Creating a bean after application initialization and inserting it into
the context so that it becomes a Spring-managed bean.

https://docs.spring.io/spring/docs/4.3.14.RELEASE/spring-framework-reference/htmlsingle/#beans-definition

    In addition to bean definitions that contain information on how to
    create a specific bean, the ApplicationContext implementations also
    permit the registration of existing objects that are created outside
    the container, by users. This is done by accessing the
    ApplicationContext’s BeanFactory via the method getBeanFactory()
    which returns the BeanFactory implementation
    DefaultListableBeanFactory. DefaultListableBeanFactory supports this
    registration through the methods registerSingleton(..) and
    registerBeanDefinition(..).

(1) ApplicationContext doesn't have a .getBeanFactory() method.  I used
AbstractApplicationContext instead.

(2) getBeanFactory() doesn't return a DefaultListableBeanFactory, it
returns a ConfigurableListableBeanFactory.  I used
ConfigurableListableBeanFactory instead, seemed to work.

To run this:

./mvnw spring-boot:run

Somewhere in the output, this line should appear:

Book: Book [title=Craig Walls, author=Spring in Action]