4.4.1 The singleton scope
When a bean is a singleton, only one shared instance of the bean will be managed, and all requests for beans with
an id or ids matching that bean definition will result in that one
specific bean instance being returned by the Spring container.
To put it another way, when you define a bean definition and it is
scoped as a singleton, then the Spring IoC container will create
exactly one instance of the object defined by that bean definition.
This single instance will be stored in a cache of such singleton
beans, and all subsequent requests and references for that named bean
will result in the cached object being returned.