请问spring源码getSingleton中对ConcurrentHashMap加独占锁的意义是什么？

Question

private final Map<String, Object> singletonObjects = new ConcurrentHashMap<String, Object>(64);
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
        Object singletonObject = this.singletonObjects.get(beanName);
        if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
            synchronized (this.singletonObjects) {
                singletonObject = this.earlySingletonObjects.get(beanName);

这里singletonObjects已经是线程安全的Map集合了，再对其加synchronized的意义何在，希望有大神可以详细讲解

Answer 1

这不就是个单例模式嘛...和ConcurrentHashMap有什么关系

Answer 2

避免singletonObject多次生成，而不是控制对ConcurrentHashMap的操作，是控制对singletonFactory.getObject()的操作