J2EE Design Patterns

Name

Front Controller

Goal

Create a single, central component to perform common functions (Figure A-3).

Figure A-3. Classes in the Front Controller pattern

Participants

Front controller

Singleton object that intercepts all requests and performs common functions.

Page controllers (or Actions)

Process user input, update the model, and choose views.

Model

Store application state data.

Views

Transform model data into a form appropriate for display to the user (Figure A-4).

Figure A-4. Interactions in the Front Controller pattern

Interactions

The front controller intercepts all requests and performs common functions such as logging or decryption. The front controller chooses and forwards the request to a page controller. The page controller parses user input, and translates it into appropriate updates to the model. The model applies business rules and stores the data persistently, either locally, in the business tier, or using some other remote persistence mechanism. Based on the model changes and user input, the page controller chooses a view. The view transforms the updated model data into a form suitable for the user.

Notes

In J2EE, the front controller is typically implemented as a servlet, although in some cases it may be implemented as a servlet filter.