The Struts framework depends on one or more configuration files to be able to load and create the necessary application-specific components at startup. The configuration files allow the behavior of the framework components to be specified declaratively, rather than having the information and behavior hardcoded. This gives developers the flexibility to provide their own extensions, which the framework can discover dynamically.
The configuration file is based on the XML format and can be validated against the Struts DTD struts-config_1_1.dtd. Although there are some similarities between the 1.0 and 1.1 versions of the framework with respect to the configuration file, there are at least as many differences. Fortunately, the designers of the framework have made backward compatibility a goal of the Struts 1.1 release; therefore, your 1.0 applications should continue to work properly with the new version.
Application modules were mentioned briefly in Chapter 3, but we haven’t yet fully introduced this new feature. With application modules, you can define multiple Struts configuration files, one for each supported module. Each application module can provide its own configuration information, including message resources, and be completely independent from other modules.
Application modules allow a single Struts application to be split into separate projects, making parallel development easier to accomplish. Although the functionality for application modules exists in the framework, you are not required to implement more than one (the default application module). We’ll discuss application modules further in Chapter 5, Chapter 6, and Chapter 7. For now, we’ll concentrate on configuring the default application; we’ll see how easy it is to add additional modules later.
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