一、Spring三级缓存的作用?

  解决对象之间的依赖问题

public class DefaultSingletonBeanRegistry extends SimpleAliasRegistry implements SingletonBeanRegistry {
        //1级缓存 用于存放 已经属性赋值 初始化后的 单列BEAN
        private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);
        //2级缓存 用于存在已经实例化，还未做代理属性赋值操作的 单例
        private final Map<String, Object> earlySingletonObjects = new HashMap<>(16);
        //3级缓存 存储单例BEAN的工厂
        private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);
        //已经注册的单例池里的beanName
        private final Set<String> registeredSingletons = new LinkedHashSet<>(256);
        //正在创建中的beanName
        private final Set<String> singletonsCurrentlyInCreation =
                Collections.newSetFromMap(new ConcurrentHashMap<>(16));
        //缓存查找bean  如果1级没有，从2级获取,也没有,从3级创建放入2级
        protected Object getSingleton(String beanName, boolean allowEarlyReference) {
            Object singletonObject = this.singletonObjects.get(beanName); //1级
            if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
                synchronized (this.singletonObjects) {
                    singletonObject = this.earlySingletonObjects.get(beanName); //2级
                    if (singletonObject == null && allowEarlyReference) {
                        //3级缓存  在doCreateBean中创建了bean的实例后，封装ObjectFactory放入缓存的
                        ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
                        if (singletonFactory != null) {
                            //创建未赋值的bean
                            singletonObject = singletonFactory.getObject();
                            //放入到二级缓存
                            this.earlySingletonObjects.put(beanName, singletonObject);
                            //从三级缓存删除
                            this.singletonFactories.remove(beanName);
                        }
                    }
                }
            }
            return singletonObject;
        }   
    }