spring源码-缓存中获取单例bean

spring 3.2 源码如下：

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);
                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);
                    }
                }
            }
        }
        return (singletonObject != NULL_OBJECT ? singletonObject : null);
    }

说明：
首先尝试从singletonObjects里面获取实例，如果获取不到再从earlySingletonObjects中获取，如果还是获取不到，再尝试从singletonFactories里面获取beanName对应的objectFactory，然后调用getObject来创建bean。并放到earlySingletonObjects里面去，并从singletonFactories里去掉。

这里涉及到不少Map

/** Cache of singleton objects: bean name --> bean instance */
    private final Map singletonObjects = new ConcurrentHashMap(64);

    /** Cache of singleton factories: bean name --> ObjectFactory */
    private final Map> singletonFactories = new HashMap>(16);

    /** Cache of early singleton objects: bean name --> bean instance */
    private final Map earlySingletonObjects = new HashMap(16);

    /** Set of registered singletons, containing the bean names in registration order */
    private final Set registeredSingletons = new LinkedHashSet(64);

1.singletonObjects : 用于保存beanName和创建bean实例之间的关系
2.singletonFactories ： 用于保存beanName和创建bean的工厂之间的关系
3.earlySingletonObjects ：也是保存beanName和bean实例之间的关系，与singletonObjects 的不同之处在于，当一个单例bean被放在这里后，那么当bean还在创建过程中，就可以通过getBean的方法获取了，其目的是用来检测循环引用。
4.registeredSingletons ： 用于保存当前所有已注册的bean。