Running a Spring Boot App and CommandLineRunner

Running a Spring Boot Application

Using the Main Method

@SpringBootApplication
public class DemoApplication {

    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }
}

Using Maven

mvn spring-boot:run

Running the Packaged JAR

java -jar app.jar

CommandLineRunner

CommandLineRunner executes code immediately after the Spring Boot application has started.

@Component
public class StartupRunner implements CommandLineRunner {

    @Override
    public void run(String... args) {
        System.out.println("Application started!");
    }
}

Common Use Cases

• Data initialization
• Cache preloading
• Startup validation
• Scheduled initialization tasks

CommandLineRunner vs ApplicationRunner

Interface	Parameter
CommandLineRunner	String... args
ApplicationRunner	ApplicationArguments

Key Takeaways

• mvn spring-boot:run is convenient for local development; java -jar app.jar is how it actually runs in production once packaged.
• CommandLineRunner and ApplicationRunner both run once, right after the application context is fully started — see the startup lifecycle for exactly where this fits.
• Use ApplicationRunner over CommandLineRunner when you need structured access to arguments (ApplicationArguments) rather than a raw String[].