message-driven-bean

RMI-IIOP requires tight coupling between client and server — the client must wait (block) for a response, both must be online simultaneously, and it only supports 1-to-1 communication. Enterprise systems need asynchronous, loosely coupled, many-to-many messaging where the client doesn’t wait and the server can be offline.

A Message-Driven Bean is a special EJB component that receives messages (JMS or other types) from a destination and processes them asynchronously. It has no client-visible interfaces (no Home, Remote, or Local interfaces) — clients communicate only via messages sent to a destination.

1. MDB implements a message listener interface (e.g., javax.jms.MessageListener for JMS)
2. The container invokes onMessage(Message msg) when a message arrives at the destination
3. MDB has no identity (stateless) — container pools instances and assigns any available instance to handle a message
4. onMessage() is weakly typed — receives generic Message type, must use instanceof to determine actual message type
5. No return value to client — interaction is fire-and-forget (asynchronous)
6. Exceptions cannot be sent back to client — only system exceptions handled by container
7. From EJB 2.1+, MDBs can consume non-JMS messages via J2EE Connector Architecture (JCA) resource adapters

• Stateless — no conversational state, like stateless session beans
• No interfaces (Home, Remote, Local) — only message listener interface
• Implements javax.jms.MessageListener (for JMS) or other listener interfaces
• onMessage() is the only business method — handles all incoming messages
• Supports concurrent processing — container can assign multiple messages to multiple MDB instances
• Loosely coupled — client and MDB don’t need to be online simultaneously

• Built from: JMS — MDBs typically consume JMS messages
• Built from: EJB Container — container manages MDB lifecycle and pooling
• Contrasts with: Session Bean — session beans have interfaces and synchronous calls; MDB has no interfaces and async messages
• Contrasts with: Entity Bean — entity beans represent data; MDBs process messages
• Related: Stateless Session Bean — both are stateless and pooled
• Related: Poison Message — MDB rollback behavior can cause poison messages

• onMessage() cannot return values to clients (asynchronous fire-and-forget)
• Application exceptions cannot be propagated to clients — only system exceptions go to container
• Weak typing means you must check message type with instanceof at runtime
• MDBs cannot be called directly via RMI — only through message destinations
• From EJB 2.1+, can consume non-JMS messages via JCA resource adapters