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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.

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: ActionForm beans are actually closer to the View than the Model.

The Struts framework generally assumes that you have defined an ActionForm bean (that is, a Java class extending the ActionForm class) for each input form required in your application. ActionForm beans are sometimes just called "form beans". If you declare such beans in your ActionMapping configuration file (see " Building the Controller Components"), the Struts controller servlet will automatically perform the following services for you, before invoking the appropriate Action method:

  • Check in the user's session for an instance of a bean of the appropriate class, under the appropriate key.
  • If there is no such session scope bean available, a new one is automatically created and added to the user's session.
  • For every request parameter whose name corresponds to the name of a property in the bean, the corresponding setter method will be called. This operates in a manner similar to the standard JSP action <jsp:setProperty> when you use the asterisk wildcard to select all properties.
  • The updated ActionForm bean will be passed to the Action Class perform() method when it is called, making these values immediately available.

When you code your ActionForm beans, keep the following principles in mind:

  • The ActionForm class itself requires no specific methods to be implemented. It is used to identify the role these particular beans play in the overall architecture. Typically, an ActionForm bean will have only property getter and property setter methods, with no business logic.
  • The ActionForm object also offers a standard validation mechanism. If you override a "stub" method, and provide error messages in the standard application resource, Struts will automatically validate the input from the form (using your method). See "Action Form Validation" for details. Of course, you can also ignore the ActionForm validation and provide your own in the Action object.
  • Define a property (with associated getXxx() and setXxx() methods) for each field that is present in the form. The field name and property name must match according to the usual JavaBeans conventions. For example, an input field named username will cause the setUsername() method to be called.
  • Think of your ActionForm beans as a firewall between HTTP and the Action. Use the validate method to ensure all required properties are present, and that they contain reasonable values. An ActionForm that fails validation will not even be presented to the Action for handling.
  • You may also place a bean instance on your form, and use nested property references. For example, you might have a "customer" bean on your Action Form, and then refer to the property "customer.name" in your JSP view. This would correspond to the methods customer.getName() and customer.setName(string Name) on your customer bean. See the Tag Library Developer Guides for more about nested syntax.
  • Caution: If you nest an existing bean instance on your form, think about the properties it exposes. Any public property on an ActionForm that accepts a single String value can be set with a query string. It may be useful to place such beans inside a thin "wrapper" that exposes only the properties required. This wrapper can also provide a filter to be sure runtime properties are not set to inappropriate values.

You should note that a "form", in the sense discussed here, does not necessarily correspond to a single JSP page in the user interface. It is common in many applications to have a "form" (from the user's perspective) that extends over multiple pages. Think, for example, of the wizard style user interface that is commonly used when installing new applications. Struts encourages you to define a single ActionForm bean that contains properties for all of the fields, no matter which page the field is actually displayed on. Likewise, the various pages of the same form should all be submitted to the same Action Class. If you follow these suggestions, the page designers can rearrange the fields among the various pages, often without requiring changes to the processing logic.

2.4 System State Beans

The actual state of a system is normally represented as a set of one or more JavaBeans classes, whose properties define the current state. A shopping cart system, for example, will include a bean that represents the cart being maintained for each individual shopper, and will (among other things) include the set of items that the shopper has currently selected for purchase. Separately, the system will also include different beans for the user's profile information (including their credit card and ship-to addresses), as well as the catalog of available items and their current inventory levels.

For small scale systems, or for state information that need not be kept for a long period of time, a set of system state beans may contain all the knowledge that the system ever has of these particular details. Or, as is often the case, the system state beans will represent information that is stored permanently in some external database (such as a CustomerBean object that corresponds to a particular row in the CUSTOMERS table), and are created or removed from the server's memory as needed. Entity Enterprise JavaBeans are also used for this purpose in large scale applications.

2.5 Business Logic Beans

You should encapsulate the functional logic of your application as method calls on JavaBeans designed for this purpose. These methods may be part of the same classes used for the system state beans, or they may be in separate classes dedicated to performing the logic. In the latter case, you will usually need to pass the system state beans to be manipulated to these methods as arguments.

For maximum code re-use, business logic beans should be designed and implemented so that they do not know they are being executed in a web application environment. If you find yourself having to import a javax.servlet.* class in your bean, you are tying this business logic to the web application environment. Consider rearranging things so that your Action classes (part of the Controller role, as described below) translate all required information from the HTTP request being processed into property setter calls on your business logic beans, after which a call to an execute() method can be made. Such a business logic class can be reused in environments other than the web application for which they were initially constructed.

Depending on the complexity and scope of your application, business logic beans might be ordinary JavaBeans that interact with system state beans passed as arguments, or ordinary JavaBeans that access a database using JDBC calls. For larger applications, these beans will often be stateful or stateless Enterprise JavaBeans (EJBs) instead.

2.6 Accessing Relational Databases

Struts can define the datasources for an application from within its standard configuration file. A simple JDBC connection pool is also provided. See The Action Mappings Configuration File section and the Utilities Developer Guide for details.

After the datasource is defined, here is an example of establishing a connection from within a Action perform method.

public ActionForward
       perform(ActionMapping mapping,
               ActionForm form,
               HttpServletRequest request,
               HttpServletResponse response)
{
 try {
   javax.sql.DataSource dataSource =
     servlet.findDataSource(null);
   java.sql.Connection myConnection =
     dataSource.getConnection();

   //do what you wish with myConnection
 } catch (SQLException sqle) {
   getServlet().log("Connection.process", sqle);
 } finally {

   //enclose this in a finally block to make
   //sure the connection is closed
   try {
     myConnection.close();
   } catch (SQLException e) {
     getServlet().log("Connection.close", e);
   }
 }
}

Note that the Struts generic connection pool is an optional component. Many Struts applications use other connection pools for better performance, especially with high-volume production systems.

Next: Building View Components


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