Revise singleton registry for lenient locking (fallback instead of deadlock)

Closes gh-23501

spring-beans/src/test/java/org/springframework/beans/factory/BeanFactoryLockingTests.java

@@ -0,0 +1,65 @@
+/*
+ * Copyright 2002-2024 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.beans.factory;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.beans.factory.support.DefaultListableBeanFactory;
+import org.springframework.beans.factory.support.RootBeanDefinition;
+import org.springframework.beans.testfixture.beans.TestBean;
+
+/**
+ * @author Juergen Hoeller
+ * @since 6.2
+ */
+class BeanFactoryLockingTests {
+
+	@Test
+	void fallbackForThreadDuringInitialization() {
+		DefaultListableBeanFactory beanFactory = new DefaultListableBeanFactory();
+		beanFactory.registerBeanDefinition("bean1", new RootBeanDefinition(ThreadDuringInitialization.class));
+		beanFactory.registerBeanDefinition("bean2", new RootBeanDefinition(TestBean.class));
+		beanFactory.getBean(ThreadDuringInitialization.class);
+	}
+
+
+	static class ThreadDuringInitialization implements BeanFactoryAware, InitializingBean {
+
+		private BeanFactory beanFactory;
+
+		private volatile boolean initialized;
+
+		@Override
+		public void setBeanFactory(BeanFactory beanFactory) {
+			this.beanFactory = beanFactory;
+		}
+
+		@Override
+		public void afterPropertiesSet() throws Exception {
+			Thread thread = new Thread(() -> {
+				beanFactory.getBean(TestBean.class);
+				initialized = true;
+			});
+			thread.start();
+			thread.join();
+			if (!initialized) {
+				throw new IllegalStateException("Thread not executed");
+			}
+		}
+	}
+
+}