@PostConstruct and @PreDestroy — what do they do?

@PostConstruct and @PreDestroy are JSR-250 lifecycle annotations that mark a method to run right after dependency injection finishes and right before the bean is destroyed, respectively. They're the modern, framework-agnostic way to hook init and cleanup — no Spring interface required.

@PostConstruct

Marks a no-arg method that runs after the constructor and after all dependencies are injected, but before the bean is considered ready. This is the correct place for initialization that needs injected collaborators — something you can't do in the constructor, because at construction time field/setter injection hasn't happened yet.

@Component
public class CacheWarmer {
    private final ProductRepository repo;
    CacheWarmer(ProductRepository repo) { this.repo = repo; }

    @PostConstruct
    void warm() {
        // repo is guaranteed injected here; not safe in a setter-injected field at construction
        cache.putAll(repo.findHotProducts());
    }
}

In the lifecycle order it fires before InitializingBean.afterPropertiesSet() and any custom init-method.

@PreDestroy

Marks a method that runs when the container shuts down, before the bean is destroyed — the spot to release resources: close pools, flush buffers, stop schedulers.

@PreDestroy
void shutdown() { executor.shutdownNow(); }

The critical caveats

Package matters in Boot 3.x. With Spring 6 / Boot 3 on Jakarta EE 9+, these are jakarta.annotation.PostConstruct / jakarta.annotation.PreDestroy — not the old javax.annotation.*. Using the javax import silently does nothing.

Prototype beans never get @PreDestroy. The container creates and wires a prototype, then forgets it, so it never calls destruction callbacks. You must clean up prototype-scoped resources yourself.

Destruction only runs on graceful shutdown. A kill -9 or JVM crash skips @PreDestroy entirely — never rely on it for correctness-critical persistence.