[英文版] Understanding JavaServer Pages Model 2 architecture （Exploring the MVC design pattern）--1

Despite its relatively recent introduction, JavaServer Pages (JSP) technology is well on its way to becoming the preeminent Java technology for building applications that serve dynamic Web content. Java developers love JSP for myriad 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.
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.