This chapter covered a lot of ground describing the basic architecture of an EJB system. At this point, you should understand that beans are business object components. The home interfaces define life-cycle methods for creating, finding, and destroying beans, and the remote and local interfaces define the public business methods of the bean. Message-driven beans do not have component interfaces. The bean class is where the state and behavior of the bean are implemented.

There are three basic kinds of beans: entity, session, and message-driven. Entity beans are persistent and represent a person, place, or thing. Session beans are extensions of the client and embody a process or a taskflow that defines how other beans interact. Session beans are not persistent: they receive their state from the client, and they live only as long as the client needs them. Message-driven beans are integration points that allow other applications to interact with EJB applications using JMS or, in EJB 2.1, some other J2eeCA 1.5-compliant resource. Message-driven beans, like stateless session beans, are not persistent and do not maintain conversational state.

The EJB object and EJB home are conceptual constructs that delegate method invocations to session and entity beans from the client and help the container to manage the enterprise bean at runtime. The clients of entity and session beans do not interact with the instances of the bean class directly. Instead, the client software interacts with stubs, which are connected to the EJB object and EJB home. The EJB object implements the remote and/or local interface and expands the bean class’s functionality. The EJB home implements the home interface and works closely with the container to create, locate, and remove beans.

Beans interact with their containers through the well-defined bean-container contract. This contract provides callback methods, the EJBContext, and the JNDI environment-naming context. The callback methods notify the bean class that it is involved in a life-cycle event. The EJBContext and JNDI environment-naming context provide the bean instance with information about its environment.