EJB 3.0 - Yet Another Introduction

Enterprise Java Bean 3.0
Yet Another Introduction…

Kelum Senanayake

Tag-Cloud

ResourceInjection
StatelessSessionBean

EnterpriseJavaBeans
EntityBean StatefulSessionBean
MessageDrivenBean
JavaEE POJO
JavaPersistenceAPI
Annotations Transactions

Enterprise Java Beans (EJB)
• A platform for building portable, reusable, and
scalable business applications.
• Allows to focus on building business logic
without having to spend time on building
infrastructure code for services such as
transactions, security, automated persistence.
• An EJB is a piece of Java code that executes in
a specialized runtime environment.

EJB as a component …
• A component should effectively encapsulate
application behavior.
• All we need to know is what to pass in and
what to expect back.
• Components can be reusable.
• Three types of EJB components:
– Session beans
– Message-driven beans
– Entities

EJB as a framework …
• EJB components live in a container.
• Together, the components, or EJBs, and the
container can be viewed as a framework.
• EJB framework provides valuable services for
enterprise application development

EJB History
• EJB 1.0 (1998-03-24)
– Defined the responsibilities of EJB Container provider
– Defined the distinct “EJB Roles”
• EJB 1.1 (1999-12-17)
– XML deployment descriptors
– Session Beans, Entity Beans
– Remote interface
• EJB 2.0 (2001-08-22)
– Message-Driven Beans
– Entity 2.x and EJB QL

EJB History …contd
• EJB 2.1 (2003-11-24)
– EJB Timer Service
– Web service support
• EJB 3.0 (2006-05-11)
– POJO, Annotations
– Dependency injection
• EJB 3.1 (2009-12-10)
– Local view without interface
– .war packaging of EJB components
– Singletons (Singleton Session Beans)
– EJB Lite: definition of a subset of EJB
– @Asynchronous for session beans

EJB 3.0 vs EJB 2.1
• EJB 3.0 is much faster than EJB 2
• An EJB 2.1 session bean must implement the
SessionBean interface
• EJB 3.0 session bean class includes only business
methods.
• EJB 3.0 interfaces are POJI business interfaces
and do not require home and component
interfaces.
• EJB 2.1 must have the deployment descriptor. But
optional in EJB 3.0. Annotations are added to the
language.

EJB 3.0 vs EJB 2.1 …contd
• EJB 3 introduced persistence API for database
access. In EJB 2 you can use Entity bean.
• An EJB 2.1 Entity EJB bean class must implement
the EntityBean interface and must provide
implementation to the ejbCreate() and
ejbPostCreate() methods.
• EJB 2.1 entity bean includes the home,
component, local home and local component
interfaces that extend the EJBHome, EJBObject,
EJBLocalHome and EJBObject interfaces
respectively.

EJB 3.0 vs EJB 2.1 …contd
• An EJB 2.1 bean must define resource-ref in ejb-
jar.xml to lookup resources. An EJB 3.0 bean can
use either dependency injection or JNDI lookup.
• An EJB 2.1 message-driven must implement the
javax.ejb.MessgeDrivenBean interface.
• Standardized Simplified persistence with POJO
– You couldn’t send an EJB 2 entity bean across the wire
in different tiers.
– They are permanently attached to the database.
– You have to write data transfer objects

Why choose EJB 3.0?
• Ease of use
– POJO programming, annotations in favor of verbose
XML, heavy use of sensible defaults & JPA
• Integrated solution stack
– EJB 3 offers a complete stack of server solutions
– Persistence, messaging, lightweight scheduling,
remoting, web services, dependency injection (DI) and
interceptors
– You won’t have to spend a lot of time looking for
third-party tools

Why EJB 3.0 …contd
• Open Java EE standard
– EJB is a critical part of the Java EE standard
– EJB 3 has an open, public API specification
– The open standard leads to broader vendor
support
– You don’t have to depend on a proprietary solution

• Broad vendor support
– You are not at the mercy of the ups and downs of
a particular company or group of people
– Vendors have historically competed against one
another by providing value-added nonstandard
features
• Stable, high-quality code base
– Clustering, load balancing, and failover support
with no changes to code, no third-party tool
integration, and relatively simple configuration

• Unit-testable POJO components
– All EJB 3 components are POJOs, they can easily
be executed outside the container
– It is possible to unit-test all component business
logic using testing frameworks
• Annotations and descriptors are not mutually
exclusive
– Deployment descriptor entries override
configuration values hard-coded into EJB
components

The Container
• When you build a simple Java class, you need a
Java Virtual Machine (JVM) to execute it.
• Think of the container as simply an extension of
the basic idea of a JVM.
• JVM transparently manages memory on your
behalf.
• The container transparently provides EJB
component services
– Transactions, security management
– Remoting and web services support

Container …contd
• A Java EE container is an application server
solution that supports EJB 3, a web container,
and other Java EE APIs and services.
• JPA is completely pluggable and separable.
– Persistence provider and container in an EJB 3
solution need not come from the same vendor
– You could use Hibernate inside a BEA WebLogic
container

Dependency injection vs. JNDI lookup
• With EJB 2, you have to write the same few lines
of boilerplate code many times to do a JNDI
lookup.
• In EJB 3, JNDI lookups have been turned into
simple configuration using metadata-based
dependency injection (DI)
• JNDI : It’s the responsibility of the client to do a
lookup and obtain a reference to the object
• You may think DI is the opposite of JNDI
– It is the responsibility of the container to inject an
object based on the dependency declaration

Building business logic with
Session Beans

Getting to know Session Beans
• So what is a Session?
– A session is a connection between a client and a
server that lasts for a finite period of time
• Session beans centers on the idea that each
request by a client to complete a distinct business
process is completed in a session.
• Recall that session beans come in two flavors:
Stateful and Stateless
• A bean may maintain its state between calls, in
which case it is stateful, or it may be a one-time
call, in which case it’s stateless.

@Stateless Session Bean
• No more Home interfaces!
• A POJI Business Interface
– Annotated with @Remote or @Local
• A POJO Implementation
– Annotated with @Stateless
• No more deployment descriptors! (optional)

@Stateful Session Bean
• A POJI Business Interface
– Annotated with @Remote or @Local
– Annotated with @Stateful
• No more deployment descriptors! (optional)

Working with multiple business
interfaces
• You cannot mark the same interface with
more than one access type annotation.
• However, a business interface can extend
another interface.
• You can create a set of interfaces utilizing OO
inheritance to avoid code duplication

Callback methods
• @PostConstruct
• @PostActivate
• @PrePassivate
• @PreDestroy

Messaging with message-driven beans

Diving into the Java Persistence API

Java Persistence API
• Separate specification document
• Produced by EJB 3.0 Expert Group (JSR-220)
• Available in & outside the Java EE container
• The API itself, defined in the javax.persistence
package
• The Java Persistence Query Language (JPQL)
• Object/relational metadata
• Gavin King (founder of Hibernate) represented
JBoss on JSR220

@Entity Beans
• No Business Interfaces!
– Annotated with @Entity
• A simple deployment descriptor
• Entities need not use getter- and setter-based
properties.

EntityManager
• EntityManager is the bridge
between the OO and
relational worlds.
• Knows how to store a POJO
entity into the database,
read, update & delete.
• Factory for Query

Query

entityManager.createNativeQuery()

Named queries
• They improve reusability of queries.
• They improve maintainability of code
– Queries are not scattered among the business logic.
• They can enhance performance because they
are prepared once and can be efficiently reused.
• Can be defined either in the entity using
annotations, or in the XML file defining O/R
mapping metadata.

Understanding transactions
• A transaction is a grouping of tasks that must be
processed as an inseparable unit.
• Every task that is part of the transaction must
succeed in order for the transaction to succeed.
All-or-nothing
• The Transaction Manager is a component that,
coordinates a transaction over multiple
distributed resources
• EJB provides through the Java Transaction API
(JTA)

Container-managed transactions
• In a CMT, the container starts, commits, and
rolls back a transaction on our behalf
• Can be done through annotations or the
deployment descriptor
• EJB context: accessing the runtime
environment
• The javax.ejb.EJBContext interface is
essentially your backdoor into the mystic
world of the container

Bean-managed transactions
• The greatest strength of CMT is also its
greatest weakness.
• CMT, you are limited to having the transaction
boundaries set at the beginning and end of
business methods and relying on the
container to determine when a transaction
starts, commits, or rolls back.
• BMT, on the other hand, allows you to specify
exactly these details programmatically

The pros and cons of BMT
• BMT is verbose, complex, and difficult to
maintain and some times error prone.
• With BMT, you can fine-tune your transaction
boundaries so that the data held by your code
is isolated for the shortest time possible
• BMT can never join an existing transaction.
– Existing transactions are always suspended
– Significantly limiting flexible component reuse

References
• [1] D. Panda, R. Rahman, and D. Lane, EJB 3 in Action, 1st
ed. Manning Publications, 2007.

• [2] “Enterprise JavaBeans - Wikipedia, the free
encyclopedia.” [Online]. Available:
http://en.wikipedia.org/wiki/Ejb. [Accessed: 10-Mar-2012].

• [3] “What I’m Learning: Differences between EJB 3.0 and
EJB 2.1.” [Online]. Available:
http://mytechnicaldocs.blogspot.com/2011/08/differences
-between-ejb-30-and-ejb-21.html. [Accessed: 10-Mar-
2012].

EJB 3.0 - Yet Another Introduction

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