Auto-Configuration

How does Spring Boot know that the booking service needs a Tomcat server and a JSON mapper, but not a database connection? Nobody wrote that down. The service has one controller, a build file with one starter, and not a single @Bean method of its own.

The answer is a list of candidate classes and a set of tests on each one. Spring Boot keeps a candidate only if the classes it needs are present and you have not already defined its bean. The sections below show where the candidates come from, what one looks like, how its tests work, and a plain-Java evaluator that makes the same decisions.