Benefits of the 3-Tier Scenario

A modern n-tier application architecture, such as that provided by J2EE, involves the separation of functionality both by using layers and tiers and also the use of components within those layers (and objects within those components).

Now, presentational developers need not know anything of the business rules in the system, and any changes to any of the layers should not impact the effectiveness of any of the others. This aids in maintenance of the system and promotes scalability and extensibility. The separation into components helps with the division of tasks even further.

With the advent of the Internet, enabling more businesses to deliver goods and services online, it is easier to deliver functionality to customers and business users. There may be issues with particular versions of browsers, but compared to the situation where thick client applications would need to be distributed and installed on each client machine, the relative merit of distributing functionality using the Internet remains overwhelming considering the potential user base companies are striving to reach.

It is this emphasis on the Internet that many enterprise service vendors are seeking to exploit. Most organizations have some form of Web presence and many are trying to use this to offer services to their customers. Since such services interact directly with the customer, their levels of reliability and usability must be high. Such Web-based applications are now common currency, and the world is evolving further. Web services are being discussed as the next generation of n-tier development, allowing applications to be created from components distributed across the Internet. As this model evolves, the distributed Internet becomes the computer.

Enterprise applications can be Web centric, but need not be. To cover Web-centric programming, this book shows how to integrate Servlets (see Day 12, “Servlets”) and Java Server Pages (JSPs) (see Day 13, “JavaServer Pages”) into Enterprise applications. Within an organization, or even when creating business-to-business (B2B) links, enterprise applications need not use Servlets or JSPs. In this case, clients may connect directly to business components, in the shape of Enterprise JavaBeans (see Day 4, “Introduction to EJBs”), over RMI or CORBA.

As the provision of functionality over the Internet gains importance, most companies will expose the functionality of their internal applications as part of a Web-based application. As functionality is exposed, it becomes important to maintain the integrity of the data in the corporate systems. Transactions provide a common mechanism for doing this. Transactions are covered in detail in Day 8, “Transactions and Persistence.”