J2EE Part 2

Although a J2EE application can consist of the three or four tiers, J2EE multitiered applications are generally considered to be three-tiered applications because they are distributed over three different locations: client machines, the J2EE server machine, and the database.

              


The above shows multi-tiered applications.

J2EE CLIENTS: can be web client or application client. Web clients consists of dynamic web pages generated by web components running in web-tier and web browser that renders the dynamic pages received from server in presentable format. Applet is a small client application that executes in JVM installed in web browser. Web clients are also called as 'thin clients' as they do not do heavy weight operations such as database queries or implementation of complex business logic. Another type of client is application client. Application client makes use of typical swing or Abstract Window Toolkit(AWT) APIs to provide a rich user interface.
The difference between standalone client and J2EE client is that J2EE client can connect to J2EE services.


Now we know that the client communicates to the business tier(meaning business layer on another computing system... therefore 'tier') either directly or through the presentation tier(meaning presentation layer on another computing system... through JSPs and servelets).


J2EE SERVER:
Web Component: They can be servelets or JSPs. Servelets are java classes having embedded html pages for dynamically processing requests and constructing responses. JSPs on the other hand are html pages having embedded java code.
Business Component: It contains the business logic or the implementation part... implementation of all the business rules.
JavaBeans Component: They can be on client side for managing data flow between applet or any other client to the components on J2EE server or it can be on J2EE server side for managing data flow between J2EE server components and database.






DATABASE SERVER:
EIS Tier: It includes Enterprise Resource Planning(ERP) and handles information systems. It manages all the database connectivity stuff.


Finally we get:





Coffee Break
Noramlly J2EE used is 3 tiered architecture.
The tiers are J2EE client, J2EE server and database server.
Client tier can be web client or application client.
JavaBeans components are NOT considered as J2EE components by specification.
J2EE server consists of web tier and business tier. 
Database server consists of EIS tier.