Components of J2EE

S Gokul

Java 2 for enterprise edition adopts a multi-tier application programming

model for developing flexible and reusable business applications and components.

By providing a basic infrastructure for lower level services like maintaining

resource pools, handling threads, managing transactions, balancing heavy loads

and scaling components, J2EE allows the developer to fully concentrate on

developing his or her business logic code — without bothering much about

complex middleware services that are hard to handle.

J2EE Application Programming Model



Before proceeding further, let us look at the basic structure of the J2EE
Application Programming Model spanning across multiple tiers. Each tier houses

its own set of components - which work in synchronization - to provide the

enterprise services to various client requests. Click here

to view image.

Client tier components



Clients for the J2EE enterprise environment can either be external clients or
internal clients.

External clients include all Internet users, who log on to the enterprise’s

eBusiness portal to avail the services offered via the Net. They also include

all mobile clients who access a similar set of services via their WAP phones.

Internal clients represent various in house departmental staff of the

enterprise, who typically make use of Intranet (which is again a Web client) or

desktop client-server GUI applications to access another set of enterprise

services.

Thus, the varieties of requests pouring into the J2EE enterprise middleware

can emerge from one or more of the following clients:

• Internet Web browsers
• Intranet Web browsers
• Desktop GUI applications
• WAP devices

J2EE responds to Web-based Internet and Intranet requests by rendering the

processed business information as HTML Web pages. Sometimes, these HTML pages

may make use of Applets and Javascripts for additional functionality. Mobile WAP

clients need to be responded by means of WML pages.

The responsibility of serving out the processed business information as HTML/WML

pages is handled by the presentation logic tier — which uses HTTP Web Servers

and WAP Servers to issue out the deliverables via the network.

Desktop GUI applications usually bypass the presentation logic tier and

correspond with business logic components directly (and hence the direct

communication link, in the diagram) — because presentation logic is typically

embedded inside the client application itself. Kindly note that this kind of

direct access to business logic components is permissible only when all internal

enterprise clients are assumed to be fully trusted.

Presentation logic components (or Web components)



The presentation logic tier is meant for visually presenting the processed data
to the clients and not for doing any business processing.

Components of this tier are typically based on request-response models. i.e.

they receive the requests emanating from the clients, pass them on to business

logic components for processing, receive the processed results and then send

them back to clients, in the required format. For example, they can receive a

http request from a client browser, forward the request to an EJB for

processing, receive the processed business data, format them as HTML pages and

send them back to the client browser for display.

The presentation logic components of J2EE infrastructure for a given business

environment can be:

HTML/XML/WML pages

• Pages written to different markup language specifications to serve
  
  different clients
• HTML and XML pages can be read by Web browsers and WML pages can be
  
  interpreted by WAP enabled mobile phones
• These pages reside in HTTP or WAP Web Servers

Java Servlets

• Java Servlets are standard server side Java extensions to the basic Web
  
  server framework
• They provide additional functionality to the server and enable dynamic
  
  content creation and delivery
• Programmers can develop their own ‘servlets’ for their business
  
  applications by extending the basic servlet provided by the Servlet API
• Servlets reside in Servlet Engines, which are Java Servlet API
  
  implementations for different commercial Web servers