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

spring-context/src/test/java/org/springframework/context/annotation/BackgroundBootstrapTests.java

@@ -16,6 +16,9 @@
 
 package org.springframework.context.annotation;
 
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.Timeout;
 
@@ -67,7 +70,7 @@ class BackgroundBootstrapTests {
 	@Test
 	@Timeout(10)
 	@EnabledForTestGroups(LONG_RUNNING)
-	void bootstrapWithStrictLockingThread() {
+	void bootstrapWithStrictLockingFlag() {
 		SpringProperties.setFlag(DefaultListableBeanFactory.STRICT_LOCKING_PROPERTY_NAME);
 		try {
 			ConfigurableApplicationContext ctx = new AnnotationConfigApplicationContext(StrictLockingBeanConfig.class);
@@ -79,6 +82,42 @@ class BackgroundBootstrapTests {
 		}
 	}
 
+	@Test
+	@Timeout(10)
+	@EnabledForTestGroups(LONG_RUNNING)
+	void bootstrapWithStrictLockingInferred() throws InterruptedException {
+		AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext();
+		ctx.register(InferredLockingBeanConfig.class);
+		ExecutorService threadPool = Executors.newFixedThreadPool(2);
+		threadPool.submit(() -> ctx.refresh());
+		Thread.sleep(500);
+		threadPool.submit(() -> ctx.getBean("testBean2"));
+		Thread.sleep(1000);
+		assertThat(ctx.getBean("testBean2", TestBean.class).getSpouse()).isSameAs(ctx.getBean("testBean1"));
+		ctx.close();
+	}
+
+	@Test
+	@Timeout(10)
+	@EnabledForTestGroups(LONG_RUNNING)
+	void bootstrapWithStrictLockingTurnedOff() throws InterruptedException {
+		SpringProperties.setFlag(DefaultListableBeanFactory.STRICT_LOCKING_PROPERTY_NAME, false);
+		try {
+			AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext();
+			ctx.register(InferredLockingBeanConfig.class);
+			ExecutorService threadPool = Executors.newFixedThreadPool(2);
+			threadPool.submit(() -> ctx.refresh());
+			Thread.sleep(500);
+			threadPool.submit(() -> ctx.getBean("testBean2"));
+			Thread.sleep(1000);
+			assertThat(ctx.getBean("testBean2", TestBean.class).getSpouse()).isNull();
+			ctx.close();
+		}
+		finally {
+			SpringProperties.setProperty(DefaultListableBeanFactory.STRICT_LOCKING_PROPERTY_NAME, null);
+		}
+	}
+
 	@Test
 	@Timeout(10)
 	@EnabledForTestGroups(LONG_RUNNING)
@@ -128,6 +167,24 @@ class BackgroundBootstrapTests {
 		ctx.close();
 	}
 
+	@Test
+	@Timeout(10)
+	@EnabledForTestGroups(LONG_RUNNING)
+	void bootstrapWithCustomExecutorAndStrictLocking() {
+		SpringProperties.setFlag(DefaultListableBeanFactory.STRICT_LOCKING_PROPERTY_NAME);
+		try {
+			ConfigurableApplicationContext ctx = new AnnotationConfigApplicationContext(CustomExecutorBeanConfig.class);
+			ctx.getBean("testBean1", TestBean.class);
+			ctx.getBean("testBean2", TestBean.class);
+			ctx.getBean("testBean3", TestBean.class);
+			ctx.getBean("testBean4", TestBean.class);
+			ctx.close();
+		}
+		finally {
+			SpringProperties.setProperty(DefaultListableBeanFactory.STRICT_LOCKING_PROPERTY_NAME, null);
+		}
+	}
+
 
 	@Configuration(proxyBeanMethods = false)
 	static class UnmanagedThreadBeanConfig {
@@ -220,6 +277,27 @@ class BackgroundBootstrapTests {
 	}
 
 
+	@Configuration(proxyBeanMethods = false)
+	static class InferredLockingBeanConfig {
+
+		@Bean
+		public TestBean testBean1() {
+			try {
+				Thread.sleep(1000);
+			}
+			catch (InterruptedException ex) {
+				Thread.currentThread().interrupt();
+			}
+			return new TestBean("testBean1");
+		}
+
+		@Bean
+		public TestBean testBean2(ConfigurableListableBeanFactory beanFactory) {
+			return new TestBean((TestBean) beanFactory.getSingleton("testBean1"));
+		}
+	}
+
+
 	@Configuration(proxyBeanMethods = false)
 	static class CircularReferenceAgainstMainThreadBeanConfig {
 
@@ -377,13 +455,13 @@ class BackgroundBootstrapTests {
 
 		@Bean(bootstrap = BACKGROUND) @DependsOn("testBean3")
 		public TestBean testBean1(TestBean testBean3) throws InterruptedException {
-			Thread.sleep(3000);
+			Thread.sleep(6000);
 			return new TestBean();
 		}
 
 		@Bean(bootstrap = BACKGROUND) @Lazy
 		public TestBean testBean2() throws InterruptedException {
-			Thread.sleep(3000);
+			Thread.sleep(6000);
 			return new TestBean();
 		}
 

spring-core/src/main/java/org/springframework/core/SpringProperties.java

@@ -118,7 +118,18 @@ public final class SpringProperties {
 	 * @param key the property key
 	 */
 	public static void setFlag(String key) {
-		localProperties.put(key, Boolean.TRUE.toString());
+		localProperties.setProperty(key, Boolean.TRUE.toString());
+	}
+
+	/**
+	 * Programmatically set a local flag to the given value, overriding
+	 * an entry in the {@code spring.properties} file (if any).
+	 * @param key the property key
+	 * @param value the associated boolean value
+	 * @since 6.2.6
+	 */
+	public static void setFlag(String key, boolean value) {
+		localProperties.setProperty(key, Boolean.toString(value));
 	}
 
 	/**
@@ -131,4 +142,19 @@ public final class SpringProperties {
 		return Boolean.parseBoolean(getProperty(key));
 	}
 
+	/**
+	 * Retrieve the flag for the given property key, returning {@code null}
+	 * instead of {@code false} in case of no actual flag set.
+	 * @param key the property key
+	 * @return {@code true} if the property is set to the string "true"
+	 * (ignoring case), {@code} false if it is set to any other value,
+	 * {@code null} if it is not set at all
+	 * @since 6.2.6
+	 */
+	@Nullable
+	public static Boolean checkFlag(String key) {
+		String flag = getProperty(key);
+		return (flag != null ? Boolean.valueOf(flag) : null);
+	}
+
 }

spring-jdbc/src/main/java/org/springframework/jdbc/core/StatementCreatorUtils.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2024 the original author or authors.
+ * Copyright 2002-2025 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.
@@ -86,7 +86,7 @@ public abstract class StatementCreatorUtils {
 	private static final Map<Class<?>, Integer> javaTypeToSqlTypeMap = new HashMap<>(64);
 
 	@Nullable
-	static Boolean shouldIgnoreGetParameterType;
+	static Boolean shouldIgnoreGetParameterType = SpringProperties.checkFlag(IGNORE_GETPARAMETERTYPE_PROPERTY_NAME);
 
 	static {
 		javaTypeToSqlTypeMap.put(boolean.class, Types.BOOLEAN);
@@ -115,11 +115,6 @@ public abstract class StatementCreatorUtils {
 		javaTypeToSqlTypeMap.put(java.sql.Timestamp.class, Types.TIMESTAMP);
 		javaTypeToSqlTypeMap.put(Blob.class, Types.BLOB);
 		javaTypeToSqlTypeMap.put(Clob.class, Types.CLOB);
-
-		String flag = SpringProperties.getProperty(IGNORE_GETPARAMETERTYPE_PROPERTY_NAME);
-		if (flag != null) {
-			shouldIgnoreGetParameterType = Boolean.valueOf(flag);
-		}
 	}