How to build a framework

Java Framework Design

• Article: Java Framework Design
• Building a Framework
• Artikel: A ”Framework” for Object Oriented Frameworks Design
• Book: Framework Design Guidelines

Notes

Framework Structure

• frozen spot to denote a common (or shared) aspect of the family
• hot spot to denote a variable aspect of the family
• frozen spots
  • set of abstract and concrete base classes that collaborate in some structure
  • common behaviour
    • implemented by a fixed, concrete template method in a baseclass
• hot spots
  • represented by a group of abstract hook methods
  • template method calls a hook method to invoke a function that is specific to one family member
  • hot spot is realized in a framework as a hot spot subsystem
    • consists of an abstract base class, concrete subclasses of that base class, and perhaps other related classes
• two principles for framework construction
  • unification principle
    • inheritance to implement hot spot subsystem
    • template methods and hook methods in same abstract base class.
    • hook methods are implemented in subclasses of the base class.
  • separation principle
    • delegation to implement hot spot subsystem
    • template methods in concrete context class
    • hook methods in separate abstract class and implemented in its subclasses
• useful patterns
  • Template Method
    • uses unification principle
  • Strategy pattern
    • uses separation principle
• which principle to choose
  • An application of a framework that uses the separation principle may execute slightly less efficiently than a unification-based framework, but separation may enable the application to adapt itself at runtime by merely changing object references
• inversion of control
  • its template methods “call down” to the client-supplied hook methods

Roles

During framework-based application development, users fall into four categories or roles:

• framework developer
  • build framework 'backbone'
• component developer
  • builds new components and integrates them in the framework
• application developer
  • assembles components into a custom application
• end user
  • uses the application

Rules of Thumb

• control visibility to keep published parts of the Framework small
• use Interfaces
• provide well-divined "hook points" that permit extensibility in places where you intend it to occur
• prevent extension in all other places