What Boot actually is
If you have written any Spring at all, you have met the container — the engine that reads your classes, builds one object per bean, and wires them together. That engine is powerful. On its own, it is also demanding: it does nothing until you tell it exactly what to build.
Spring Boot does not replace that engine. It sits on top of it. Boot's whole job is to remove the setup work around the container — the long lists of beans you declare, the library versions you juggle, the web server you install — and swap it for sensible defaults you can override whenever you disagree.
That last clause is the entire philosophy, and it has a name: convention over configuration. Boot makes a reasonable guess for almost everything, so an app runs with near-zero setup. When a guess is wrong, you override just that one piece — and nothing else changes.
Let me show what that setup used to cost, because Boot only makes sense once you feel the problem it removes.
