What's the difference between ApplicationContextInitializer, ApplicationRunner, and CommandLineRunner?

ApplicationContextInitializer runs very early, BEFORE the context is refreshed — used to programmatically customize the context (e.g. adding property sources) before any bean exists yet. ApplicationRunner and CommandLineRunner both run AFTER the context is fully started with every bean ready, differing only in how they receive command-line arguments: CommandLineRunner gets a raw String[] array, while ApplicationRunner gets a parsed ApplicationArguments object with named-option support (e.g. --name=value).