MDB

Internal

• JEE
• JMS

Overview

A Message-Driven Bean (MDB) is an asynchronous message consumer, invoked by the container as a result of the arrival of a message at the JMS destination serviced by the message-driven bean.

To a client, an MDB is a message consumer that implements some business logic running on the server. A client accesses an MDB by sending messages to the JMS destination associated with the MDB container of that type. MDBs are anonymous. They have no client-visible identity. MDB instances have no conversational state either, meaning they do not maintain state for a specific client. All bean instances are equivalent when they are not involved in servicing a client message. An MDB instance is created by the container to handle the processing of the messages for which the MDB is the consumer. Its lifetime is controlled by the container.

API

The @MessageDriven Annotation

An MDB must be annotated with the @javax.ejb.MessageDriven annotation, or denoted in its deployment descriptor as a message driven bean.

Code

An MDB must implement the appropriate listener interface for the messaging type the MDB supports. If the MDB implements javax.jms.MessageListener interface, that distinguishes the MDB as a JMS MDB. If the class does not implement the interface, the MDB must specify the message listener interface using the @MessageDriven annotation, with its "messageListenerInterface" element, or the messaging-type deployment descriptor element.

The onMessage() method of the message listener is called by the container when a message arrives. The method must contain the business logic to handle the message.

Examples

The simplest possible working MDB example is available here:

https://github.com/NovaOrdis/playground/tree/master/jee/ejb/mdb

Another example that showcases most of the configuration and API details discussed in this article is available here:

https://github.com/NovaOrdis/playground/tree/master/jee/ejb/mdb-full

Client's Access to MDBs

Clients cannot access a specific MDB, they can only send messages to destinations serviced by MDBs.

A client looks up the destination in JNDI. The client's JNDI name space may be configured to include destinations serviced by MDBs installed in multiple containers located on multiple machines on a network. The actual locations of the MDBs are transparent to the clients sending messages into them.