The J2EE platform uses a distributed multitiered application model for enterprise applications. Application logic is divided into components according to function, and the various application components that make up a J2EE application are installed on different machines depending on the tier in the multitiered J2EE environment to which the application component belongs. Figure 1-1 shows two multitiered J2EE applications divided into the tiers described in the following list. The J2EE application parts shown in Figure 1-1 are presented in J2EE Components.

Separation of static from dynamic content: With servlets, the logic for generation of the dynamic content is an intrinsic part of the servlet itself, and is closely tied to the static presentation templates responsible for the user interface. Thus, even minor changes made to the UI typically result in the recompilation of the servlet. This tight coupling of presentation and content results in brittle, inflexible applications. However, with JSP, the logic to generate the dynamic content is kept separate from the static presentation templates by encapsulating it within external JavaBeans components. These are then created and used by the JSP page using special tags and scriptlets. When a page designer makes any changes to the presentation template, the JSP page is automatically recompiled and reloaded into the web server by the JSP engine.

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The sample page you'll be creating adds 2 + 2 and displays the result in the browser.

JSP pages are created by putting a add.jsp file in the document directory. The JSP page is a mixture of HTML and Java code. Resin's JSP engine compiles the .jsp file into a Java servlet and then runs the servlet to produce the output.

Resin's web server stores its pages in a directory named doc. On this machine it's /var/www/hosts/www.caucho.com/webapps/ROOT/

Create the text file doc/test/add.jsp (/var/www/hosts/www.caucho.com/webapps/ROOT/test/add.jsp) with your favorite text editor: notepad, emacs, vi, or whatever. It should contain the following text:

Errors happen. There's a reason the patron saint of programmers is St. Murphy. Here we'll just explain Resin's error reporting to simplify tracking down errors.

There are three types of bugs you'll find in JSP code: parse errors, runtime exceptions, and functional errors.

Parse errors are easy, like grammatical errors. In the example, we've forgotten to add the second `2