Because an understanding of the Model-View-Controller architecture is crucial to understanding Struts, this section takes a closer look at each of its parts. As a point of interest, MVC is based on an older graphical user interface (GUI) design pattern that has been around for some time, with its origins in the Smalltalk world. Many of the same forces behind MVC for GUI development apply nicely to Web development.
In the MVC architecture, model components provide an interface to the data and/or services used by an application. This way, controller components don't unnecessarily embed code for manipulating an application's data. Instead, they communicate with the model components that perform the data access and manipulation. Thus, the model component provides the business logic. Model components come in many different forms and can be as simple as a basic Java bean or as intricate as Enterprise JavaBeans (EJBs) or Web services.
View components are used in the MVC architecture to generate the response to the browser. Thus, a view component provides what the user sees. Oftentimes the view components are simple JSPs or HTML pages. However, you can just as easily use WML, a templating engine such as Velocity or FreeMarker, XML with XSLT, or another view technology altogether for this part of the architecture. This is one of the main design advantages of MVC. You can use any view technology that you'd like without impacting the Model (or business) layer of your application.
At the core of the MVC architecture are the controller components. The Controller is typically a servlet that receives requests for the application and manages the flow of data between the Model layer and the View layer. Thus, it controls the way that the Model and View layers interact. The Controller often uses helper classes for delegating control over specific requests or processes.