Introduce spring.locking.strict=true flag for 6.1.x style bean creation locking

Closes gh-34303

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

@@ -76,6 +76,7 @@ import org.springframework.core.NamedThreadLocal;
 import org.springframework.core.OrderComparator;
 import org.springframework.core.Ordered;
 import org.springframework.core.ResolvableType;
+import org.springframework.core.SpringProperties;
 import org.springframework.core.annotation.MergedAnnotation;
 import org.springframework.core.annotation.MergedAnnotations;
 import org.springframework.core.annotation.MergedAnnotations.SearchStrategy;
@@ -128,6 +129,17 @@ import org.springframework.util.StringUtils;
 public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFactory
 		implements ConfigurableListableBeanFactory, BeanDefinitionRegistry, Serializable {
 
+	/**
+	 * System property that instructs Spring to enforce string 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.
+	 * @since 6.2.6
+	 * @see #preInstantiateSingletons()
+	 */
+	public static final String STRICT_LOCKING_PROPERTY_NAME = "spring.locking.strict";
+
+	private static final boolean lenientLockingAllowed = !SpringProperties.getFlag(STRICT_LOCKING_PROPERTY_NAME);
+
 	@Nullable
 	private static Class<?> jakartaInjectProviderClass;
 
@@ -1031,7 +1043,8 @@ public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFacto
 	@Override
 	@Nullable
 	protected Boolean isCurrentThreadAllowedToHoldSingletonLock() {
-		return (this.preInstantiationPhase ? this.preInstantiationThread.get() != PreInstantiation.BACKGROUND : null);
+		return (lenientLockingAllowed && this.preInstantiationPhase ?
+				this.preInstantiationThread.get() != PreInstantiation.BACKGROUND : null);
 	}
 
 	@Override

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

@@ -20,11 +20,15 @@ import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.Timeout;
 
 import org.springframework.beans.factory.ObjectProvider;
+import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
+import org.springframework.beans.factory.support.DefaultListableBeanFactory;
 import org.springframework.beans.testfixture.beans.TestBean;
 import org.springframework.context.ConfigurableApplicationContext;
+import org.springframework.core.SpringProperties;
 import org.springframework.core.testfixture.EnabledForTestGroups;
 import org.springframework.scheduling.concurrent.ThreadPoolTaskExecutor;
 
+import static org.assertj.core.api.Assertions.assertThat;
 import static org.springframework.context.annotation.Bean.Bootstrap.BACKGROUND;
 import static org.springframework.core.testfixture.TestGroup.LONG_RUNNING;
 
@@ -56,6 +60,21 @@ class BackgroundBootstrapTests {
 		ctx.close();
 	}
 
+	@Test
+	@Timeout(5)
+	@EnabledForTestGroups(LONG_RUNNING)
+	void bootstrapWithStrictLockingThread() {
+		SpringProperties.setFlag(DefaultListableBeanFactory.STRICT_LOCKING_PROPERTY_NAME);
+		try {
+			ConfigurableApplicationContext ctx = new AnnotationConfigApplicationContext(StrictLockingBeanConfig.class);
+			assertThat(ctx.getBean("testBean2", TestBean.class).getSpouse()).isSameAs(ctx.getBean("testBean1"));
+			ctx.close();
+		}
+		finally {
+			SpringProperties.setProperty(DefaultListableBeanFactory.STRICT_LOCKING_PROPERTY_NAME, null);
+		}
+	}
+
 	@Test
 	@Timeout(5)
 	@EnabledForTestGroups(LONG_RUNNING)
@@ -148,6 +167,28 @@ class BackgroundBootstrapTests {
 	}
 
 
+	@Configuration(proxyBeanMethods = false)
+	static class StrictLockingBeanConfig {
+
+		@Bean
+		public TestBean testBean1(ObjectProvider<TestBean> testBean2) {
+			new Thread(testBean2::getObject).start();
+			try {
+				Thread.sleep(1000);
+			}
+			catch (InterruptedException ex) {
+				throw new RuntimeException(ex);
+			}
+			return new TestBean();
+		}
+
+		@Bean
+		public TestBean testBean2(ConfigurableListableBeanFactory beanFactory) {
+			return new TestBean((TestBean) beanFactory.getSingleton("testBean1"));
+		}
+	}
+
+
 	@Configuration(proxyBeanMethods = false)
 	static class CircularReferenceBeanConfig {
 

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

@@ -39,6 +39,7 @@ import org.springframework.lang.Nullable;
  * @author Juergen Hoeller
  * @since 3.2.7
  * @see org.springframework.beans.StandardBeanInfoFactory#IGNORE_BEANINFO_PROPERTY_NAME
+ * @see org.springframework.beans.factory.support.DefaultListableBeanFactory#STRICT_LOCKING_PROPERTY_NAME
  * @see org.springframework.core.env.AbstractEnvironment#IGNORE_GETENV_PROPERTY_NAME
  * @see org.springframework.expression.spel.SpelParserConfiguration#SPRING_EXPRESSION_COMPILER_MODE_PROPERTY_NAME
  * @see org.springframework.jdbc.core.StatementCreatorUtils#IGNORE_GETPARAMETERTYPE_PROPERTY_NAME