[java] Running code after Spring Boot starts

I want to run code after my spring-boot app starts to monitor a directory for changes.

I have tried running a new thread but the @Autowired services have not been set at that point.

I have been able to find ApplicationPreparedEvent, which fires before the @Autowired annotations are set. Ideally I would like the event to fire once the application is ready to process http requests.

Is there a better event to use, or a better way of running code after the application is live in spring-boot?

The "Spring Boot" way is to use a CommandLineRunner. Just add beans of that type and you are good to go. In Spring 4.1 (Boot 1.2) there is also a SmartInitializingBean which gets a callback after everything has initialized. And there is SmartLifecycle (from Spring 3).

Best way you use CommandLineRunner or ApplicationRunner The only difference between is run() method CommandLineRunner accepts array of string and ApplicationRunner accepts ApplicationArugument.

You can extend a class using ApplicationRunner , override the run() method and add the code there.

import org.springframework.boot.ApplicationRunner;

@Component
public class ServerInitializer implements ApplicationRunner {

    @Override
    public void run(ApplicationArguments applicationArguments) throws Exception {

        //code goes here

    }
}

Try this one and it will run your code when the application context has fully started.

 @Component
public class OnStartServer implements ApplicationListener<ContextRefreshedEvent> {

    @Override
    public void onApplicationEvent(ContextRefreshedEvent arg0) {
                // EXECUTE YOUR CODE HERE 
    }
}

I really like the suggestion for usage of the EventListener annotation by @cahen (https://stackoverflow.com/a/44923402/9122660) since it is very clean. Unfortunately I could not get this to work in a Spring + Kotlin setup. What does work for Kotlin is adding the class as a method parameter:

@EventListener 
fun doSomethingAfterStartup(event: ApplicationReadyEvent) {
    System.out.println("hello world, I have just started up");
}

It is as simple as this:

@EventListener(ApplicationReadyEvent.class)
public void doSomethingAfterStartup() {
    System.out.println("hello world, I have just started up");
}

Tested on version 1.5.1.RELEASE

ApplicationReadyEvent is really only useful if the task you want to perform is not a requirement for correct server operation. Starting an async task to monitor something for changes is a good example.

If, however your server is in a 'not ready' state until the task is completed then it's better to implement SmartInitializingSingleton because you'll get the callback before your REST port has been opened and your server is open for business.

Don't be tempted to use @PostConstruct for tasks that should only happen once ever. You'll get a rude surprise when you notice it being called multiple times...

With spring configuration :

@Configuration
public class ProjectConfiguration {
    private static final Logger log = 
   LoggerFactory.getLogger(ProjectConfiguration.class);

   @EventListener(ApplicationReadyEvent.class)
   public void doSomethingAfterStartup() {
    log.info("hello world, I have just started up");
  }
}

Best way to execute block of code after Spring Boot application started is using PostConstruct annotation.Or also you can use command line runner for the same.

1. Using PostConstruct annotation

@Configuration
public class InitialDataConfiguration {

    @PostConstruct
    public void postConstruct() {
        System.out.println("Started after Spring boot application !");
    }

}

2. Using command line runner bean

@Configuration
public class InitialDataConfiguration {

    @Bean
    CommandLineRunner runner() {
        return args -> {
            System.out.println("CommandLineRunner running in the UnsplashApplication class...");
        };
    }
}

you can use @Component

@RequiredArgsConstructor
@Component
@Slf4j
public class BeerLoader implements CommandLineRunner {
    //declare 

    @Override
    public void run(String... args) throws Exception {
        //some code here 

    }

just implement CommandLineRunner for spring boot application. You need to implement run method,

public classs SpringBootApplication implements CommandLineRunner{

    @Override
        public void run(String... arg0) throws Exception {
        // write your logic here 

        }
}

Have you tried ApplicationReadyEvent?

@Component
public class ApplicationStartup 
implements ApplicationListener<ApplicationReadyEvent> {

  /**
   * This event is executed as late as conceivably possible to indicate that 
   * the application is ready to service requests.
   */
  @Override
  public void onApplicationEvent(final ApplicationReadyEvent event) {

    // here your code ...

    return;
  }
}

Code from: http://blog.netgloo.com/2014/11/13/run-code-at-spring-boot-startup/

This is what the documentation mentions about the startup events:

...

Application events are sent in the following order, as your application runs:

An ApplicationStartedEvent is sent at the start of a run, but before any processing except the registration of listeners and initializers.

An ApplicationEnvironmentPreparedEvent is sent when the Environment to be used in the context is known, but before the context is created.

An ApplicationPreparedEvent is sent just before the refresh is started, but after bean definitions have been loaded.

An ApplicationReadyEvent is sent after the refresh and any related callbacks have been processed to indicate the application is ready to service requests.

An ApplicationFailedEvent is sent if there is an exception on startup.

...

Providing an example for Dave Syer answer, which worked like a charm:

@Component
public class CommandLineAppStartupRunner implements CommandLineRunner {
    private static final Logger logger = LoggerFactory.getLogger(CommandLineAppStartupRunner.class);

    @Override
    public void run(String...args) throws Exception {
        logger.info("Application started with command-line arguments: {} . \n To kill this application, press Ctrl + C.", Arrays.toString(args));
    }
}

Why not just create a bean that starts your monitor on initialization, something like:

@Component
public class Monitor {
    @Autowired private SomeService service

    @PostConstruct
    public void init(){
        // start your monitoring in here
    }
}

the init method will not be called until any autowiring is done for the bean.

Use a SmartInitializingSingleton bean in spring > 4.1

@Bean
public SmartInitializingSingleton importProcessor() {
    return () -> {
        doStuff();
    };

}

As alternative a CommandLineRunner bean can be implemented or annotating a bean method with @PostConstruct.