O'Reilly - Java Message Service

Figure 8.1. The EJB 2.0 bean components
Java Message Service
Application developers create custom enterprise beans by implementing one of the main
bean interfaces and developing the bean according to conventions and policies dictated by
the EJB specification. Entity beans are usually used for modeling business concepts that
have persistent data and may be accessed by many clients concurrently. Entity beans might
be used to model customers, orders, vehicles, locations, and similar objects. Session beans
model business processes that may or may not have session state. Session beans might be
used to model business concepts like a securities broker, an online shopping cart, loan
calculation, medical claim processor - any process or mediator-type business concept.
Message-driven beans are used to model stateless JMS consumers. A message-driven bean
will have a pool of instances at runtime, each of which is a MessageListener. The bean
instances can concurrently consume hundreds of messages delivered to the message-driven
bean, which makes the message-driven bean scalable. Similar to session beans, messagedriven
beans model business processes by coordinating the interaction of other beans and
resources according to the messages received and their payloads.
In addition to the Java classes that define a bean, every bean has an XML configuration
file called a deployment descriptor. The deployment descriptor allows the bean developer
to declare many of a bean's runtime behaviors including transaction policies, access control
policies, and the resources (services) available. Resources ( JMS, JDBC, JavaMail, etc.)
that are declared in the deployment descriptor are accessed via JNDI from the bean's
environment naming context (ENC). The ENC is simply a default read-only JNDI
namespace that is available to every bean at runtime. Each bean deployment has its own
123