4.1 Reuse in Web Applications

Reusability is a driving factor in object-oriented languages, including Java. Java is full of built-in frameworks meant to promote reuse. JavaBeans provides an explicit component model, while Swing and AWT both provide reusable widget libraries. Throughout the software industry, the adoption of object-oriented programming languages in the 90s led to across-the-board development of a number of frameworks for reuse.

Sadly, there is no standard component framework in the relatively young field of J2EE web applications. While the JavaBeans component model is often used to communicate model data, there is no universal mechanism for reusing parts of views or controllers. Until quite recently, web application developers have had to build their own framework or settle for applications that were not intrinsically reusable. This problem has been recently recognized, and frameworks such as Struts^[2] have gained popularity as a solution (see the sidebar "Frameworks and Patterns"). These frameworks allow controllers and views to be developed using standard interfaces and connected in standard ways.

^[2] An impressive list of open source frameworks is available at http://www.waferproject.org.

Frameworks and Patterns Most web application frameworks implement at least the MVC pattern, and many even claim to be "pattern-based." In general, frameworks force applications written to them to conform to certain patterns. Recognizing these patterns can be a big help in designing applications that work well in a given framework. Here's a look at two popular frameworks and the patterns they implement: Jakarta Struts is probably the best-known web application framework. It is based on what they call a "Model 2" approach, an implementation of MVC in which you write the model and view components, and Struts provides the controller. The Struts controller is based on the Service to Worker pattern, featuring a front controller, a dispatcher configured via XML files, and a code framework for embedding application logic into actions. Struts also provides a number of utility classes and tag libraries, including Tiles, an implementation of the Composite View pattern. JavaServer Faces (JSF) is a new web application framework from Sun. The primary goal of JSF is to provide a UI framework for web applications, allowing the development of custom, reusable UI widgets that maintain state across requests. As such, JSF is centered around the Composite View pattern, and includes an extensive tag library for developing and using these components. JSF also provides a simple implementation of the Service to Worker pattern, including a front controller, an XML-configured navigation handler, and support for actions.