A MVC Model 1 architecture consists of a Web browser directly accessing Web-tier JSP pages. The JSP pages access Web-tier JavaBeans that represent the application model, and the next view to display (JSP page, servlet, HTML page, and so on) is determined either by hyperlinks selected in the source document or by request parameters. A Model 1 application control is decentralized, because the current page being displayed determines the next page to display. In addition, each JSP page or servlet processes its own inputs (parameters from GET or POST). In some Model 1 architectures, choosing the next page to display occurs in scriptlet code, but this usage is considered poor form.
A MVC Model 2 architecture introduces a controller servlet between the browser and the JSP pages or servlet content being delivered. The controller centralizes the logic for dispatching requests to the next view based on the request URL, input parameters, and application state. The controller also handles view selection, which decouples JSP pages and servlets from one another. Model 2 applications are easier to maintain and extend, because views do not refer to each other directly. The Model 2 controller servlet provides a single point of control for security and logging, and often encapsulates incoming data into a form usable by the back-end MVC model. For these reasons, the Model 2 architecture is recommended for most interactive applications.