🔸 TL;DR 🧾
▪️ Session Beans = managed business services (stateless/stateful/singleton)
▪️ @Asynchronous = easy background execution under container control
▪️ MDBs = message consumers for event-driven processing
▪️ Transactions = one of EJB’s strongest features (container-managed by default)
EJB can feel “old school”… but the ideas are timeless: server-side components, managed lifecycle, transactions, async, and messaging — all handled by the container ✅ If you work with Jakarta EE apps (or legacy Java EE), understanding EJB is still a practical skill.
🔸 SESSION BEANS: THE “SERVICE LAYER” OF EJB 🧠
▪️ Stateless: no client-specific state → best for scalable business services
▪️ Stateful: keeps conversational state per client → useful for multi-step workflows
▪️ Singleton: one instance for the app → shared state + scheduled jobs/caches
🔸 SIMPLE SESSION BEAN: THE CORE IDEA ⚙️
▪️ You write a class annotated like @Stateless / @Stateful
▪️ The container injects it with @EJB (or @Inject in CDI contexts)
▪️ The container manages pooling, lifecycle, security, and transactions
1) You write a session bean (business service)
2) Your web layer (Servlet here) calls the bean
(Optional) CDI-style injection instead of @EJB
🔸 INVOKING SESSION BEANS FROM A WEB APP 🌐
▪️ Classic setup: a Servlet / JSF / JAX-RS resource calls an injected EJB
▪️ Your web layer stays thin, the EJB holds business logic
▪️ The container takes care of the “enterprise plumbing” (tx, threads, resources)
🔸 ASYNCHRONOUS METHOD CALL ⏱️
Need “fire-and-forget” or background work without blocking the request thread?
▪️ Use @Asynchronous on a method
▪️ Return void for fire-and-forget, or a Future/CompletionStage style result (depending on stack)
▪️ The container runs it in managed threads (safer than manual new Thread())
Service
Resource
🔸 MESSAGE-DRIVEN BEANS (MDB): EVENT CONSUMERS 📩
MDBs are EJBs designed to consume messages (typically JMS).
▪️ The container delivers messages to your bean automatically
▪️ Great for decoupling, buffering spikes, and async processing
🔸 CONTAINER-MANAGED VS BEAN-MANAGED (MDB & TX) 🤝
▪️ Container-managed: you rely on annotations/config → simplest, most common
▪️ Bean-managed: you explicitly control things (like transactions) → more code, more responsibility Rule of thumb: use container-managed unless you have a strong reason not to.
🔸 TRANSACTIONS IN EJB (STATEFUL VS STATELESS) 💳
EJB makes transactions boringly reliable (that’s a compliment 😄):
▪️ Stateless: each call is usually independent → clean, predictable tx boundaries
▪️ Stateful: can span multi-step workflows → tx boundaries can be more subtle
▪️ Use transaction attributes (like “required / requires new”) to express intent clearly
▪️ Keep tx short: do the DB work, commit, move on ✅
🔸 TAKEAWAYS 🎯
▪️ EJB is less about hype, more about production-grade primitives
▪️ Prefer stateless for scalability; use stateful only when workflow state is truly needed
▪️ Use container-managed transactions & messaging first — go bean-managed only when necessary
▪️ EJB concepts map well to modern patterns: services, async jobs, event consumers, tx boundaries
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