Application Controller

See Core J2EE Patterns, 2nd Edition for full description of this pattern and its strategies.

Problem

You want to centralize and modularize action and view management.

Forces

• You want to reuse action and view-management code.
• You want to improve request-handling extensibility, such as adding use case functionality to an application incrementally.
• You want to improve code modularity and maintainability, making it easier to extend the application and easier to test discrete parts of your request-handling code independent of a web container.

Solution

Use an Application Controller to centralize retrieval and invocation of request-processing components, such as commands and views.

Class Diagram

Sequence Diagram

Strategies

• Command Handler Strategy
• View Handler Strategy
• Transform Handler Strategy
• Navigation and Flow Control Strategy
• Message Handling Strategies
• Custom SOAP Message Handling Strategy
• JAX RPC Message Handling Strategy

Consequences

• Improves modularity
• Improves reusability
• Improves extensibility

Related Patterns

• Front Controller
  A Front Controller uses an Application Controller to perform action and view management.
• Service Locator
  A Service Locator performs service location and retrieval. A Service Locator is a coarser object, often uses sophisticated infrastructure for lookup, and doesn’t manage routing. It also doesn’t address view management.
• Command Processor [POSA1]
  A Command Processor manages command invocations, providing invocation scheduling, logging, and undo/redo functionality.
• Command Pattern [GoF]
  A Command encapsulates a request in an object, separating the request from its invocation.
• Composite Pattern [GoF]
  A Composite represents objects as part-whole hierarchies, treating individual objects and compositions of objects uniformly.
• Application Controller [PEAA]
  Martin Fowler’s description of Application Controller [PEAA] seems to focus on controlling a user’s navigation through an application using a state machine, as described in the Navigation and Flow Control strategy. However, the Application Controller [PEAA] and our documentation of Application Controller have the same core intent.

Excerpted from Core J2EE Patterns, 2nd Edition. See the book for full description of the pattern including class diagrams, sequence diagrams and sample code.