Understanding JavaServer Pages Model 2 architectur

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Understanding JavaServer Pages Model 2 architecture
Exploring the MVC design pattern
By Govind Seshadri, JavaWorld.com, 12/29/99
Despite its relatively recent introduction, JavaServer Pages (JSP) technology is well on its way to becoming the preeminent【卓越的】 Java technology for building applications that serve dynamic Web content. Java developers love JSP for myriad【种种】 reasons. Some like the fact that it brings the "write once, run anywhere" paradigm to interactive Web pages; others appreciate the fact that it is fairly simple to learn and lets them wield Java as a server-side scripting language. But most concur on one thing -- the biggest advantage of using JSP is that it helps effectively separate presentation from content. In this article, I provide an in-depth look at how you can gain optimal separation of presentation from content by using the JSP Model 2 architecture. This model can also be seen as a server-side implementation of the popular Model-View-Controller (MVC) design pattern. Please note that you should be familiar with the basics of JSP and servlet programming before continuing on, as I do not address any syntax issues in this article.
Server-Side Java: Read the whole series!   

Welcome to the server-side Java series

Create forward-compatible beans in EJB, Part 1

Understanding JavaServer Pages Model 2 Architecture

So, what's wrong with servlets?
While JSP may be great for serving up dynamic Web content and separating content from presentation, some may still wonder why servlets should be cast aside for JSP. The utility of servlets is not in question. They are excellent for server-side processing, and, with their significant installed base, are here to stay. In fact, architecturally speaking, you can view JSP as a high-level abstraction of servlets that is implemented as an extension of the Servlet 2.1 API. Still, you shouldn't use servlets indiscriminately; they may not be appropriate for everyone. For instance, while page designers can easily write a JSP page using conventional HTML or XML tools, servlets are more suited for back-end developers because they are often written using an IDE -- a process that generally requires a higher level of programming expertise. When deploying servlets, even developers have to be careful and ensure that there is no tight coupling between presentation and content. You can usually do this by adding a third-party HTML wrapper package like htmlKona to the mix. But even this approach, though providing some flexibility with simple screen changes, still does not shield you from a change in the presentation format itself. For example, if your presentation changed from HTML to DHTML, you would still need to ensure that wrapper packages were compliant with the new format. In a worst-case scenario, if a wrapper package is not available, you may end up hardcoding the presentation within the dynamic content. So, what is the solution? As you shall soon see, one approach would be to use both JSP and servlet technologies for building application systems.

Differing philosophies
The early JSP specifications advocated two philosophical approaches for building applications using JSP technology. These approaches, termed the JSP Model 1 and Model 2 architectures, differ essentially in the location at which the bulk of the request processing was performed. In the Model 1 architecture, shown in Figure 1, the JSP page alone is responsible for processing the incoming request and replying back to the client. There is still separation of presentation from content, because all data access is performed using beans. Although the Model 1 architecture should be perfectly suitable for simple applications, it may not be desirable for complex implementations. Indiscriminate usage of this architecture usually leads to a significant amount of scriptlets or Java code embedded within the JSP page, especially if there is a significant amount of request processing to be performed. While this may not seem to be much of a problem for Java developers, it is certainly an issue if your JSP pages are created and maintained by designers -- which is usually the norm on large projects. Ultimately, it may even lead to an unclear definition of roles and allocation of responsibilities, causing easily avoidable project-management headaches.

Figure 1: JSP Model 1 architecture
The Model 2 architecture, shown in Figure 2, is a hybrid approach for serving dynamic content, since it combines the use of both servlets and JSP. It takes advantage of the predominant strengths of both technologies, using JSP to generate the presentation layer and servlets to perform process-intensive tasks. Here, the servlet acts as the controller and is in charge of the request processing and the creation of any beans or objects used by the JSP, as well as deciding, depending on the user's actions, which JSP page to forward the request to. Note particularly that there is no processing logic within the JSP page itself; it is simply responsible for retrieving any objects or beans that may have been previously created by the servlet, and extracting the dynamic content from that servlet for insertion within static templates. In my opinion, this approach typically results in the cleanest separation of presentation from content, leading to clear delineation of the roles and responsibilities of the developers and page designers on your programming team. In fact, the more complex your application, the greater the benefits of using the Model 2 architecture should be.


Figure 2: JSP Model 2 architecture
In order to clarify the concepts behind the Model 2 architecture, let's walk through a detailed implementation of it: a sample online music store called Music Without Borders.
SUBHEAD_BREAK: 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 .
Listing 1:
EShop.jsp
Music Without Borders

  Music Without Borders




CD:

  Yuan | The Guo Brothers | China | 4.95
  Drums of Passion | Babatunde Olatunji | Nigeria | 6.95
  Kaira | Tounami Diabate| Mali | 6.95
  The Lion is Loose | Eliades Ochoa | Cuba | 3.95
  Dance the Devil Away | Outback | Australia | 4.95
  Record of Changes | Samulnori | Korea | 2.95
  Djelika | Tounami Diabate | Mali | 4.95
  Rapture | Nusrat Fateh Ali Khan | Pakistan | 2.95
  Cesaria Evora | Cesaria Evora | Cape Verde | 6.95
  Ibuki | Kodo | Japan | 3.95

Quantity:






Listing 2:
Cart.jsp
0)) {
%>

ALBUM
ARTIST
COUNTRY
PRICE
QUANTITY




  
  
  
  
  
  
   
   
   '>
   
   
      
     
   
  
  
  
   
   
  
  
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:
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.


Figure 3: Music Without Borders, main view
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:

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.


Figure 4: Music Without Borders, shopping cart view
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
import java.util.*;
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import shopping.CD;
public class ShoppingServlet extends HttpServlet {
  public void init(ServletConfig conf) throws ServletException  {
    super.init(conf);
  }
  public void doPost (HttpServletRequest req, HttpServletResponse res)
      throws ServletException, IOException {
    HttpSession session = req.getSession(false);
    if (session == null) {
      res.sendRedirect("http://localhost:8080/error.html");
    }
    Vector buylist=
      (Vector)session.getValue("shopping.shoppingcart");
    String action = req.getParameter("action");
    if (!action.equals("CHECKOUT")) {
      if (action.equals("DELETE")) {
        String del = req.getParameter("delindex");
        int d = (new Integer(del)).intValue();
        buylist.removeElementAt(d);
      } else if (action.equals("ADD")) {
        //any previous buys of same cd?
        boolean match=false;
        CD aCD = getCD(req);
        if (buylist==null) {
          //add first cd to the cart
          buylist = new Vector(); //first order
          buylist.addElement(aCD);
        } else { // not first buy
          for (int i=0; i
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
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;
  }
}
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
Music Without Borders Checkout

  Music Without Borders Checkout





ALBUM
ARTIST
COUNTRY
PRICE
QUANTITY












      
      
TOTAL
$
      



Shop some more!

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!


Figure 5: Music Without Borders, checkout view
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.
Author BioGovind Seshadri is an Enterprise Java Guru for jGuru.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 the community better understand server-side Java technology. Together, JavaWorld and jGuru are jointly producing articles, free educational Web events, and working together on the JavaWorld bookstore and Web-based training.
Listing 6:
error.html
  Sorry, there was an unrecoverable error!
  Please try again.
Deploying Music Without BordersI 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.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 using
  http://localhost:8080/examples/jsp/shopping/EShop.jsp
  as the URL

Leveraging【杠杆作用】 JSP and servletsIn 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.


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