Avoid full singleton lock for getSingleton(beanName, false)

Closes gh-25667

(cherry picked from commit d8c420a)

Author: jhoeller

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

@@ -81,7 +81,7 @@ public class DefaultSingletonBeanRegistry extends SimpleAliasRegistry implements
 	private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);
 
 	/** Cache of early singleton objects: bean name to bean instance. */
-	private final Map<String, Object> earlySingletonObjects = new HashMap<>(16);
+	private final Map<String, Object> earlySingletonObjects = new ConcurrentHashMap<>(16);
 
 	/** Set of registered singletons, containing the bean names in registration order. */
 	private final Set<String> registeredSingletons = new LinkedHashSet<>(256);
@@ -178,16 +178,24 @@ public Object getSingleton(String beanName) {
 	 */
 	@Nullable
 	protected Object getSingleton(String beanName, boolean allowEarlyReference) {
+		// Quick check for existing instance without full singleton lock
 		Object singletonObject = this.singletonObjects.get(beanName);
 		if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
-			synchronized (this.singletonObjects) {
-				singletonObject = this.earlySingletonObjects.get(beanName);
-				if (singletonObject == null && allowEarlyReference) {
-					ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
-					if (singletonFactory != null) {
-						singletonObject = singletonFactory.getObject();
-						this.earlySingletonObjects.put(beanName, singletonObject);
-						this.singletonFactories.remove(beanName);
+			singletonObject = this.earlySingletonObjects.get(beanName);
+			if (singletonObject == null && allowEarlyReference) {
+				synchronized (this.singletonObjects) {
+					// Consistent creation of early reference within full singleton lock
+					singletonObject = this.singletonObjects.get(beanName);
+					if (singletonObject == null) {
+						singletonObject = this.earlySingletonObjects.get(beanName);
+						if (singletonObject == null) {
+							ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
+							if (singletonFactory != null) {
+								singletonObject = singletonFactory.getObject();
+								this.earlySingletonObjects.put(beanName, singletonObject);
+								this.singletonFactories.remove(beanName);
+							}
+						}
 					}
 				}
 			}