J2EE architecture is essentially a distributed architecture
for developing multi-tier enterprise applications. Here, applications are
divided into multiple tiers: the client-tier to provide a user interface, middle-tier
modules to provide services and business logic, and backend EIS to provide data
management. Figure illustrates the typical J2EE environment.
In the above figure the client tier could consist of various
client types such as web clients, Standalone Java clients, PDAs, and hand held
PCs. The client can access the application from inside or outside the corporate
The middle tier provides services to the client through the
web tier or the EJB tier.
The web tier is used to connect web clients or Java clients
from outside the firewall to the EIS.
EJB tier is used to connect Java clients inside the firewall
and web clients (through the web tier) to the EIS.