to lock (false).
+				return switch (preInstantiation) {
+					case MAIN -> (Boolean.TRUE.equals(this.strictLocking) ? null : true);
+					case BACKGROUND -> false;
+				};
+			}
+			if (Boolean.FALSE.equals(this.strictLocking) ||
+					(this.strictLocking == null && !getThreadNamePrefix().equals(this.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).
+				return true;
+			}
+		}
+		// Traditional behavior: forced to always hold a full lock.
+		return null;
 	}
 
 	@Override
@@ -1076,6 +1105,7 @@ public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFacto
 
 		this.preInstantiationPhase = true;
 		this.preInstantiationThread.set(PreInstantiation.MAIN);
+		this.mainThreadPrefix = getThreadNamePrefix();
 		try {
 			for (String beanName : beanNames) {
 				RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
@@ -1088,6 +1118,7 @@ public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFacto
 			}
 		}
 		finally {
+			this.mainThreadPrefix = null;
 			this.preInstantiationThread.remove();
 			this.preInstantiationPhase = false;
 		}
@@ -1183,6 +1214,12 @@ public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFacto
 		}
 	}
 
+	private static String getThreadNamePrefix() {
+		String name = Thread.currentThread().getName();
+		int numberSeparator = name.lastIndexOf('-');
+		return (numberSeparator >= 0 ? name.substring(0, numberSeparator) : name);
+	}
+
 
 	//---------------------------------------------------------------------
 	// Implementation of BeanDefinitionRegistry interface

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

@@ -272,7 +272,7 @@ public class DefaultSingletonBeanRegistry extends SimpleAliasRegistry implements
 						// Thread-safe exposure is still guaranteed, there is just a risk of collisions
 						// when triggering creation of other beans as dependencies of the current bean.
 						if (logger.isInfoEnabled()) {
-							logger.info("Creating singleton bean '" + beanName + "' in thread \"" +
+							logger.info("Obtaining singleton bean '" + beanName + "' in thread \"" +
 									Thread.currentThread().getName() + "\" while other thread holds " +
 									"singleton lock for other beans " + this.singletonsCurrentlyInCreation);
 						}
@@ -443,12 +443,16 @@ public class DefaultSingletonBeanRegistry extends SimpleAliasRegistry implements
 
 	/**
 	 * Determine whether the current thread is allowed to hold the singleton lock.
-	 * <p>By default, any thread may acquire and hold the singleton lock, except
-	 * background threads from {@link DefaultListableBeanFactory#setBootstrapExecutor}.
-	 * @return {@code false} if the current thread is explicitly not allowed to hold
-	 * the lock, {@code true} if it is explicitly allowed to hold the lock but also
-	 * accepts lenient fallback behavior, or {@code null} if there is no specific
-	 * indication (traditional behavior: always holding a full lock)
+	 * <p>By default, all threads are forced to hold a full lock through {@code null}.
+	 * {@link DefaultListableBeanFactory} overrides this to specifically handle its
+	 * threads during the pre-instantiation phase: {@code true} for the main thread,
+	 * {@code false} for managed background threads, and configuration-dependent
+	 * behavior for unmanaged threads.
+	 * @return {@code true} if the current thread is explicitly allowed to hold the
+	 * lock but also accepts lenient fallback behavior, {@code false} if it is
+	 * explicitly not allowed to hold the lock and therefore forced to use lenient
+	 * fallback behavior, or {@code null} if there is no specific indication
+	 * (traditional behavior: forced to always hold a full lock)
 	 * @since 6.2
 	 */
 	@Nullable