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1) Session Beans
Session beans
are intended to allow the application author to easily implement
portions of application code in middleware
and
to simplify access to this code.
Session
beans are divided into two types:
a) Stateless Session
Bean
b) State
full Session Bean
a) Stateless Session
Bean
A stateless session bean does not maintain a conversational state
with the client. When a client invokes
the
methods of a stateless bean, the bean’s instance
variables may contain a state specific to that client,
but
only for the duration of the invocation. When the method is
finished, the client-specific state
should not be retained.
b) State full Session
Bean
The state of an object consists of the values of its instance
variables. In a stateful session bean, the
instance variables represent the state of a unique
client-bean session. Because the client interacts
(“talks”)
with its bean, this state
is often called the conversational state. The state is retained
for the duration of
the client-bean
session.
2) Entity Beans
An entity bean represents a business object in a persistent
storage mechanism. Some examples of business objects are
customers, orders, and products. In the J2EE SDK, the persistent
storage mechanism is a relational database.
3) Message driven
beans
A message-driven bean is an enterprise
bean that allows J2EE applications to process messages
asynchronously. It acts as a
JMS message listener, which
is similar to an event listener except that it receives messages
instead of events.
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