n a previous article, I introduced you to the Spring framework, showed you how to use Spring's basic functionality to create objects, and how to do some simple database interactions. In this follow-up article I will introduce you to the Spring model-view- controller (MVC) framework by walking you through the creation of a simple stock-trading Web application.

MVC is a pattern that helps separate presentation from business logic. In short, in an MVC application all Web requests are handled by controllers. A "controller" is responsible for interpreting the user's request and interacting with the application's business objects in order to fulfill the request. These business objects are represented as the "model" part of the MVC. Based on the outcome of the request execution, the controller decides which "view" to forward the model to. The view uses the data in the model to create the presentation that is returned to the user.

If you've ever worked on a Web application, chances are that you've worked with either a custom MVC framework or an existing framework, such as Struts. Struts is in fairly widespread use in the Java world, but the Spring MVC framework promises to provide a simpler alternative to Struts.

In this article I will walk you through the development of a Web-based stock-trading application. This application will have three primary workflows each of which will cover a different type of controller available in Spring. After reading this article, you should have enough technical and business knowledge to build a full-fledged stock-trading application and compete head-on with existing discount brokerage firms. OK, not really, but you will at least have enough knowledge to get you started on your own Web projects using Spring.

You can download the Spring distribution from http://www.springframework.org/download.html. If you want to see the application in action, download the source code for this article, which includes a deployable war file.

Author's Note: The code in this article has only been tested on Tomcat 5.5.

The Platform
There are many different Web application servers and development environments to choose from. I won't describe any single environment here because I want to focus the discussion on the Spring framework and not the underlying tools. However, I developed this sample application using Tomcat 5.5, Java 1.5, Spring 1.1, and Eclipse 3.0 using the Sysdeo Tomcat plugin. I have used Spring with other application servers and with different versions of Java 1.4.x without any big surprises, so you should be safe if you have a slightly different environment.

Getting Started
Before diving into Spring, set up a mechanism for deploying your application code into your Web server and set up an application context if necessary. In Tomcat 5.5, the application context automatically takes on the name of the war file (or expanded war directory) that you deploy. My war file is called "tradingapp.war ."

Here is the basic directory structure that you need to create for your Web app:

 /WEB-INF /src (java classes will go in here) /jsp (application jsps will go in here) /lib (jar files that our app depends on will go in here) /classes (compiled class files will go in here) 

In order to test out the application server setup, create a file called index.jsp (as follows) and put it in the root of your application directory:

 /index.jsp    Trading App Test   

Figure 1. Clear the Deck: Pull up the index.jsp page to make sure that you can retrieve a simple JSP page.

Try to pull up the page by going to http://localhost:8080/tradingapp/index.jsp (see Figure 1). (The port may vary depending on the application server you are using.)

On most application servers, users can access files in the root directory of the context. Hence, you were able to hit the index.jsp file directly. In an MVC architecture, you want the controller to handle all incoming requests. In order to do this, it is a good idea to keep your JSP files in a place where they are not directly accessible to the user. That is why we will put all of our application JSP files in the WEB-INF/jsp directory. We'll come back to this soon, but for now let's find out how to access a Spring controller.