Continue with pre-instantiation when current bean is in creation already

Closes gh-34349

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

@@ -1089,8 +1089,15 @@ else if (logger.isInfoEnabled()) {
 						"without bootstrap executor configured - falling back to mainline initialization");
 			}
 		}
+
 		if (!mbd.isLazyInit()) {
-			instantiateSingleton(beanName);
+			try {
+				instantiateSingleton(beanName);
+			}
+			catch (BeanCurrentlyInCreationException ex) {
+				logger.info("Bean '" + beanName + "' marked for pre-instantiation (not lazy-init) " +
+						"but currently initialized by other thread - skipping it in mainline thread");
+			}
 		}
 		return null;
 	}

spring-context/src/test/java/org/springframework/context/annotation/BackgroundBootstrapTests.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.
@@ -19,11 +19,14 @@
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.Timeout;
 
+import org.springframework.beans.factory.BeanCurrentlyInCreationException;
+import org.springframework.beans.factory.ObjectProvider;
 import org.springframework.beans.testfixture.beans.TestBean;
 import org.springframework.context.ConfigurableApplicationContext;
 import org.springframework.core.testfixture.EnabledForTestGroups;
 import org.springframework.scheduling.concurrent.ThreadPoolTaskExecutor;
 
+import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
 import static org.springframework.context.annotation.Bean.Bootstrap.BACKGROUND;
 import static org.springframework.core.testfixture.TestGroup.LONG_RUNNING;
 
@@ -33,6 +36,17 @@
  */
 class BackgroundBootstrapTests {
 
+	@Test
+	@Timeout(5)
+	@EnabledForTestGroups(LONG_RUNNING)
+	void bootstrapWithUnmanagedThread() {
+		ConfigurableApplicationContext ctx = new AnnotationConfigApplicationContext(UnmanagedThreadBeanConfig.class);
+		ctx.getBean("testBean1", TestBean.class);
+		assertThatExceptionOfType(BeanCurrentlyInCreationException.class).isThrownBy(  // late - not during refresh
+				() -> ctx.getBean("testBean2", TestBean.class));
+		ctx.close();
+	}
+
 	@Test
 	@Timeout(5)
 	@EnabledForTestGroups(LONG_RUNNING)
@@ -45,7 +59,35 @@ void bootstrapWithCustomExecutor() {
 	}
 
 
-	@Configuration
+	@Configuration(proxyBeanMethods = false)
+	static class UnmanagedThreadBeanConfig {
+
+		@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() {
+			try {
+				Thread.sleep(2000);
+			}
+			catch (InterruptedException ex) {
+				throw new RuntimeException(ex);
+			}
+			return new TestBean();
+		}
+	}
+
+
+	@Configuration(proxyBeanMethods = false)
 	static class CustomExecutorBeanConfig {
 
 		@Bean
@@ -58,7 +100,7 @@ public ThreadPoolTaskExecutor bootstrapExecutor() {
 		}
 
 		@Bean(bootstrap = BACKGROUND) @DependsOn("testBean3")
-		public TestBean testBean1(TestBean testBean3) throws InterruptedException{
+		public TestBean testBean1(TestBean testBean3) throws InterruptedException {
 			Thread.sleep(3000);
 			return new TestBean();
 		}