If you’re building Jakarta EE apps, CDI (Contexts and Dependency Injection) is the “glue” that makes your code modular, testable, and clean.

Here’s the mental model + the key building blocks you’ll use daily. 👇

🔸 TLDR ✅

▪️ Beans are managed objects (CDI creates them).

▪️ @Inject wires dependencies without new.

▪️ Qualifiers disambiguate “which implementation?”.

▪️ Scopes control lifecycle (request, session, app…).

🔸 NAMED BEANS 🏷️

A named bean is a CDI bean that also has a name you can reference (commonly in UI layers like JSF).

▪️ @Named exposes the bean under a name (default: class name with lowercase first letter)

▪️ Useful when a template / UI expression needs a reference

🔸 DEPENDENCY INJECTION 💉

Instead of manually creating dependencies, you ask CDI to provide them.

▪️ Use @Inject to request a dependency

▪️ CDI selects a matching bean and manages its lifecycle

▪️ Your code becomes easier to test and refactor

🔸 QUALIFIERS 🎯

When multiple beans match the same type, CDI needs help choosing the right one.

▪️ Create a custom @Qualifier

▪️ Annotate both the implementation and the injection point

▪️ Makes intent explicit (“use the fast one”, “use the mock one”, etc.)

🔸 NAMED BEANS SCOPES ⏳

Scope = lifecycle + where the instance “lives”.

Common CDI scopes:

▪️ @Dependent → default; “tied to injection point” (new instance often)

▪️ @RequestScoped → one instance per HTTP request 🌐

▪️ @SessionScoped → one per user session 👤 (must be Serializable)

▪️ @ApplicationScoped → one per app (singleton-ish) 🏢

▪️ @ConversationScoped → longer than request, shorter than session (less used)

Rule of thumb:

▪️ Stateless service → @ApplicationScoped (or @Dependent)

▪️ Web/request data → @RequestScoped

▪️ User state → @SessionScoped (careful with memory)

🔸 CDI EVENTS (FIRING, HANDLING, ORDERING) 📣

Events = publish/subscribe inside your app (decoupled communication).

Fire an event:

Observe an event:

Ordering observers:

▪️ Use @Priority to control observer order (lower value runs first)

Bonus patterns you’ll see:

▪️ Async events with fireAsync(...) (when supported by your CDI version/runtime) ⚡

▪️ Filtering with qualifiers on events (e.g., @Urgent) to target specific listeners 🎯

🔸 TAKEAWAYS 🧠

▪️ CDI is about wiring + lifecycle (not just injection).

▪️ If injection becomes ambiguous → add a qualifier.

▪️ Pick scopes intentionally → it’s a performance & memory decision.

▪️ Events are great for cross-cutting concerns (audit, metrics, notifications) without tight coupling.

#JakartaEE #CDI #DependencyInjection #Java #EnterpriseJava #BackendDevelopment #CleanCode #SoftwareArchitecture #JavaDevelopers #Microservices

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