Java™ Application Development on Linux - Dator

138Chapter 5The Sun Microsystems Java Software <strong>Development</strong> Kitof software systems. This second kind used to be the only kind, but now it may,in a sense, be the minority of applications, but the most critical to an operation.This second class of system has come to be called enterprise systems.In enterprise computing, we have the same problem we opened with: Howdo you make the information and resources available to everyone who needsthem at the lowest cost? And the answer is (as it always is) “that depends.”These days, one of the most common solutions is to use a Web server topublish an application. This works well for a great many applications and it ismuch easier to do than many other methods. That explains its popularity. TheWeb interface is quite limited, however, so for more user interface intensiveapplications, client-server computing evolved. To us techies, all of this stuff isclient-server. In this context however, client-server refers to a 2-tier systemwhere the UI and logic exist in a GUI application on a PC and commonresources are in an SQL database all the clients share.This is also commonly used, but becomes expensive in a couple of cases.The first is when the database itself becomes a bottleneck because the numberof users grows and grows but only one database can exist. The second is simplythe cost of maintenance. Since the logic exists in the client, any change to thelogic requires updating the software on all clients. Even when a scheme for automatingthis exists, it is still time-consuming and costly to get all the changesout to all users simultaneously. There are workarounds for both of these issues,but here we are concerned with a different solution altogether.So, how can we have a UI richer than with a Web application but avoidthe pitfalls of the traditional 2-tier client-server computing? The answer is toseparate the UI from the business logic and the business logic from the underlyingdata store. This results in 3 tiers—presentation, business logic, and data.Much of this book will concern itself with 3-tier computing solutions.Java has four major architectures for building 3-tier solutions. One of themis RMI. 175.8.1.2 RMI BasicsRMI works by sharing an interface between the client and the server. The interfacegroups together the methods that a client may call on a server. A class is17. The others are Enterprise JavaBeans, servlets, and JavaServer Pages. The latter two are Webbased,and therefore suffer from the UI deficiencies of Web forms, but Sun calls them part ofEnterprise Java, so we will too.