spring如何解决循环引用的

https://zhuanlan.zhihu.com/p/84267654

https://www.imooc.com/article/34150

引用大牛的总结：

以下吸收归纳总结：

spring实例化一个Bean的时候：

          实例化目标Bean（此时首先放入singletonFacties中）

          注入其依赖属性

核心：

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

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

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