How to execute code on Spring application start-up

The blog is originally published on my blog jsblogs.github.io

---

• Introduction
• Technologies used
• Application Events
• ApplicationRunner

Introduction

Have you ever encountered a situation where you've to perform some tasks immediately after the Spring/SpringBoot application starts. i.e.
Initialize some data into database, initialize application level constants, make an API call, etc.

There are several ways to achieve it. Here I'm gonna discuss about:

1. Application events
2. ApplicationRunner

Technologies used

1. Java 11
2. Spring Boot 2.2.4
3. Gradle 6.0.1

Application events

The Spring framework triggers various events. For our use case we'll be more interested in ContextStartedEvent and ContextRefreshedEvent.
ContextStartedEvent event triggered at the time of context gets started.
ContextRefreshedEvent event triggered at the time of context gets started or refreshed.

@Component
public class EventHandler {
    @EventListener(ContextStartedEvent.class)
    public void handleContextStartEvent(ContextStartedEvent e) {
        // Write your code here
    }

    @EventListener(ContextRefreshedEvent.class)
    public void handleContextRefreshEvent(ContextRefreshedEvent e) {
        // Write your code here
    }
    // Or you can handle both the events in 1 method  

    @EventListener({ContextStartedEvent.class, ContextRefreshedEvent.class})
    public void handleBoth(ApplicationContextEvent e) {
        if (e instanceof ContextStartedEvent) {

        } else {

        }
    }
}

ApplicationRunner

SpringBoot provides an interface called ApplicationRunner, any bean implementing this interface should run when that contained in the SpringApplication.

@Component
public class DBInitializer implements ApplicationRunner {

    private final UserRepository userRepository;

    private DBInitializer(UserRepository userRepository) {
        this.userRepository = userRepository;
    }

    @Override
    public void run(ApplicationArguments args) throws Exception {
        // Initialize user here
    }
}

or the above can be used

@Configuration
public class Config {

@Bean
    public ApplicationRunner initializeUser(UserRepository userRepository) {
        return args -> {
            // Initialize user here
        };
    }
}

ApplicationRunner provides ApplicationArguments in the run method which is used to get command line arguments by invoking getSourceArgs().
You can also get the parsed arguments using this class. i.e.

Let's say you've passed command line arguments like
--source /usr/local --print-only --target /tmp/local

So the method call to

1. getOptionNames() in ApplicationArguments will return set of arguments - ['source', 'print-only', 'target']
2. containsOption(String name) checks if argument contains
3. getOptionValues(name) returns list of option values. getOptionValues('source') will return list - ['/usr/local']