How to Execute Code After Spring Boot Startup and Monitor a Directory for Changes?

Executing Code Upon Spring Boot Startup

When building applications with Spring Boot, developers often encounter the need to execute specific tasks once the application has fully initialized. This article addresses this scenario, specifically focusing on monitoring a directory for changes after Spring Boot startup.

The use of threads for this purpose is problematic, as Spring's dependency injection framework (Autowiring) may not have fully initialized the services required by the monitoring task. Instead, Spring Boot provides an alternative approach using application events.

The ApplicationPreparedEvent event is dispatched before dependency injection is complete, making it unsuitable for our purpose. Fortunately, Spring Boot offers a more appropriate event:

ApplicationReadyEvent

This event triggers once the application is fully initialized and ready to process HTTP requests. By subscribing to this event, we can ensure that the code for monitoring the specified directory only executes when the application is operational.

The following code snippet demonstrates how to utilize the ApplicationReadyEvent:

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

This method will execute after the Spring Boot application has started and is ready to handle requests. The flexibility of the ApplicationReadyEvent allows for the execution of any desired code after application startup.

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