Wizard API updated!
Tim Boudreau has released a new version of the Swing Wizard library (version 0.997) that fixes the WizardException bug reported in JavaWorld's recent Open Source Java Project profile. The article's examples have been reworked to test out the new, improved WizardException. Thanks, Tim, for this helpful fix!
Open Source Java Projects: The Wizard API

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HMVC: The layered pattern for developing strong client tiers

This hierarchical model eases the development of a Java-based client tier

By Jason Cai, Ranjit Kapila and Gaurav Pal, JavaWorld.com, 07/21/00

The task of designing and developing the client tier of an n-tier Web architecture often challenges developers. This is particularly true in the Web world, where the sheer variety of servers, deployment platforms, and protocols turns the challenge into a headache. A client-tier architect must address a number of questions:

  • How should I structure my GUI?
  • How will users interact with my GUI?
  • How should I separate server-side/transport data formats from my GUI?
  • How should I provide sound mechanisms for event management, application flows, and widget control?


In order to understand some of these key issues, we must differentiate between the presentation layer (or client tier) and the GUI layer. The GUI layer deals with a small subset of the whole presentation layer, namely the UI widgets and the immediate effects of user actions -- a JTextField and its ActionListener, for example. The presentation layer needs to deal with application flows and server interaction in addition to providing GUI services. The terms presentation layer and client tier are used interchangeably in this article.

Framework-based approach

To mitigate the risk associated with creating a robust client tier, developers have produced several frameworks and design patterns with varying degrees of success. The Model-View-Controller (MVC) paradigm remains one of the more enduring patterns. However, the traditional MVC scope falls short when it comes to the control of GUI elements (widgets). MVC does not handle the complexities of data management, event management, and application flows. As an adaptation of the MVC triad, the HMVC -- Hierarchical-Model-View-Controller -- paradigm seeks to redress some of the above-mentioned issues. We developed this pattern during the course of our work in the field. HMVC provides a powerful yet easy-to-understand layered design methodology for developing a complete presentation layer. While MVC provides an efficient framework for developing GUI interaction, HMVC scales it to the entire client tier. Some key benefits of a responsibility-based, layered architecture include:

  • Defined intralayer communication and isolation from higher layers
  • Defined interlayer communication with minimal coupling
  • Localization of exposure to third-party code


This article explores the application of the HMVC design pattern in the development of a Java-based client-tier infrastructure.

Note: The entire source code for this article can be downloaded as a zip file from the Resources section below.

Model view controller -- MVC

Developers primarily use MVC in Smalltalk for implementing GUI objects. Numerous GUI class libraries and application frameworks have reused and adopted the pattern. As the MVC paradigm offers an elegant and simple means for solving UI-related problems in an object-oriented way, its popularity is justified. MVC provides clearly defined roles and responsibilities for its three constituent elements -- model, view, and controller. The view manages the screen layout -- that is, what the user interacts with and sees on the screen. The model represents the data underlying the object -- for example, the on-off state of a check box or the text string from a text field. Events cause the data in the model to change. The controller determines how the user interacts with the view in the form of commands.

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