Model View Controller

MVC provides a good abstraction and framework for combining GUI and the underlying application logic. However, different frameworks provides slightly different infrastructure for implementing MVC. Here's a summary of them.
 
Ruby on Rails:

You can implement a similar framework in Java:

• Model - The model represents data and the rules that govern access to and updates of this data. In enterprise software, a model often serves as a software approximation of a real-world process.
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• View - The view renders the contents of a model. It specifies exactly how the model data should be presented. If the model data changes, the view must update its presentation as needed. This can be achieved by using a push model, in which the view registers itself with the model for change notifications, or a pull model, in which the view is responsible for calling the model when it needs to retrieve the most current data.
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• Controller - The controller translates the user's interactions with the view into actions that the model will perform. In a stand-alone GUI client, user interactions could be button clicks or menu selections, whereas in an enterprise web application, they appear as GET and POST HTTP requests. Depending on the context, a controller may also select a new view -- for example, a web page of results -- to present back to the user.

Interaction Between MVC Components

1. The view registers as a listener on the model. Any changes to the underlying data of the model immediately result in a broadcast change notification, which the view receives. This is an example of the push model described earlier. Note that the model is not aware of the view or the controller -- it simply broadcasts change notifications to all interested listeners.
2. The controller is bound to the view. This typically means that any user actions that are performed on the view will invoke a registered listener method in the controller class.
3. The controller is given a reference to the underlying model.

Modifying the MVC Design

A more recent implementation of the MVC design places the controller between the model and the view. This design is common in the Apple Cocoa framework. 

The primary difference between this design and the more traditional version of MVC is that the notifications of state changes in model objects are communicated to the view through the controller. Hence, the controller mediates the flow of data between model and view objects in both directions. View objects, as always, use the controller to translate user actions into property updates on the model. In addition, changes in model state are communicated to view objects through an application's controller objects.