--Ralph Waldo Emerson

Layering with dynamic templates

In the first part of this book, we stressed the importance of layering an application to isolate the effects of change. By compartmentalizing an application, we can change one piece without disrupting the other pieces. The same concept can be applied within the presentation layer to separate the look and feel from the actual content.

• One approach to separating layout from content is the use of dynamic JSP includes. The JSP specification provides for both static and dynamic includes. The standard JSP action for a dynamic include is <jsp:include> .
• We make use of dynamic includes by breaking the server page into several fragments, each with its own job to do. A background template can set the default format and layout, and page fragments can be included at runtime to provide the content. A dynamic include folds the output of the included page into the original page. It acts like a switch that moves processing over to a page fragment and then back to the caller. As shown in See The effect of the <jsp:include> action on the processing of a request, the included template is processed normally, just as if it had been called on its own.

The Tiles framework, which we explore in this chapter, uses a more advanced form of the JSP include action. In a Tiles application, the background, or layout, template usually defines the position of a header, menu body, content, and footer. Other pages are then included to fill each of these positions. If the header changes, then only that template file need be changed. The change is automatically reflected in all pages that include that template. The effects of changes are minimized--and the hobgoblins appeased.

• Standard HTML components, like Cascading Style Sheets (CSSs), also work well with dynamic templates. A style sheet can help keep the templates internally consistent and further minimizes the effects of change.

Template consequences

Every technology comes bundled with compromises. Here are some consequences--pro, con, and mixed--that come with using dynamic templates in your application:

• The JSP include technology is well established and reliable, and tends to scale well in larger applications. The underlying technology for including dynamic templates is part of the core Java Servlet API.
• Most containers are optimized for JSPs and standard features like servlet include.
• The included pages typically output HTML fragments and are not synoptically complete. This can prevent you from maintaining templates with standard HTML editors, which expect markup to be part of a complete, stand-alone page.
• Most sites recompile JSP pages when the source changes. Templates create more pages to be monitored for such changes.
• Templates actually reuse code that would otherwise be duplicated from page to page. This can result in a significantly smaller footprint and conserve server resources.