Customizing Spring Boot Startup — Banner, Listeners, and FailureAnalyzer

Spring Boot's startup pipeline has extension points at every phase — before the context exists, before beans are created, after everything is ready, and when startup fails. Knowing which hook fires when determines which customization goes where.

Before the context is created

• SpringApplication.addListeners(new MyListener()) registers listeners that fire for early events like ApplicationStartingEvent and ApplicationEnvironmentPreparedEvent
• EnvironmentPostProcessor — add property sources from external systems (Vault, AWS Secrets Manager) before @Value injection evaluates them
• ApplicationContextInitializer — set active profiles, add property sources, or modify the context before beans are registered

After the context is ready

• @EventListener(ApplicationReadyEvent.class) — fires when the application is fully started; the same timing as CommandLineRunner and ApplicationRunner
• @EventListener(ApplicationStartedEvent.class) — fires after context refresh but before runners

When startup fails

• FailureAnalyzer — implement this interface to produce a human-readable message explaining a startup failure; Spring Boot ships analyzers for port-in-use, missing beans, and failed conditions
• Your analyzer catches a specific exception type and returns an explanation with a suggested action — replace stack traces with actionable diagnostics for your team
• Register in META-INF/spring/org.springframework.boot.diagnostics.FailureAnalyzer (Boot 3.x) or spring.factories (older)

Banner

• Cosmetic but has operational value — identifies the app name and version in aggregated logs
• Disable with spring.main.banner-mode=off if CI logs need to stay clean

In an interview, the strong answer is: `FailureAnalyzer` for actionable error messages, `EnvironmentPostProcessor` for early property injection, listeners via `spring.factories` for pre-context events — three hooks for three different timing requirements.