单例 Bean 中注入原型 Bean
Spring Bean 之间的注入和调用不可避免的会出现单例 Bean 和原型 Bean 互相调用的场景。在原型 Bean 中注入单例 Bean 不会出现什么问题，但是返回来在单例 Bean 中注入原型 Bean 不做特殊处理的话就会出现一些奇怪的问题。
比如 Spring 官网对该场景的描述
In most application scenarios, most beans in the container are singletons. When a singleton bean needs to collaborate with another singleton bean or a non-singleton bean needs to collaborate with another non-singleton bean, you typically handle the dependency by defining one bean as a property of the other. A problem arises when the bean lifecycles are different. Suppose singleton bean A needs to use non-singleton (prototype) bean B, perhaps on each method invocation on A. The container creates the singleton bean A only once, and thus only gets one opportunity to set the properties. The container cannot provide bean A with a new instance of bean B every time one is needed.