Ashi's presentation on EJB 3.0

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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