Understanding J2EE Application Containers and Their Roles in Java Enterprise Applications, Slides of Java Programming

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^ The^ application

server

maintains

control

and^ provides

services

through

an^ interface

or^ framework

known

as^ a

container  J2EE^ application

components

use^ the

protocols and

methods of

the^ container

to^ access

other^

application

components

and^ services

provided

by^ the

server.

^ Contract

specifies

how^ components

should^ be

developed

and^ deployed.  How^ components

can^ access

services^

provided

by

container.  Interfaces

and^ Classes,

developer

writes^ the

classes^ that

implements

this^ interfaces. ^ Extends

the^ classes

and^ provides

appropriate

implementation

of^ various

methods

and^ rest^

will^ be

done^ by^ container.  Container

takes^ the

responsibility

of^ instantiating,

initializing

and^ invoking

the^ components.

Application

does^ not^

directly^ instantiate

or^ invokes

components.

^ Server

itself^ Provide

runtime

support

for

J2EE^ application

components ^ Application

client container

^ Applet

container ^ Web^ container ^ EJB^ container

^ An applet container, meaning a-browser,-usually with the Java Plug-in

^ Servlet

container ^ Servlet

engine ^ A^ J2EE

container

that^ hosts

web^ applications

.

^ Extends

the^ web

server^ functionality

by^ providing

developers

the^ environment

to^ run^ servlets

and

JavaServer

Pages^ (JSPs). ^ Automatically

instantiates

and^ initializes

the

Servlets

on^ application

startup^

or^ when

invoked

for

the^ first

time.^ Container

calls^ the

service()

method

when^ user

requests

the^ servlet.

^ The^ EJB

server

provides

an^ environment

that^ ‐ Supports

the^ execution

of^ applications

developed

using^

Enterprise

JavaBeans™

(EJB)^ components.  Enterprise

beans

typically

contain

the

business

logic^ for

a^ J2EE

application.

^ The^ EJB

server^ must

provide^

one^ or^ more

EJB

containers.  An^ EJB^ container

manages

the^ enterprise

beans

contained

within^ it. ^ Responsible

for ‐ Registering

the^ object ‐ Providing

a^ remote

interface

for^ the^ object

‐ Creating

and^ destroying

object^ instances ‐ Checking

security

for^ the^ object ‐ Managing

the^ active

state^ for

the^ object

‐ Coordinating

distributed

transactions.

^ Manage

memory

consumption

^ Processing ^ Containers

pool^ resources ^ Lifecycles

of^ all^ the

beans

When^ a

bean^ is

not^ being

used,^ a

container

will^ place

it^ in^ a^ pool

to^ be^ reused

by

another

client,

or^ possibly

evict^ it

from

memory

and^ only

bring^ it

back^ when

its

needed.

A^ container

may^ provide

the^ following

services:  Lifecycle

management ^ Bean^

Pooling ^ Transactions ^ Security ^ Connection

pooling ^ Instance

pooling

^ A^ Web

Server is

a^ server

capable

of

‐ Receiving HTTP requests ‐ Interpreting

them ‐ Processing the

corresponding

HTTP

Responses ‐ Sending them

to^ the

appropriate

clients

(Web^ Browsers). Example:

Apache

Web^ Server.

^ It's^ a^

server^

used^ to

communicate

with

Web^ Browsers

as^ its^ clients. ^ Communication

protocol used

in^ this

case^ is

HTTP

(HyperText

Transfer

Protocol).  This^ is

why^ a^

Web^ Server

is^ also

called

an

HTTP^

Server.

^ A^ Web^

Container

is^ a^ J2EE^

compliant

implementation

which^ provides

an^ environment

for^ the^ Servlets and

JSPs^ to

run.^ Putting

it^ differently

we^ can^ say

that^ a^ Web

Container

is^ combination

of^ a^ Servlet Engine

and^ a^ JSP

Engine.^ If

an

HTTP^ Request

refers^ to^ a^ Web^ Component

(typically

a

Servlet or

a^ JSP)^ then

the^ request

is^ forwarded

to^ the^ Web

Container

and^ the^ result

of^ the^ request

is^ sent^ back

to^ Web

Server,^ which

uses^ that

result^ to^

prepare^ the

HTTP

Response

for^ the^ particular

HTTP^ Request.

Example:

Tomcat^ is

a^ typical^

Web^ Container.

A^ typical

setup^ would

be^ to^ have

Apache^ HTTP

Server^ as

the^ Web^

Server^ and

Tomcat^ as

the^ Web^

Container.

20

A^ Web^ Server

that^ Support

Servlets^

and^ Java^ Server

Pages

(JSP)