CommandLineRunner and ApplicationRunner — Spring Technical Interview Questions & Answers

Running code once at application startup.

CommandLineRunner and ApplicationRunner

  • CommandLineRunner and ApplicationRunner run code once after the app starts.
  • Useful for seed data, warmup, or printing startup info.
  • Both run when the context is ready; they differ mainly in argument style.
  • Keep them light — heavy work belongs in services/jobs.
  • Startup hooks for one-time initialization logic.

Technical Interview Tip: Used for one-time startup tasks.

Code Examples for CommandLineRunner and ApplicationRunner

CommandLineRunner and ApplicationRunner — working example

// Typical layout after Initializr (Maven):
// src/main/java/.../DemoApplication.java
// src/main/resources/application.properties
// pom.xml → spring-boot-starter-web

import org.springframework.boot.CommandLineRunner;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration
class BootHooks {
  @Bean
  CommandLineRunner seed() {
    return args -> System.out.println("app ready: " + String.join(",", args));
  }
}

CommandLineRunner and ApplicationRunner — create projects via Initializr/STS; run the @SpringBootApplication main class or package an executable jar.

How to Explain CommandLineRunner and ApplicationRunner to an Interviewer

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