2. Building Model Components
2.1 Overview
Many requirements documents used for building web applications focus on the View. However, you should ensure that the processing required for each submitted request is also clearly defined from the Model's perspective. In general, the developer of the Model components will be focusing on the creation of JavaBeans classes that support all of the functional requirements. The precise nature of the beans required by a particular application will vary widely depending on those requirements, but they can generally be classified into several categories discussed below. However, a brief review of the concept of "scope" as it relates to beans and jsp is useful first.
很多对于创建web应用的需求文档关注于View。然而，
2.2 JavaBeans and Scope
Within a web-based application, JavaBeans can be stored in (and Accessed from) a number of different collections of "attributes". Each collection has different rules for the lifetime of that collection, and the visibility of the beans stored there. Together, the rules defining lifetime and visibility are called the scope of those beans. The JavaServer Pages (JSP) Specification defines scope choices using the following terms (with the equivalent servlet API concept defined in parentheses):
page - Beans that are visible within a single JSP page, for the lifetime of the current request. (Local variables of the service method)
request - Beans that are visible within a single JSP page, as well as to any page or servlet that is included in this page, or forwarded to by this page. (Request attributes)
session - Beans that are visible to all JSP pages and servlets that participate in a particular user session, across one or more requests. (Session attributes)
application - Beans that are visible to all JSP pages and servlets that are part of a web application. (Servlet context attributes)
It is important to remember that JSP pages and servlets in the same web application share the same sets of bean collections. For example, a bean stored as a request attribute in a servlet like this:
MyCart mycart = new MyCart(...);
request.setAttribute("cart", mycart);
is immediately visible to a JSP page which this servlet forwards to, using a standard action tag like this:
<jsp:useBean id="cart" scope="request"
class="com.mycompany.MyApp.MyCart"/>
2.3 ActionForm Beans
Note: While ActionForm beans often have properties that correspond to properties in your Model beans, the form beans themselves should be considered a Controller component. As sUCh, they are able to transfer data between the Model and View layers.
