Understanding JavaServer Pages Model 2 architecture
Exploring the MVC design pattern
By Govind Seshadri, JavaWorld.com, 12/29/99
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Listing 4:
CD.java
package shopping; public class CD { String album; String artist; String country; float price; int quantity; public CD() { album=""; artist=""; country=""; price=0; quantity=0; } public void setAlbum(String title) { album=title; } public String getAlbum() { return album; } public void setArtist(String group) { artist=group; } public String getArtist() { return artist; } public void setCountry(String cty) { country=cty; } public String getCountry() { return country; } public void setPrice(float p) { price=p; } public float getPrice() { return price; } public void setQuantity(int q) { quantity=q; } public int getQuantity() { return quantity; } }
Noticethat we have also included additional intelligence within the servlet,so that it understands that, if a previously added CD is reselected, itshould simply increase the count for that CD bean within the shoppingcart. The controller servlet also processes actions triggered fromwithin Cart.jsp,such as the user deleting items from the shopping cart, or proceedingto the checkout counter. Observe that the controller always hascomplete control over which resources should be invoked in response tospecific actions. For example, changes made to the state of theshopping cart, such as additions or deletions, cause the controllerservlet 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 theuser 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
<%@ page session="true" import="java.util.*, shopping.CD" %> Music Without Borders Checkout

---

<% Vector buylist = (Vector) session.getValue("shopping.shoppingcart"); String amount = (String) request.getAttribute("amount"); for (int i=0; i < buylist.size();i++) { CD anOrder = (CD) buylist.elementAt(i); %> <% } session.invalidate(); %>

ALBUM	ARTIST	COUNTRY	PRICE	QUANTITY
<%= anOrder.getAlbum() %>	<%= anOrder.getArtist() %>	<%= anOrder.getCountry() %>	<%= anOrder.getPrice() %>	<%= anOrder.getQuantity() %>
		TOTAL	$<%= amount %>

Shop some more!

Checkout.jspsimply extracts the shopping cart from the session and the total amountfor the request, and then displays the selected items and their totalcost. Figure 5 shows the client view upon checkout. Once the user goesto the checkout counter, it is equally important to get rid of thesession object. That is taken care of by having a session.invalidate()invocation at the end of the page. This process is necessary for tworeasons. First, if the session is not invalidated, the user‘s shoppingcart is not reinitialized; if the user then attempts to commenceanother round of shopping upon checkout, her shopping cart willcontinue to hold items that she has already purchased. The secondreason is that if the user simply left the site upon checkout, thesession object will not be garbage collected and will continue to takeup valuable system resources until its lease period expires. Since thedefault session-lease period is about 30 minutes, this can quickly leadto 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 systemresources!

Figure 5: Music Without Borders, checkout view
Noticethat all the resources for this application are session aware, sincethe model here is stored within the session. Consequently, you mustensure that the user does not somehow access the controller directly,even by mistake. You can take care of this by implementing theautomatic client redirection to the error page (shown in Listing 6)when the controller detects the absence of a valid session.
Author Bio
Govind Seshadri is an Enterprise Java Guru forjGuru.com, and the author of Enterprise Java Computing --Applications and Architecture from Cambridge University Press(1999). Learn more about Govind at jGuru.com.JavaWorld and jGuru have formed a partnership to help thecommunity better understand server-side Java technology. Together,JavaWorld and jGuru are jointly producing articles, freeeducational Web events, and working together on theJavaWorld bookstore and Web-based training. Listing 6:
error.html

Sorry, there was an unrecoverable error!
Please try again.

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 theResources 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.jsp to \jswdk-1.0.1\examples\jsp\shopping
Copy Cart.jsp to \jswdk-1.0.1\examples\jsp\shopping
Copy Checkout.jsp to \jswdk-1.0.1\examples\jsp\shopping
Compile the .java files by typing javac *.java
Copy ShoppingServlet.class to \jswdk-1.0.1\webpages\Web-Inf\servlets
Create shopping directory under \jswdk-1.0.1\examples\Web-Inf\jsp\beans
Copy CD.class to \jswdk-1.0.1\examples\Web-Inf\jsp\beans\shopping
Copy error.html to \jswdk-1.0.1\webpages
Once your server has been started, you should be able to access the application usinghttp://localhost:8080/examples/jsp/shopping/EShop.jsp as the URL
Leveraging JSP and servlets
Inthis example, we have examined in detail the level of control andflexibility provided by the Model 2 architecture. In particular, we‘veseen how the best features of servlets and JSP pages can be exploitedto maximize the separation of presentation from content. Properlyapplied, the Model 2 architecture should result in the concentration ofall 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 forsimpler applications.