J2EE Application Components

J2EE applications are made up of components. A J2EE
component is a self-contained functional software unit that
is assembled into a J2EE application with its related
classes and files and communicates with other components.
The J2EE specification defines the following J2EE components:

    * Application clients and applets are client components.
    * Java Servlet and JavaServer Pages (JSP) technology
components are web components.
    * Enterprise JavaBeans (EJB) components (enterprise
beans) are business components.

J2EE components are written in the Java programming language
and compiled in the same way as any Java programming
language program. The difference when you work with the J2EE
platform, is J2EE components are assembled into a J2EE
application, verified that they are well-formed and in
compliance with the J2EE specification, and deployed to
production where they are run and managed by the J2EE server.
Client Components

A J2EE application can be web-based or non-web-based. An
application client executes on the client machine for a
non-web-based J2EE application, and a web browser downloads
web pages and applets to the client machine for a web-based
J2EE application.
Application Clients

An application client runs on a client machine and provides
a way for users to handle tasks such as J2EE system or
application administration. It typically has a graphical
user interface created from Project Swing or Abstract Window
Toolkit (AWT) APIs, but a command-line interface is
certainly possible.

Application clients directly access enterprise beans running
in the business tier. However, if the J2EE application
client requirements warrant it, an application client can
open an HTTP connection to establish communication with a
servlet running in the web tier.
Web Browsers

The user's web browser downloads static or dynamic Hypertext
Markup Language (HTML), Wireless Markup Language (WML), or
Extensible Markup Language (XML) web pages from the web
tier. Dynamic web pages are generated by servlets or JSP
pages running in the web tier.

There are four types of J2EE component:

Enterprise Applications:-
              These components provide a top-level 
structure which contains any number of other J2EE 
components (except other J2EE Enterprise Applications). The 
JAR file holding an enterprise application has the 
suffix .ear, so Enterprise Applications are frequently 
referred to as EARs. 
Web Applications:-
              These J2EE components may contain Servlets, 
Java ServerPages, Tag Library Definitions and plain Java 
classes or resources as required for proper operation. The 
documents and Java classes of Web Applications frequently 
create markup language (HTML, WML, XML, etc.) responses to 
requests from web browsers or business applications. The 
JAR file holding a Web application has the suffix .war, so 
WebApps are frequently referred to as WARs. 
Enterprise JavaBean Applications:-
              These components hold server-side business 
logic packaged within Enterprise JavaBeans. 
Resource Applications:-
              These J2EE components hold Java classes that 
act as drivers or communication gateways to Enterprise 
Information Systems (EISs). It is highly likely that the 
manufacturer of an EIS packages its driver into a J2EE 
resource archive (RAR).