将初始化代码添加到 Spring 引导应用程序的正确方法是什么？

Question

TLDR: I want my Spring Boot application to run some initialization code when it starts. The code needs access to Spring beans and values.

I'm writing a Spring Boot application that will consume multiple messages from a queue concurrently. In order to do this, it needs to instantiate multiple consumer objects. Does Spring have a good way to instantiate a configurable number of instances of the same class?

The queue client I have to use acts as a thread pool. It creates one thread for every consumer object I give it. The consumer objects only receive one message at a time, and they have to fully process and acknowledge the message before they can receive another one. The consumers aren't thread-safe, so I can't just use a singleton instance.

I considered the approach below, but it doesn't feel right to me. It seems like an abuse of the @Component annotation because the Initializer instance isn't used after it's constructed. What's a better way to do it?

@Component
public class Initializer {

    public Initializer(ConsumerRegistry registry, @Value("${consumerCount}") int consumerCount) {
        for (int i = 0; i < consumerCount; i++) {
            // Each registered consumer results in a thread that consumes messages.
            // Incoming messages will be delivered to any consumer thread that's not busy.
            registry.registerConsumer(new Consumer());
        }
    }

}

Answer 1

An ApplicationListener would fit your need. It gets notified on the registered event eg when the ApplicationContext is ready. You will have full access to all your Beans and injection.

@Component
public class StartupApplicationListener implements ApplicationListener<ApplicationReadyEvent> {

    @Inject
    private ConsumerRegistry registry;

    @Inject
    @Value("${consumerCount}")
    private int consumerCount;

    @Override
    public void onApplicationEvent(ApplicationReadyEvent event) {
        //do your logic
        for (int i = 0; i < consumerCount; i++) {
            // Each registered consumer results in a thread that consumes messages.
            // Incoming messages will be delivered to any consumer thread that's not busy.
            registry.registerConsumer(new Consumer());
        }
    }
}