Spring5 源码阅读笔记（1.4.1）getSingleton(beanName) 从缓存里拿单例实例

在 doGetBean 方法里我们先尝试从缓存里拿实例：

Object sharedInstance = getSingleton(beanName);

跟源码：
类 DefaultSingletonBeanRegistry

跟 getSingleton：

@Nullable
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
	//先从一级缓存拿
	Object singletonObject = this.singletonObjects.get(beanName);
	//如果bean正在创建。堆内存有了，属性还没有DI（依赖注入）
	if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
		synchronized (this.singletonObjects) {
			//从二级缓存中拿
			singletonObject = this.earlySingletonObjects.get(beanName);

			//如果还拿不到，并且允许bean提前引用（解决循环依赖）
			if (singletonObject == null && allowEarlyReference) {
				//从三级缓存中拿到对象工厂
				ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
				if (singletonFactory != null) {
					//从工厂中拿到对象
					singletonObject = singletonFactory.getObject();
					//升级到二级缓存
					this.earlySingletonObjects.put(beanName, singletonObject);
					//删除三级缓存
					this.singletonFactories.remove(beanName);
				}
			}
		}
	}
	return singletonObject;
}

介绍同样是这个类里的几个容器：

//一级缓存
/** Cache of singleton objects: bean name to bean instance. */
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);
//三级缓存
/** Cache of singleton factories: bean name to ObjectFactory. */
private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);
//二级缓存
/** Cache of early singleton objects: bean name to bean instance. */
private final Map<String, Object> earlySingletonObjects = new HashMap<>(16);
//正在实例化的bean
/** Names of beans that are currently in creation. */
private final Set<String> singletonsCurrentlyInCreation =
		Collections.newSetFromMap(new ConcurrentHashMap<>(16));

至于为什么要搞三个缓存，很重要的一个作用就是解决循环依赖。
如何解决，暂时不在这个小节讲述。