Bean-获取、循环依赖

三级缓存中获取单例Bean

    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;
    }

singletonObjects：beanName -> beanInstance
earlySingletonObjects：beanName -> beanInstance
singletonFactories：beanName -> ObjectFactory

定义
在创建bean的过程中发现当前bean已经在“当前创建池”里时，将抛出BeanCurrentlyInCreationEception表示循环依赖。
构造器循环依赖（createBeanInstance）
加入singletonFactories缓存的前提是执行了构造器，所以构造器的循环依赖无法解决
setter循环依赖（populateBean）
对于setter注入造成的依赖是通过Spring容器提前暴露刚完成构造器注入但未完成其他步骤的bean来完成的，而且只能解决单例作用域的bean循环依赖。