Spring对加载的bean之间循环依赖的处理
根据下面文档的叙述，简言之：
- 对于相互之间通过构造函数注入相互循环依赖的情况，Spring会抛出BeanCurrentlyInCreationException错误。
- 如果AB两个beans是通过属性注入的循环依赖(Singleton)，spring会通过先对其中一个bean赋值之后再对另外一个bean赋值，来打破循环依赖。因此不会有问题。实际上是用默认构造函数初始化A之后，放入earlyBeanFactory，使之可以被获取，然后实现循环依赖的处理。
3.对于prototype的bean，spring无法处理循环依赖，因为spring不缓存prototype的bean
Circular dependencies
If you use predominantly constructor injection, it is possible to create an unresolvable circular dependency scenario.
For example: Class A requires an instance of class B through constructor injection, and class B requires an instance of class A through constructor injection. If you configure beans for classes A and B to be injected into each other, the Spring IoC container detects this circular reference at runtime, and throws a >BeanCurrentlyInCreationException.
One possible solution is to edit the source code of some classes to be configured by setters rather than constructors. Alternatively, avoid constructor injection and use setter injection only. In other words, although it is not recommended, you can configure circular dependencies with setter injection.
Unlike the typical case (with no circular dependencies), a circular dependency between bean A and bean B forces one of the beans to be injected into the other prior to being fully initialized itself (a classic chicken/egg scenario).
