"You mean," I said, "that it takes five or six tries to get an idea right?"
"Yes," he said, "at least that."
"But," I replied, "you also said that if a newly introduced product doesn't catch
on in the first two or three times, then it is dead?"
"Yup," he said.
"Then new products are almost guaranteed to fail, no matter how good the idea!"
"Now you understand," said the designer.1
Over the last 10 years, the software industry has sub-divided and attempted to
standardize the interconnecting pieces that comprise client server computing. As the
pieces evolve, they become more de-coupled from each other growing into independent,
stand-alone objects with standardized interfaces. The goal is to develop independent
objects that can dynamically communicate with any other object across the Internet.
Many ideas have been proposed for an object-oriented Internet. Currently there
are three leading contenders. They are Common Object Request Broker Architecture
(CORBA), Distributed Component Object Model (DCOM), and Enterprise JavaBeans (EJB).
This node describes the EJB design for an object-oriented Internet using
client/server technology. Enterprise JavaBeans provides a framework for extending sever-side
functionality by separating and encapsulating business-logic from the system level
I. Description of EJB
Enterprise JavaBeans is an evolutionary step combining client/server, distributed
computing, and objects into a framework to ease the development of large business
applications. In earlier technology the programmer had to know about the internal
implementations of various parts of the client/server architecture in order to develop an
application. EJB allows for greater encapsulation and specialization of the components
of client/server computing.
EJB provides a framework for developing runtime extensions to server
functionality by encapsulating business-logic in stand-alone objects. The objects can
then be configured at runtime to meet specific application needs. In addition, the specification
separates out the low-level, system activity into a standard set of services so developers
do not have to re-invent the wheel every time they write an application. The system services
are encapsulated with a standard interface and placed in the middleware.
There are two goals EJB seeks to achieve. The first is to make developing
distributed client/server applications easier by separating system services from the
application business-logic. The second is to enhance the move of business-logic
processing to the server from the client machine. The business-logic functions can be run
on the server more efficiently and thus avoid a slow downloading process if sent to
the client over the network.
II. The Architecture of EJB
There are three basic parts of the EJB architecture. They are the EJB Component,
the EJB Container, and the EJB Object. An Enterprise JavaBean is a component, written
in Java, that encapsulates business-logic. The component executes within an EJB
Container that resides on the server. The client communicates with the EJB Component
using the EJB Object. The latter is a "remote interface" that is downloaded to the client.
It then handles the communication between the client and the EJB Container.
III. Outline of Runtime EJB Activity
- A. EJB Component
- The EJB Component is implemented as one of two different models. The
first model is called a Session Bean. It is a model of a transient communication
between the client and the server. There is no persistence beyond the duration of
the interaction. Nothing is stored for later use about the interaction even though a
session bean may provide database transaction services to the client.
The second model of an EJB component is an Entity Bean. The entity
bean represents data in a database along with the methods to process that data.
An entity bean is long-lived. It has persistence because its state is stored in a
database and is obtainable in a later client - server transaction.
- B. EJB Container
- The EJB Container manages EJB Components. It is loaded and run on the
server. The EJB Container will in turn handle the object life cycle of the EJB
Components for which it is responsible. The container provides all the system
services to the component so the latter may focus on just the business-logic of the
application. The services include transaction and resource management,
versioning, scalability, persistence, and security to name a few.
- C. EJB Object
- The EJB Object is downloaded to the client during the initialization of an
interaction between the client and server. The EJB Object provides a set of
method calls to the business logic of the EJB Container and Component. The
communication between the client and the EJB Component appears seamless to
the client because it only needs to invoke the appropriate methods available in the
EJB Object. The EJB Object handles all the system level communication over the
network to the EJB Container, which in turn communicates with the EJB Component.
- Client uses the Java Naming and Directory Interface (JNDI) to locate the Home-Interface
for the EJB Component class it wants.
- Client calls one of the create() methods to create the server-side object.
- The client-side Home-Interface makes a remote call to the EJB Container on
- The EJB Container creates the EJB Component, either a Session Bean or an
Entity Bean. The EJB Component will reside on the server. Then the EJB
Container creates and returns to the client an EJB Object.
- The Client now interacts with the EJB Object and thereby with the server-side
bean to complete a set of business services.
- When finished, the Client will request the EJB Container shut down the EJB
Component. The EJB Component will be inactivated or destroyed, resources
will be released, and then finally the EJB Object, on the client-side, will be
1. Donald A. Norman, The Design of Everyday Things (New York, NY: Doubleday, 1988) page 29.
Johnson, Mark, "A Beginners Guide to Enterprise JavaBeans", Java World, October:
Norman, Donald A., The Design of Everyday Things, New York: Doubleday, 1988.
Nova Laboratories, "The Developers Guide to Understanding Enterprise JavaBeans
Applications", Bordentwon, NJ, 2000. www.nova-labs.com
Orfali, R., and D. Harkney, Client/Server Programming with Java and CORBA, 2nd ed.,
New York: John Wiley, 1998.
Orfali, R., D. Harkney, and J. Edwards, Client/Server Survival Guide, 3rd ed., New York:
John Wiley, 1999.
Roth, Bill, "An Introduction to Enterprise JavaBeans Technology", Sun Microsystems,
Inc., 2000, Developer.java.sun.com
Sun Microsystems, Inc., "Enterprise JavaBeans Technology", 2000, Java.sun.com/products/ejb
Thomas, Anne, "Enterprise JavaBeans Technology: Server Component Model for the
Java Platform", Boston: Patricia Seybold Group White Paper, 1998.