The document provides an overview of Enterprise Java Beans (EJB) including basic concepts, the different types of EJB components (session beans, entity beans, message-driven beans), and how to create an EJB. It discusses when to use EJBs, the role of containers, and the evolution of EJB specifications over multiple releases. Key topics covered include the lifecycles and uses of different bean types, persistence management options, and how messages are processed asynchronously using message-driven beans.

1. Understanding EJB
By:
Asha Pathik
Aniruddha Ray

2. Saastha Infotech
Agenda
• Overview and Basic Concepts
– What is EJB?
– When to use EJB
– Containers
– Evolution of EJB
• EJB Components
– Types of Enterprise Beans
– Session Beans
– Entity Beans
– Message-driven Beans
• Creating an EJB
• Summary
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Overview and Basic concepts
What is EJB?
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What is EJB ( Enterprise Java Beans )?
• EJB is the J2EE standard for developing and
deploying server-side distributed components in
java
• It defines a contract between components and
application servers that enables any component to
run on any compliant server
– it is the ubiquitous industry standard
– portability is possible
– rapid application development
– physically, EJB is two things
• a specification
• a set of Java interfaces
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Overview and Basic concepts
When to use EJB
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When to use EJB
• For large scale applications where resources and
data are distributed.
• When the application is running on different servers
at many locations.
• Where scalability is critical.
• Where transactions are required to ensure data
integrity
• When a variety of clients need to handled
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Overview and Basic concepts
Evolution of Enterprise JavaBeans
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Evolution of Enterprise Java Beans – Part 1
• EJB release 1.0 focused on the following aspects:
– defined the distinct “EJB roles” that are assumed by the component
architecture
– defined the client view of enterprise beans
– defined the developer’s view of enterprise beans
– defined the responsibilities of EJB Container provider and Server
Provider
– defined the format of ejb.jar file, EJB’s unit of deployment
• EJB release 1.1 augmented these with following:
– provided better support for application assembly and deployment
– specified in greater detail the responsibilities of the individual EJB
roles
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Evolution of Enterprise Java Beans – Part 2
• EJB release 2.0 focused on the following aspects:
– defined message-driven bean and the integration with the JMS
– provided local client view and support for efficient, lightweight
access to EJB from local clients
– defined new architecture for container persistence
– support for management of relationships among entity beans
– declarative query syntax for finder and select methods for entity
beans with container-managed persistence
– support for additional methods in home interface
– support for run-as security identity
– provided for network interoperability among servers
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Evolution of Enterprise Java Beans – Part 3
• EJB release 2.1 focused on the following aspects:
– enabling enterprise beans to implement and utilize web services
– providing a container-managed timer services
– enhancing EJB QL with additional ORDER BY and aggregate
operators
– enhancing the message-driven bean component type to other
messaging types
• The EJB release 3.0 is focused on a simplification of the
Enterprise JavaBeans architecture from the developer’s
point of view.
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Overview and Basic concepts
Containers
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Containers
• Are the interface between a component and low-level
platform-specific functionality that supports the
component
– components are simple set of programs that are capable of
performing a particular business logic
• Provide configurable settings : like data accessibility
• Manages non-configurable settings like enterprise
bean and servlet life cycles, database connection
resource pooling, data persistence etc.
• Before a J2EE component can be executed, it must
be assembled into J2EE application and deployed
into its container
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EJB Components
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EJB Components
Session Beans
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Session Bean : Concepts
• A Session Bean is non-persistent object that
implements some business logic.
– private to one client connection
– represents an interactive session
– not recoverable after system crash or shutdown!
– when client terminates, bean terminates i.e. no longer active
• A Stateful Session Bean has Conversational State
– activation/ passivation of a Bean is possible
– E.g. items reviewed in a session at some sites.
• A Stateless Session Bean has no Conversational State
– no activation or passivation
– pooling of stateless Session Beans by the container
– very efficient
– E.g. computing value using a formula.
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Stateful Session Bean : Part(1)
Conversational Bean
• Consists of attributes and referenced objects :
– all non-transient attributes of the Bean
– referenced data in the database
– open connections to network
– references to other Beans
– etc.
• Exists during one client session
• A Conversation State is not persistent and is not
automatically rolled back when a transaction fails!
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Stateful Session Bean : Part(2)
• Passivation:
– container serializes Bean instance and saves it to disk
– preparation for Passivation is done with the method ejbPassivate:
• close all existing connections
• resolve all external references
• Activation:
– de-serialize Bean instance from disk
– afterwards, ejbActivate method is invoked to:
• re-establish external references
• re-establish connections
• If non-serializable objects are part of the state, these
methods must be implemented in the Bean class
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Stateful Session Bean : Life Cycle
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Stateless Session Bean
• All instances are equivalent
• Private to one client only during one call
• Pooled while not in use
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EJB Components
Entity Beans
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Entity Bean : Concept
• An Entity Bean represents an object-oriented view of
some entities stored in a persistent, crash-resistant
storage (usually a relational database)
– persistent and transient (modifier transient) attributes are allowed,
but only persistent attributes will be saved in database.
– each entity bean typically has an underlying table in a RDBMS(
business data), and each instance of the bean corresponds to a row
in that table.
– transactional and recoverable on a server crash.
• Shared among multiple clients ( no per-client state)
• Primary Key : a unique attribute of bean or a serializable
class (Primary Key Class) with one or more attributes
• Two Beans with same Home Interface and same
Primary Key are regarded as identical ( Bean Identity)
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Persistence Management
• Bean-Managed Persistence (BMP) : any java-accessible
data storage is possible
– implements persistence mechanisms in ejbLoad resp. ejbStore
– implements ejbActivate, ejbPassivate and ejbRemove
– ejbCreate must return a ‘real’ primary key ( e.g. return pk;)
• Container-Managed Persistence (CMP) : Container
controls access to (usually) a relational database
– empty implementation of ejbLoad, ejbStore, ejbActivate,
ejbPassivate and ejbRemove
– ejbCreate returns ‘null’ as primary key ( return null)
– Object-Relational Mapping between Bean attributes and database
entries ( Vendor –specific)
– more details later
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Primary Key
• A Primary Key of an Entity Bean consists of one or more
persistent Bean attributes
– values of primary key attributes must be unique for a Bean instance
– primary key attributes must be serializable
• The primary key class …
– is a serializable class with one or several attributes which ( together)
acts as primary key of an Entity Bean
• one can use e.g. Long or String for single primary key attributes or
specific classes
• all primary key attributes must be persistent Bean attributes
– must be declared in the Bean’s Deployment Descriptor
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Activation and Passivation
• The EJB Container can passivate unused Entity
Beans
– ejbPassivate is invoked
– used resources ( e.g. database connections, references to other
Beans etc. ) are freed
– bean instance is pooled
• The Bean is activated when used again
– an instance of this Bean is taken from the pool
– ejbActivate is invoked
– needed resources are occupied ( e.g. database connections are
re-established)
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Entity Bean : Life Cycle
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EJB Components
Message-driven Beans
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Message-Driven Bean – Concepts (1)
• Like a Stateless Session Bean, a Message-driven Bean
only provides a piece of business logic without state
management ( similar life cycle)
• A Message-driven bean must implement the interfaces
javax.ejb.MessageDrivenBean and
javax.ejb.MessageListener
– MessageListener declares a method
public void onMessage(Message msg)
which will be invoked when message arrives
• A Message-driven Bean has no client-visible interfaces
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Message-Driven Bean – Concepts (2)
• A Message-driven bean is assigned to a JMS
destination by means of the Deployment Descriptor
element
– message-driven destination
• The bean is invoked by the container, when a JMS
message to this destination arrives
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Message-Driven Bean – Concepts (3)
• A message driven bean is an enterprise bean that
allows J2EE applications to process messages
asynchronously
• It acts a JMS listener, which is similar to an event
listener except that it receives messages instead of
events
• The messages can be sent to any J2EE component
or a non-J2EE system using JMS
• It retains no data or conversational state
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Contents of an EJB
• Interfaces: The remote and home interface for
remote access. Local and local home accesses for
local access.
• Enterprise bean class: Implements the methods
defined in the above interfaces.
• Deployment descriptor: An XML file that specifies
information about the bean such as its type,
transaction attributes, etc.
• Helper classes: non-bean classes needed by the
enterprise bean class such as utility and exception
classes.
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