[英文版] Understanding JavaServer Pages Model 2 architecture （Exploring the MVC design pattern）--2
Understanding Music Without Borders
The main view, or presentation, for our Music Without Borders online store is facilitated by the JSP page EShop.jsp (shown in Listing 1). You will notice that the page deals almost exclusively with presenting the main user interface of the application to the client, and performs no processing whatsoever -- an optimal JSP scenario. Also, notice that another JSP page, Cart.jsp (shown in Listing 2), is included within EShop.jsp via the directive <jsp:include page="Cart.jsp" flush="true" />.
| Listing 1: EShop.jsp
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| Listing 2: Cart.jsp
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Here, Cart.jsp handles the presentation of the session-based shopping cart, which constitutes the model in our MVC architecture. Observe the scriptlet at the beginning of Cart.jsp:
<%
Vector buylist = (Vector) session.getValue("shopping.shoppingcart");
if (buylist != null && (buylist.size() > 0)) {
%>
Basically, the scriptlet extracts the shopping cart from the session. If the cart is empty or not yet created, it displays nothing; thus, the first time a user accesses the application, she is presented with the view shown in Figure 3.
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Figure 3: Music Without Borders, main view
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If the shopping cart is not empty, then the selected items are extracted from the cart one at a time, as demonstrated by the following scriptlet:
<%
for (int index=0; index < buylist.size(); index++) {
CD anOrder = (CD) buylist.elementAt(index);
%>
Once the variables describing an item have been created, they are then simply inserted into the static HTML template using JSP expressions. Figure 4 shows the application view after the user has placed some items in the shopping cart.
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Figure 4: Music Without Borders, shopping cart view
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The important thing to observe here is that the processing for all actions carried out within either Eshop.jsp or Cart.jsp is handled by the controller servlet, ShoppingServlet.java, which is shown in Listing 3.
| Listing 3: ShoppingServlet.java
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Every time the user adds an item within EShop.jsp, the request is posted to the controller servlet. The servlet in turn determines the appropriate action, and then processes the request parameters for the item to be added. It then instantiates a new CD bean (shown in Listing 4) representing the selection, and goes about updating the shopping cart object before placing it back within the session.
| Listing 4: CD.java
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Notice that we have also included additional intelligence within the servlet, so that it understands that, if a previously added CD is reselected, it should simply increase the count for that CD bean within the shopping cart. The controller servlet also processes actions triggered from within Cart.jsp, such as the user deleting items from the shopping cart, or proceeding to the checkout counter. Observe that the controller always has complete control over which resources should be invoked in response to specific actions. For example, changes made to the state of the shopping cart, such as additions or deletions, cause the controller servlet to forward the request after processing to the Eshop.jsp page. This in turn causes the page to redisplay the main view, along with the updated contents of the shopping cart. If the user decides to check out, the request is forwarded after processing to the Checkout.jsp page (shown in Listing 5) by means of the following request dispatcher, as shown below:
String url="/jsp/shopping/Checkout.jsp";
ServletContext sc = getServletContext();
RequestDispatcher rd = sc.getRequestDispatcher(url);
rd.forward(req,res);
| Listing 5: Checkout.jsp
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Checkout.jsp simply extracts the shopping cart from the session and the total amount for the request, and then displays the selected items and their total cost. Figure 5 shows the client view upon checkout. Once the user goes to the checkout counter, it is equally important to get rid of the session object. That is taken care of by having a session.invalidate() invocation at the end of the page. This process is necessary for two reasons. First, if the session is not invalidated, the user's shopping cart is not reinitialized; if the user then attempts to commence another round of shopping upon checkout, her shopping cart will continue to hold items that she has already purchased. The second reason is that if the user simply left the site upon checkout, the session object will not be garbage collected and will continue to take up valuable system resources until its lease period expires. Since the default session-lease period is about 30 minutes, this can quickly lead to the system running out of memory in a high-volume system. Of course, we all know what happens to an application that runs out of system resources!
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Figure 5: Music Without Borders, checkout view
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Notice that all the resources for this application are session aware, since the model here is stored within the session. Consequently, you must ensure that the user does not somehow access the controller directly, even by mistake. You can take care of this by implementing the automatic client redirection to the error page (shown in Listing 6) when the controller detects the absence of a valid session.
| Listing 6: error.html
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Deploying Music Without Borders
I will assume that you are using the latest version of JavaServer Web Development Kit (JSWDK) from Sun for running the example. If not, see the Resources section to find out where to get it. Assuming that the server is installed in \jswdk-1.0.1, its default location in Microsoft Windows, deploy the Music Without Borders application files as follows:
- Create shopping directory under
\jswdk-1.0.1\examples\jsp - Copy
EShop.jspto\jswdk-1.0.1\examples\jsp\shopping - Copy
Cart.jspto\jswdk-1.0.1\examples\jsp\shopping - Copy
Checkout.jspto\jswdk-1.0.1\examples\jsp\shopping - Compile the
.javafiles by typingjavac *.java - Copy
ShoppingServlet.classto\jswdk-1.0.1\webpages\Web-Inf\servlets - Create shopping directory under
\jswdk-1.0.1\examples\Web-Inf\jsp\beans - Copy
CD.classto\jswdk-1.0.1\examples\Web-Inf\jsp\beans\shopping - Copy
error.htmlto\jswdk-1.0.1\webpages - Once your server has been started, you should be able to access the application using http://localhost:8080/examples/jsp/shopping/EShop.jsp as the URL
Leveraging JSP and servlets
In this example, we have examined in detail the level of control and flexibility provided by the Model 2 architecture. In particular, we've seen how the best features of servlets and JSP pages can be exploited to maximize the separation of presentation from content. Properly applied, the Model 2 architecture should result in the concentration of all of the processing logic in the hands of the controller servlet, with the JSP pages responsible only for the view or presentation. However, the downside of using the Model 2 approach is its complexity. Consequently, it may be desirable to use the Model 1 approach for simpler applications.