The Enterprise Bean Component

Enterprise JavaBeans server-side components come in three fundamentally different types: entity, session, and message-driven beans. Both session and entity beans are RMI-based server-side components that are accessed using distributed object protocols. Message-driven beans process messages from non-RMI systems like Java Message Service, legacy systems, and web services. All EJB servers must at least support a JMS-based message driven bean, but they may also support other types of message-driven bean.

A good rule of thumb is that entity beans model business concepts that can be expressed as nouns. For example, an entity bean might represent a customer, a piece of equipment, an item in inventory, or even a place. In other words, entity beans model real-world objects; these objects are usually persistent records in some kind of database. Our hypothetical cruise line will need entity beans that represent cabins, customers, ships, etc.

Session beans are extensions of the client application that manage processes or tasks. A Ship bean provides methods for doing things directly to a ship, but doesn't say anything about the context under which those actions are taken. Booking passengers on the ship requires that we use a Ship bean, but it also requires a lot of things that have nothing to do with the ship itself: we'll need to know about passengers, ticket rates, schedules, and so on. A session bean is responsible for this kind of coordination. Session beans tend to manage particular kinds of activities, such as the act of making a reservation. They have a lot to do with the relationships between different entity beans. A TravelAgent session bean, for example, might make use of a Cruise, a Cabin, and a Customer—all entity beans—to make a reservation.

Similarly, message-driven beans coordinate tasks involving other session and entity beans. Message-driven beans and session beans differ primarily in how they are accessed. While a session bean provides a remote interface that defines which methods can be invoked, a message-driven bean subscribes to or listens for messages. It responds by processing the message and managing the actions that other beans take. For example, a ReservationProcessor message-driven bean would receive asynchronous messages—perhaps from a legacy reservation system—from which it would coordinate the interactions of the Cruise, Cabin, and Customer beans to make a reservation.

The activity that a session or message-driven bean represents is fundamentally transient: you start making a reservation, you do a bunch of work, and then it's finished. The session and message-driven beans do not represent things in the database. Obviously, session and message-driven beans have lots of side effects on the database; in the process of making a reservation, you might create a new Reservation by assigning a Customer to a particular Cabin on a particular Ship. All of these changes would be reflected in the database by actions on the respective entity beans. Session and message-driven beans like TravelAgent and ReservationProcessor, which are responsible for making a reservation on a cruise, can even access a database directly and perform reads, updates, and deletes to data. But there's no TravelAgent or ReservationProcessor record in the database—once the bean has made the reservation, it waits to process another.

What makes the distinction between the different types of beans difficult to understand is that it's extremely flexible. The relevant distinction for Enterprise JavaBeans is that an entity bean has persistent state; session and message-driven beans model interactions but do not have persistent state.

Tags: EJB, enterprise bean, component, java beans, entity, session, message-driven, RMI, EJB server, persistent state, interactions