ApacheCon: Struts for MVC Web Apps

According to Craig McClanahan, writing a web application is more difficult than writing a traditional application for a couple of pretty simple reasons:

1. HTTP is a stateless protocol, and applications need to maintain state
2. Since the web is world-wide, applications are expected to be internationalized

To complicate matters more, building any large scale application requires a large set of skills: presentation, application (business logic), persistence (files, databases), and application deployment (networks, firewalls, PKI). McClanahan claims we need Model-View-Controller as a fundamental organizing principle in designing and developing these applications.

McClanahan gave a brief overview of the MVC design paradigm in general, and how to use it in the web context in particular.

Overall, the presentation was right on the money, but McClanahan missed the mark when he was talking about the Back button. His claim was that web applications are fundamentally different than surfing around visiting web pages, and that we need to train users not to use the Back button on their web browsers when they’re using a web application. I disagree wholeheartedly. The Back button is a very powerful metaphor for a couple several reasons:

• The web is all about simplicity and a uniform interface. If it’s blue and underlined, you can click on it. If it looks like a GUI Widget (a button, text box, pull-down, etc.) you can use it the way you use other operating system Widgets. You can Search. And you can hit the Back button when you want to stop what you’re doing and go back to what you were doing before. In other words, web apps are not really that different from web content.
• Users are accustomed to using the Back button. They’ve already invested a bunch of energy learning how to use their browsers, and the Back button is probably the 3rd-most commonly used UI feature (right up there with from clicking on links and scrolling, probably used more often than even typing strings into text boxes).
• When a user hits the Back button, they are in control of the experience. If you try to force users to move through your web app using a specific flow because you want to be control, you’ll fail. You’ve got to respect your users; they’re not slaves to your web application. To be successful on the Internet you must recoginize that the user is always free to walk away at any moment, and hitting the Back button is just one way they can do it. Would you rather that the user close their web browser completely and start over by visiting one of your competitors?

Okay, enough ranting. Maybe I’ll buy McClanahan a drink tonight and try to convince him to spend some time reading Jakob Nielsen. I’ll get back to taking notes on the presentation now.

Sometime while I was composing my rant, McClanahan started talking about Struts. Struts focuses mostly on the Controller aspect of MVC, delegating the Model aspect to other Sun technologies like Enterprise Java Beans and JDBC, and letting you use JSP for the View component.

He used a web logon application as a motivating example to show each of the MVC layers. We saw some slides of JSP syntax for the View Layer (login.jsp), and XML config file that struts uses for the Controller layer which maps URI paths and “actions” to Java class names and parameters. One of the XML config files essentially tells Struts what classes it will need to instantiate. Another encodes rules about what the data should look like (username and password being non-empty, etc.)

It appears that Struts uses many levels of indirection to make your application as reusable as possible. For example, your business logic Java code shouldn’t import anything form org.apache.struts because that would bind it too tightly to the Struts framework. Similarly, you need to avoid importing javax.servlet into your business logic because that binds too closely to a web application. This level of abstraction aids in reusability, but it takes a lot of work to keep it clean.