What Spring Boot Adds

What Spring Boot adds on top of Spring: a mechanism that creates the beans a typical application needs, based on the libraries it finds. Plain Spring needs you to register a DispatcherServlet, start a server and configure the rest by hand, and Boot makes that disappear behind one dependency and one annotation. It covers what Boot changes, the three annotations inside @SpringBootApplication, auto-configuration and its conditions, and what happens when you define a bean yourself. It leaves out writing your own auto-configuration.