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<TITLE>Writing Advanced Applications, Chapter 2: A Multi-Tiered
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<H2>Writing Advanced Applications<BR>
<FONT SIZE="3">Chapter 2 Continued: A Multi-Tiered Application with
Enterprise Beans</FONT></H2>
<FONT SIZE="-1">[<A HREF="code.html"><<BACK</A>] [<A HREF="index.html#contents">CONTENTS</A>] [<A HREF="code3.html">NEXT>></A>]</FONT></DIV>
<P>
The proliferation of internet- and intranet-based applications
has created a great need for distributed transactional
applications that leverage the speed, security, and
reliability of server-side technology. One way to meet this
need is to use a multitiered model where a
thin-client application invokes business logic that executes on the server.
<P>
Normally, thin-client multitiered applications are hard to write because
they involve many lines of intricate code to handle transaction and
state management, multithreading, resource pooling, and other complex
low-level details. And to compound the difficulties, you have to rework
this code every time you write an application because the code is so
low-level it is not reusable.
<P>
If you could use someone's
prebuilt and pretested transaction management code or even reuse some of
your own code, you would save a lot of time an energy that you could better
spend solving the business problem. Well, Enterprise JavaBeans<FONT SIZE="-2"><SUP>TM</SUP></FONT>
technology can give you the help you need.
The Enterprise JavaBeans technology makes distributed transactional
applications easy to write because it separates the low-level
details from the business logic. You concentrate on creating the best
business solution and leave the rest to the underlying architecture.
<P>
This chapter describes how to create the example auction application
using the services provided by the Enterprise JavaBeans platform. Later
chapters will show how you can customize these services and integrate
these features into existing non-EJB applications.
<P>
<UL>
<LI><FONT FACE="Verdana, Arial, Helvetica, sans-serif">
<A HREF="#define">Enterprise Beans Defined</A></FONT>
<LI><FONT FACE="Verdana, Arial, Helvetica, sans-serif">
<A HREF="#thin">Thin-Client Programs</A></FONT>
<LI><FONT FACE="Verdana, Arial, Helvetica, sans-serif">
<A HREF="#multi">Multitiered Architecture</A></FONT>
<LI><FONT FACE="Verdana, Arial, Helvetica, sans-serif">
<A HREF="#es">Entity and Session Beans</A></FONT>
<LI><FONT FACE="Verdana, Arial, Helvetica, sans-serif">
<A HREF="#house">Auction House Workings</A></FONT>
<LI><FONT FACE="Verdana, Arial, Helvetica, sans-serif">
<A HREF="#dev">Developing and Running Applications</A></FONT>
<LI><FONT FACE="Verdana, Arial, Helvetica, sans-serif">
<A HREF="#how">How Multitiered Applications Work</A></FONT>
</UL>
<HR>
<A NAME="define"></A>
<H3>Enterprise Beans Defined</H3>
An Enterprise Bean is a simple class that provides two types of
methods: business logic and lifecycle. A client program calls the business
logic methods to interact with the data held on the server.
The container calls the lifecycle methods to manage the Bean on the server.
In addition to these two types of methods, an Enterprise Bean has an
associated configuration file, called a deployment descriptor, that is
used to configure the Bean at deployment time.
<P>
As well as being responsible for creating and deleting Beans the Enterprise
JavaBeans server also manages transactions, concurrency, security and
data persistence.
Even the connections between the client and server are provided by using
the RMI and JNDI APIs and servers can optionally provide scalabilty through
thread management and caching.
<P>
The auction house example implements a complete Enterprise
JavaBeans solution by providing only the business logic and using
the underlying services provided by the architecture. However, you may
find that the container managed services, although providing
maximum portability, do not meet all your application requirements.
The next chapters will show how you can provide these services in your
Bean instead and also use these services in non-Enterprise Bean applications.
<A NAME="thin"></A>
<H3>Thin-Client Programs</H3>
A thin client is a client program that invokes business logic
running on the server. It is called <EM>thin</EM> because most
of the processing happens on the server. In the figure below,
the servlet is the thin client. It invokes Enterprise Beans
that run on the Enterprise JavaBeans server. It also executes
logic that creates web pages that appear in the browser.
<P>
<IMG SRC="./Art/thin.gif">
<A NAME="multi"></A>
<H3>Multitiered Architecture</H3>
<IMG SRC="./Art/multi.gif" ALIGN="LEFT">
Multitier architecture or three-tier architecture
extends the standard two-tier client and server
model by placing a multithreaded application
server between the client and the database.
<P>
Client programs communicate with the database through the
application server using high-level and platform independent
calls. The application server responds to the client
requests, makes database calls as needed into the
underlying database, and replies to the client program
as appropriate.
<P>
The three tiers in the web-based auction house
example consists of the thin-client servlet, the Enterprise
JavaBeans server (the application server), and the
database server as shown in the figure.
<A NAME="es"></A>
<H3>Entity and Session Beans</H3>
There are two types of Enterprise Beans: entity Beans and session Beans.
An Enterprise Bean that implements a business entity is an <I>entity
Bean</I>, and an Enterprise Bean that implements a business task
is a <I>session</I> Bean.
<P>
Typically, an entity Bean represents one row of persistent
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