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Enterprise JavaBeans
Chapter 14
Java Programming: Advanced Topics
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Objectives
• Explore an enterprise application’s needs for secure
distributed access, scalability with high performance,
robustness, data persistence with transactional
integrity, and management of distributed and often
disparate resources
• Discover how the EJB framework provides the quality
of services enterprise applications require
• Learn how stateful and stateless session EJBs
provide components that perform business logic
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Objectives (Cont.)
• Learn how to program an EJB client
• Discuss exception handling in EJBs and
EJB clients
• Learn how entity beans represent
persistent data in Java objects
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Objectives (Cont.)
• Discuss mapping fields of entity beans to
elements in databases using CMP and BMP
• Learn how to use container-managed
relationships and EJB query language with
container-managed persistence
• Learn what Java Message Service is and learn
how message-driven beans consume
asynchronous messages
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Objectives (Cont.)
• Discuss transactional properties of EJBs
• Discuss elements of application security
that relate to EJBs
• Consider some best practices for
designing applications that use EJBs
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Enterprise Programming
• EJB specification: a description of this
environment and the framework for building
distributed objects that implement a standard
interface
• Distributed systems: software systems that
reside on several physical hosts
– the components must interoperate over a network
that may be a local area network (LAN) or the
Internet
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Enterprise Programming (Cont.)
• Scalability: the ability of a system to continue to
give high performance as the number of users
increases
• Persistence: any data that outlives any software
component
• Resource management is required whenever
the system makes high demands on databases
and other resources
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Enterprise Programming (Cont.)
• Connection pooling: keeping a set of
connections open so that different software
components can be given an open connection
for short-term exclusive use
• Security: usually of critical importance in
enterprise applications
• Transactional integrity: often the quality that
most concerns developers of enterprise
applications
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What are EJBs
• Enterprise JavaBeans (EJBs): server-side
software components that conform to the J2EE
architecture for development and deployment of
distributed systems
• J2EE-compliant application servers must provide
a run-time environment for the EJBs: an EJB
container
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What are EJBs (Cont.)
• EJBs come in three basic kinds:
– Session beans:
• can perform any kind of processing
– Entity beans:
• represent persistent data
– Message-driven beans (MDB):
• have a specialized purpose and are used with
messaging software
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What are EJBs (Cont.)
• The major components of an EJB:
– Bean class
– Home interface
– Local or Remote interface
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MVC Architecture
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EJB Containers and Services
• The role of the EJB container is to provide the
following:
– The distribution infrastructure and a naming service to
help client code locate and access EJBs.
– The ability to place EJBs in a scalable architecture
– Support for concurrent access
– Resource management, including connection pooling
– Security services in addition to the secure
environment that can be configured for applications
loaded into the application server
– Transaction managers that interact with JDBC drivers
and resource adapters
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J2EE Enterprise Application
Packaging and Deployment
• EJBs are always packaged in J2EE enterprise
applications
• Files that compose EJBs are packaged in jar
files
• An EJB jar must contain a deployment
descriptor file
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J2EE Packaging into Archive
Files
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Session EJBs
• Session EJBs can do general purpose
processing
• They are associated with the client that calls
them
• There are two types of session EJBs:
– stateful
– stateless
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Stateless Session EJBs
• Stateless session beans can be shared among
clients
• Methods defined in the javax.ejb.SessionBean
interface are call-back methods that control the
lifecycle of the bean, that called by the
container
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Stateless Session EJBs (Cont.)
• Steps to create a stateless session EJB:
– 1. Define a class that extends the SessionBean
interface
– 2. Create a home interface
– 3. Add business methods to the session bean class
and write implementations of those methods
– 4. Add a remote interface and include all the
business methods that can be called by remote
clients
– 5. Write the deployment descriptor
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Stateful Session EJBs
• Stateful session EJBs retain conversational
state between method calls and are used by
only one client
• Conversational state: information that must
be retained as long as the client is actively
interacting with the application
• Transactional state: data that must be
permanently recorded when the client activity
ends
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EJB Clients
• The EJB 2.0 specification adds interfaces to all
entity and session beans specifically for use by
co-located clients
• There are four interfaces that you can define
for a session or entity bean
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The Client Interfaces
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The Client Interfaces (Cont.)
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The Client Interfaces (Cont.)
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The Client Interfaces (Cont.)
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Writing EJB Clients
• The client starts by accessing the JNDI
namespace, by instantiating an object of type
javax.naming.InitialContext
• When the client has the EJB home, it uses a
create method to get a handle to the remote
interface
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Handling Exceptions in
EJB Clients
• Define and use application-level exception
classes to encapsulate anticipated problem
conditions
• Application-level exceptions: exceptions that
business logic can anticipate and possibly
recover from.
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Entity EJBs
• Entity beans represent persistent data
• Use entity beans as the interface between Java
components and relational or object-oriented
databases
• The container uses the primary key to locate the
data when a client requests an entity bean and
creates only one bean to represent that data in a
Java object
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EJB to Database Schema Mapping
• There are three approaches to database schema
mapping:
– Top-down mapping:
• possible when a new database is required and Java
developers are allowed to create databases
– Bottom-up mapping:
• occurs when you have a database and can design your EJBs
to match the tables and columns defined in the database
schema
– Meet-in-the-middle mapping:
• most common solution in practice
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EJB to Database Schema
Mapping (Cont.)
• The EJB specification allows two approaches:
– Container Managed Persistence (CMP):
• involves declaring the mapping between the deployment
descriptor and delegating all code generation to the
container
– Bean Managed Persistence (BMP):
• is a do-it-yourself solution
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Sample CMP Entity Bean
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EJB Query Language
• Enterprise JavaBean Query Language (EJB
QL) was introduced in the EJB 2.0 specification
to support the abstract persistence model
• EJB QL: language for expressing the
equivalent of SQL SELECT statements for
CMP beans
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Comparing EJB QL and SQL
Statements
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Using EJB QL
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Message-Driven Beans
• An message-driven bean (MDB) initiates
processing in response to an external event
• To create an MDB, build the bean class
• The MDB class must implement
javax.ejb.MessageDrivenBean and
javax.jms.MessageListener
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A Deployment Descriptor with
MDB
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A Possible Scenario that Uses
MDBs
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EJB Transactional Characteristics
• The EJB specification gives you options for
defining transactions:
– Container managed transactions (CMT)
– Bean managed transactions (BMT)
– Client demarked transaction
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EJB Transactional Levels
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EJB Transactional Levels (Cont.)
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EJB Transactional Levels (Cont.)
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EJB Security
• The concept of passing security roles down the
call stack as EJB methods call other EJB
methods is called delegation
• Role-based security can be applied to entire
EJBs or to individual methods
• EJB developers are not responsible for user
authentication or authorization (mapping the
user to a role)
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Summary
• EJBs are distributable server-side components
that run in EJB containers provided by J2EEcompliant application servers
• EJBs come in three basic kinds: session beans,
entity beans, and message-driven beans (MDB)
• EJB clients locate bean instances by looking up
the name in a JNDI server
• Stateless session beans can be shared among
clients, while stateful session EJBs are used by
only one client
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Summary (Cont.)
• Entity beans represent persistent data and can
be designed for container-managed persistence
(CMP) or bean-managed persistence (BMP)
• EJB QL is a SQL-like language for writing
database queries in terms of CMP bean classes
and fields
• The concept of passing security roles down the
call stack as EJB methods call other EJB
methods is called delegation
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