Configuration and First Project

Generating a Spring Boot project takes thirty seconds. What takes time, and what separates a project that ages well from one that turns into a minefield, is decided in the two hours that follow: where do the classes go, how is the configuration written, and how does the application behave when it moves to another environment?

The failure scenario is always the same. An application.properties that grows line by line over six months, with keys nobody remembers the effect of. A production database URL committed "temporarily." A @Value("${paiement.delai}") injected in four classes, one of which writes paiement.delais in the plural and silently picks up a default value. And on deployment day, an application that starts without errors and misbehaves.

None of these problems is fatal on its own. What they have in common is that they are detected late, when they could all have been caught at startup. That is exactly what this lesson sets up: a configuration that fails fast and loudly when it is incorrect, rather than drifting silently.

Here we generate biblio-api, the library catalog that will serve as our running example through the last lesson.