Quarkus Skill

Bean Scopes

• @ApplicationScoped — one instance per application (most common for services).
• @RequestScoped — one instance per HTTP request.
• @Singleton — like @ApplicationScoped but NOT proxied. Use only when proxy overhead matters.
• @Dependent — new instance per injection point. Avoid unless specifically needed.

Dependency Injection

• Use @Inject on fields (package-private preferred) or constructor parameters.
• @Inject is not required when there is exactly one constructor (Simplified Constructor Injection).
• @Inject is also not required when a qualifier annotation is used (e.g., @Channel("my-channel") Emitter<String> emitter).
• Use @Produces methods to create beans that need custom initialization.

Events

• Fire events with Event<T> injection and event.fire(payload).
• Observe with void onEvent(@Observes MyEvent event).
• Use @ObservesAsync for async event handling.

Lifecycle and Startup

• @Startup on a bean class forces eager initialization at startup.
• Use @PostConstruct and @PreDestroy for lifecycle hooks.

Interceptors

• Define interceptor binding with @InterceptorBinding.
• Implement interceptor with @Interceptor and @AroundInvoke.
• Lifecycle interceptors are also available: @PostConstruct, @PreDestroy, and @AroundConstruct.
• Register with @Priority(value).

Build-Time Discovery

• ArC discovers beans at build time (not runtime). Unscoped classes are NOT beans.
• Add a scope annotation to make a class a CDI bean. Note: some extensions add scope automatically (e.g. @Scheduled, @WebSocket).
• Instance<T> lookup is detected automatically. Use @Unremovable if a bean is only accessed via CDI.current() or ArC-specific APIs like Arc.container().

Testing

• Use @InjectMock to replace beans in @QuarkusTest.
• For @Singleton beans, use Mockito's @InjectSpy or configure the mock manually, since @InjectMock requires a client proxy (normal-scoped beans only).
• Use @QuarkusComponentTest for lightweight CDI-only tests (no full app startup).

Common Pitfalls

• Do NOT use new MyService() — always let CDI inject beans.
• @Singleton beans are NOT proxied — there is no client proxy indirection, which gives slightly better performance.
• Quarkus supports Simplified Constructor Injection: a no-args constructor is generated automatically if missing, and @Inject is not required when there is exactly one constructor.