C HA PTER 1 A N INTRODUC TION TO S PRIN G in Java

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The transfer classes (form beans) in Spring MVC are conventional POJOs, and the validation framework is both POJO-based and simple to configure.

Spring Web Flow can be seen as a complement to the existing Spring MVC framework, as it uses the existing view resolvers and a specialized controller to provide its functionality. Web Flow allows you to model your application behavior as a state machine: the application resides in various states, and events are raised to move the application between these states. That may sound a bit weird if you haven t seen this sort of model before, but it s actually a pretty well-accepted approach to designing certain types of web applications. Web Flow allows you to design modules of your web application as complex user journeys without arbitrary end points. Whereas Spring MVC is ideal for simple linear formbased problems, Spring Web Flow is suited to more-dynamic problems. The two can be mixed and matched as appropriate. The additional advantage of building an application by using Web Flow is the ease of design state machines are easy to model as diagrams combined with the fact that a Web Flow application can readily be packaged for reuse in other projects. The web component of our example application is built using a combination of Spring MVC and Spring Web Flow so you will have an opportunity to gauge the relative merits of these two related approaches to web application design.

Of specialized interest to Portlet developers is the Spring Portlet MVC framework. Portlet containers (portals) allow you to build a larger web application up from a set of smaller subcomponents that can reside together on the same web page. Portals usually provide a set of standard infrastructure capabilities such as user authentication and authorization. A typical portal is supplied with a large suite of standard portlets to allow users to read e-mail, manage content, maintain a calendar, and so on. This makes them attractive for creating in intranets or for customer-facing websites, where a set of basic services can be supplemented by a small suite of custom-written tools to provide an integrated environment without the expense and time constraint of creating an entirely bespoke system. Spring Portlet MVC provides an exactly analogous version of the Spring MVC framework for working within a JSR 168 compliant portlet environment. Although Spring Portlet MVC builds on the JSR 168 portlet API, the differences between Spring Portlet MVC and Spring MVC are much easier to accommodate than the differences between the portlet API and the servlet API that underlie them. In addition to minimizing the technical differences between the portlet and servlet APIs, Spring Portlet MVC provides all of the facilities to the portlet environment that Spring MVC provides to the servlet environment.

While it introduces some delightful frameworks of its own, Spring also plays nicely with existing frameworks. There is full support for the use of Apache Struts, JavaServer Faces, and Apache Tapestry in the framework. In each case, suitable classes are provided to allow you to inject dependencies into the standard implementation classes. Where possible, several approaches are offered for users who may be working under additional constraints. For example, the Struts framework can be Spring enabled by configuring your actions using either DelegatingRequestProcessor or DelegatingActionProxy. The former allows closer integration with Spring, but the latter allows you to take advantage of Spring features without giving up any custom request processors that you may be using (Struts does not allow you to configure multiple request processors). Similar support is available for most commonly used frameworks, and the approaches used for these transfer well to any other web framework that uses standard Java features and that provides for a modicum of extensibility.