What happens when you call SpringApplication.run()?

SpringApplication.run(MyApp.class, args) is a single call that performs the entire application bootstrap sequence:

1. Determine the application type — servlet-based web application, reactive web application, or a plain non-web application — inferred from what's on the classpath.
2. Prepare the Environment — load configuration from application.properties/.yml, environment variables, command-line arguments, and determine active profiles, all before any bean is created.
3. Print the banner (the ASCII-art Spring Boot banner, if not disabled) and run any registered ApplicationContextInitializers, which get a chance to further customize the context before it's refreshed.
4. Create the ApplicationContext — the concrete type matching the detected application type (e.g., AnnotationConfigServletWebServerApplicationContext for a servlet web app).
5. Refresh the context — this is where the bulk of the real work happens: component scanning discovers @Component-annotated classes, auto-configuration classes are evaluated and conditionally applied, all bean definitions are instantiated and wired together (following the bean lifecycle), and — for a web application — the embedded server starts as part of this same refresh.
6. Run runners — any beans implementing CommandLineRunner or ApplicationRunner are executed, in order, right after the context is fully ready — a common place to put one-off startup logic (seeding data, printing startup diagnostics).

Throughout this sequence, Spring Boot publishes a series of ApplicationEvents other beans can listen for via @EventListener: ApplicationStartingEvent, ApplicationEnvironmentPreparedEvent, ApplicationContextInitializedEvent, ApplicationPreparedEvent, ApplicationStartedEvent, and finally ApplicationReadyEvent — commonly used as the signal that the application is fully up and ready to serve traffic (e.g., to trigger a final cache warm-up or register with a service registry only once startup has genuinely completed).

@Component
class StartupLogger {
    @EventListener(ApplicationReadyEvent.class)
    void onReady() {
        System.out.println("Application fully started and ready to serve traffic");
    }
}